Competence No.6 (Course covered 6.2) 6.2/9 . Theoretical knowledge of construction and operation of marine boilers including materials used.
A boiler is a closed pressure vessel wherein steam is generated by boiling distilled water / feed water under pressure. All boilers have a furnace or combustion chamber where fuel is burnt to release its energy. Air is supplied to the boiler furnace to enable combustion of the fuel to take place. A large surface area between the combustion chamber and the water enables the energy of combustion, in the form of heat, to be transferred to the water. Boilers are fitted with one steam drum and one water drum each to ensure steam and water can be separated. There must also be a variety of fittings and controls to ensure that fuel oil, air and feed water supplies are matched to the demand for steam.
Fig 1 Requirement of a Marine Boiler: • It should be capable of generating the maximum quantity of steam with minimum fuel consumption • It should be light in weight and should not occupy much space • It should have safe working conditions • The initial cost, installation cost and maintenance cost of the boiler should be low. • It should be accessible for easy inspection and repair. • It should be capable of quick starting and should be able to meet rapid variations of load.
Types of boilers: • Fire tube boilers • Water tube boilers
Fire tube boilers:
In fire tube boilers, the hot gases pass though the tubes that are surrounded by water. The water is heated up and converted into steam. E.g. Cochran, Scotch boiler & Clarkson boiler. The FTB is usually chosen for low-pressure steam production on vessels requiring steam for auxiliary purposes. Operation is simple and feed water of medium quality is used.
Water tube boilers: In water tube boilers, water is circulated though the tubes and hot flue gases flow outside the tubes. e.g. Bobcock & Wilcox, Admiralty three drum, Y-160 and Foster wheeler D-type. The water tube boiler is employed for high pressure, high temperature, high capacity steam applications, e.g. providing steam for main propulsion turbines of cargo pump turbines. Firetube boilers are used for auxiliary purposes to provide smaller quantities of low pressure stem on diesel engine powered ships. Fig.2 a. Smoke uptake m. Footing b. Economizer n. Waterwall A heat exchanger that transfers Tubes welded together to form a heat from Boiler Flue Gases to Boiler Feedwater. wall. c. Steam outlet o. Waterwall Header Saturated steam from the Steam Distribution box for waterwall and Drum to the Superheater downcomers. d. Cyclone p. Back side waterwall A device inside the steam drum that q. Boiler hood is used to prevent water and solids r. Waterwall Header from passing over with the steam. Collecting box for waterwall and e. Stay tube risers. for superheater s. Riser f. Stays Tubes in which steam is generated for superheater tubes due to high convection or radiant g. Superheated steam outlet heat. The water-steam emulsion h. Superheater rises in these tubes toward the A bank of tubes, in the exhaust gas steam drum. duct after the boiler, used to heat t. Downcomer the steam above the saturation A tube through which water flows temperature. downward. These tubes are i. Superheater Headers normally not heated, and the boiler Distribution and collecting boxes for water goes through them to supply the superheater tubes. the generating tubes. j. Water drum u. Steam drum k. Burner separates the steam from the l. Waterwall Header water. Distribution box for waterwall and v. Economizer Header downcomers. Distribution box for the economizer tubes.
Construction of boilers: The construction of water tube boiler, which use small-diameter tubes and have a small steam drum, enables the generation or production of steam at high temperatures and pressures. The weight of the boiler is much less than that of the fire tube boiler and steam raising process is much quicker. This boiler has two drums namely steam drum (bigger in size) and water drum (smaller in size) and an integral furnace. This furnace is at the side of the two drums and is surrounded on all sides by walls of tubes. Between these two drums, large numbers of smaller diameter generating tubes are fitted. Tubes neighboring furnace are called fire row tubes of screen tubes which act as an upriser. Large bore down comer pipes are fitted between steam drum and water drum to ensure good natural circulation of water. In this arrangement, super heater is located between the drums, protected from the very hot furnace gases by several rows of screen tubes. Refractory material or brick work is used on the furnace floor and the burner wall. The double casing of the boiler provides a passage for the combustion air to the control of register surrounding the burner. The furnace side, the floor and roof tubes are welded into the steam and water drums. The lower water wall headers are connected by external down comer from the steam drum and upper water wall header are connected to the steam drum by riser tubes. The gases leaving the furnace pass through screen tubes which are arranged to permit flow between them. The large number of tubes results in considerable heat transfer before the gases reach the secondary superheater. The gases then flow over primary superheater and the economizer before passing to exhaust. Fig.3 Water circulation: In the steam generation process the feed water enters the boiler where it is heated and becomes steam. The feed water circulates from the steam drum to the water drum and is heated in the process. The water from the water drums rise up to the steam drum due to the thermo-convection current through uprisers consisting of generator row tubes and screen tubes. The downcomers fitted on each boiler bring down the relatively cooler water from the steam drum to the water drum to establish a positive circulation during normal operation. Some of the feed water passes through tubes surrounding the furnace, i.e. water wall and floor tubes, where it is heated and returned to the steam drum (in case furnace is cooled by water filled tubes). Large bore downcomer tubes are used to circulate feed water between the drums. The downcomer tubes pass outside of the furnace and join the steam and water drums. The steam is produced in a steam drum and drawn of as a ‘saturated steam’ which contains small quantities water particles. Alternatively the steam may pass to a superheater which is located within the boiler. Here steam is further heated and dries, i.e. all traces of water are converted into steam. This superheater steam then leaves the boiler for use in the system. The temperature of this steam will be above that of the steam in the drum. An attemperator may be fitted in the system to control the temperature of the superheated steam as per requirements.
Materials Used in Boiler Construction. Component Material Composition & Description Boiler Casing Mild steel plates (low C, Si, Mn, (Hot finished rolled plates) carbon) Steam Drum Mild steel plates (low C, Si, Mn (Hot finished) carbon) TS-430-490MN/m2 Fire Row & Gen. Cold drawn seamless (low C, Si, Mn, S, P (cold drawn) Row Tubes carbon steel) S/H Tubes Cr, Molybdenum alloy steel Cr, Mo, C, Si, Mn, Mi (cold finished) S/H Tube Support Heat resistant austenitic C, Si, Mn, Ni, Cr, P, S (hot finished) steel Steam Piping (S/H Cr-Mo low carbon alloy C, Si, Mn, P, S, Ni, Cr, Mo (cold range upto 9500F) steel finished) Economiser Tubes Cold drawn seamless steel C, Si, Mn, P (stud resistance welded to tubes)
Water Drum Mild steel plates (low C, Si, Mn (hot finished rolled plate) carbon)
6.2/10. List the services provided by boilers and the typical pressures used.
• For main engine propulsion/turbines (in case of steam ships) • For power generation (to run steam turbo generators) • For running auxiliaries (in case of steam ships) • For soot blowing and for the steam atomized burners. • For fresh water generation (Evaporators) • For fire major fighting (steam drenching) • For heating duties (ME fuel oil heater, Galley supply, Purifier, Calorifier, Galley, Accommodation heating, Sea chests tracer lines for pipeline heating) • For cargo heating • For fuel treatment plant tank coil heating • For deck machineries • For running Cargo pump turbines • For operating bilge, stripping and other steam driven pumps. • For tank washing in tanker ships and general cleaning. • For using as a steam ejector media for ejector pumps and vacuum devices • For Driving steam driven deck machineries like winches etc.,
Pressures used:
The working pressure used in marine boilers will vary from boiler to boiler as required. Still the normal working pressure of boilers used is as below:-
For Steam ships High pressure 60 bar and above For Motor ships Low pressure 6-15 bar Medium pressure 17-30 bar For Tanker Vessels Medium pressure 17-30 bar. 6.2/11. Define a) Fire tube boiler b) Water tube boiler c) Packaged boiler and briefly explain the differences and why one type of boiler is preferred over other
Fire tube boilers: In fire tube boilers, the hot gases pass though the tubes that are surrounded by water. The water is heated up and converted into steam. e.g. Cochran, Scotch boiler & Clarkson boiler. The FTB is usually chosen for low-pressure steam production on vessels requiring steam for auxiliary purposes. Operation is simple and feed water of medium quality is used.
Water tube boilers: In water tube boilers, water is circulated though the tubes and hot flue gases flow outside the tubes. e.g. Bobcock & Wilcox, Admiralty three drum, Y-160, Foster wheeler D-type. The water tube boiler is employed for high pressure, high temperature, and high capacity steam applications.
Package boiler: Where relatively small, intermittent steam demands are to be met, use is often made of package boilers. This term is usually applied to self contained units mounted on a single bedplate and comprising a steam generating section, feed water system and pump, fuel oil system and pump, together with a forced draught fan. In addition suitable control equipment will also be required. This package now only needs connections Fig.4 to the ship’s electrical supply and other necessary services to become operational. Fully automatic controls are provided and located in a control panel at the side of the boiler.
Differences between fire tube and water tube boilers:
S No Fire tube boilers Water tube boilers a. Hot flue gases pass through tubes Water is circulated through tubes that which are immersed in water are surrounded by hot flue gases b. Pressure range is limited to 25 bar This can generate steam at a pressure of 200 bar c. Raising of steam is slow Raising of steam is more rapid
d. Reduced evaporation since heating Evaporation rate is high since water is surface area is less circulated though tubes e. Bursting of even one tube affects Bursting of one or two tubes does not the function of boiler very much affect the function of the boiler very much f. The chances of bursting is less The chances of bursting is more
g. Suitable for rapid changes in load Preferable for large load fluctuations like locomotive boiler extending over longer durations h. Space occupied per kg of steam Space occupied per kg of steam generation is less generation is more i. It is not suitable for power plants It is best suitable for power plants since reduced evaporation j. Construction of this boiler is Cheaper for the same capacity of fire costlier and difficult tube boilers. k. The drums are protected form flame impingement or direct heat.
6.2/12. Explain why shells of cylindrical forms are preferred and why end plates of spherical types are to be preferred over flat end plates
When a force is applied to a curved plate as shown in fig, internal forces are set up which enable the plate to withstand the force without undue distortion. The stress acting on a circumferential area will be equally distributed throughout its area, hence the force acting per unit area gets distributed and load bearing capability is increased. The cylindrical end joint need not be as strong as longitudinal joint. The cylindrical shape has an advantage of reduction in space consumption and no supporting stays are required. Whereas in case of flat plate the force applied tries to bend the plate until equilibrium is obtained, thus comes the requirement of stays. The pressure vessels are often given hemispherical ends but, if this is not possible, any flat surfaces must be stayed or of sufficient thickness to resist the pressure without undue distortion. Forc
Bursting Stress acts perpendicular to any radius Component of stress to balance
Fig.5 Stress in a curved plate
Internal stays
Hemispherical end plates no Flat end plates internal stays must be internal stays required fitted to support them
6.2/13. What are different types of stays used in boiler and why?
Stay tubes are used in marine boilers to support boiler tubes in construction to resist the pressure without undue distortion. Types of Stays: • Girder stays • Gusset stays
Girder stays: Girder stays are used to support the top of the combustion chamber, transmitting the bending stresses from the top Girde wrapper plate onto the r vertical tube plate and back Stays plate of the chamber. Gusset stays: The Cochran is typical tank boiler of vertical type suitable for producing small amounts of low pressure steam for auxiliary purposes. In this boiler, the combustion chamber top requires support, and this is provided by means of a gusset stay which transfers the stresses from the flat top of the chamber into the boiler shell. Gusset stay
6 .2/14 . Explain the advantages of using corrugated furnaces
Furnaces are corrugated for strength; the arrangement also gives increased heating surface area as compared to a plain furnace of similar dimensions. Various types of corrugation are available, but the suspension bulb type is preferred since for a given working pressure and furnace diameter the material thickness can be less than any other form of corrugation, hence heat transfer will be improved.
6.2/15. Describe how tubes are expanded in tube plates and explain the differences in following:- a) Plain tube b) Stay tube c) Single flow tube (d) Swirl flow tube (e) Thimble tube
Attachment of tubes in water tube boilers Tubes can be attached to drums and headers by expanding or by welding. In most cases the generating, screen and water wall tubes are expanded into plain seats, and then bell mouthed. The tube ends must be cleaned, and then carefully drifted, or roller expanded into the holes in the tube plate. They must project through the tube plate by at least 6mm. To prevent tubes pulling out of the tube plate, they must be bell-mouthed. In case of tubes with larger diameters, such as down comers, it is usual to use grooved seats. Super heater tubes are also usually expanded and bell-mouthed up to steam temp of about 450⁰ C, above this the tubes are often attached by welding. Piping systems
Machinery space pipe work is made up of assorted straight lengths and bends joined by flanges with an appropriate gasket or joint between or very small bore piping may use compression couplings. The piping material will be chosen to suit the system conditions.
The pipes are supported and held in by hangers or pipe clips in such a way as to minimize vibration. Steam pipes or pipes in systems with considerable temperature variation may be supported on spring hangers which permit a degree of movement and are called ‘load hangers’. An alternative to spring hangers is the use of expansion loops of piping or an expansion joint.
Expansion of pipes
An expansion piece is fitted in a pipe line which is subject to considerable temperature variations. One type consists of a bellows arrangement which will permit movement in several directions and absorb variation. The fitting must be selected according to the variation in system temperatures and installed to permit the expansion and contraction required in the system.
Plain tube The plain or common tubes are used in the boiler between steam and water drum to generate steam and are expanded into the tube plates at both ends. The tubes have a diameter of about 65mm with a thickness of 5mm. The front end of the tube often swelled out to allow for easier tube removal. The back end of the tube is bell-mouthed after expansion, or may be spot-welded.
Stay tubes Fire tube boiler has a number of flat surfaces which require support. Stays are fitted in the steam drum and in the annular water space, which together with a number of tubes provide the necessary support is called stay tubes. These tubes being screwed and then expanded into both tube plates. The thickness of these stay tubes varies according to the load to be supported, but must not be less than 5mm the base of the thread. After the tubes has been screwed and then expanded into the tube plates, nuts are usually fitted at the front end but not in the combustion chamber to avoid overheating. Welding can be used after screwing the stay tubes into the tube plates but the tubes must be expanded before and after welding.
Swirl flow tube The vertical smoke tubes are known as swirlyflow tubes, they have a special twist along the greater part of their length, only a short portion at each end being left plain to allow for expansion. It is claimed that these tubes are more efficient than normal plain smoke tubes in that they cause the gases passing through to swirl so coming into more intimate contact with the tube wall and therefore increasing the rate of heat transfer.
Thimble tube The boiler was developed to generate steam by causing a prolonged series of spasmodic ebullitions to take place in a series of horizontal tapered thimble tubes heated externally, without any special means being provided for circulation within the tube. It enhances the heat transfer between the tubes and feed water to produce steam. It consists of an outer shell enclosing a cylindrical furnace surmounted by the combustion chamber into which the thimble tubes project. These are expanded and bell-mouthed into a cylindrical tube plate forming the combustion chamber. These boilers will operate for long periods without internal cleaning although, if an undue amount of scale forms inside the thimble tubes, it is very difficult to remove. Thus reasonable quality of feed water should be provided. The formation of scale will subject the thimble tubes to a certain amount of overheating. The tubes have a diameter of about 100mm.
Stay tube, screwed into plate fitted with nut and expanded
Stay tube within nest, expanded before and after welding Margin stay tube, expanded before and after welding
[[[[[
Plain tube, expanded
Fig.5 Scotch boiler tubes
6.2/16. Sketch the path of water circulation and gas paths in boilers Path of Water circulation and gas paths: 6 .2/17 . List all boiler mountings: a) on shell b) internal and describe briefly their purposes
Boiler Mountings Various valves and other fittings are required for the proper working of the boiler. Those attached directly to the pressure parts of the boiler are referred to as boiler mountings which are being performed either directly, or indirectly, by means of extended rods, spindles. Boiler Mountings fitted on boiler are safety valves, main steam stop valves, aux sup steam stop valves, sat steam stop valve, main feed check valve, aux feed check valve, water gauges, pressure gauges, air cocks, running down valves, blow down cocks, super heater header drain valves, robot feed regulator, boiler sampling cocks and soot blowers. (a) Safety valve. These are fitted to protect the boiler form the effects of overpressure. As per international regulations, at least two safety valves are fitted to each boiler, but in practice it is usual to fit three safety valves – two on the steam drum, and one on the super heater outlet header. This is fitted to release excess steam pressure from the boiler.
(b) Main steam stop valves This is mounted on the superheater outlet header, and enables the boiler to be isolated from the steam drum. Two may be fitted to control the passage of steam from the boiler to the main steam range and of SDNR type to prevent steam flowing into a damaged boiler in the event of loss of pressure due to a burst tube.
(c) Auxiliary superheated steam stop valve Fitted to supply steam to auxiliary superheated steam range to run the auxiliaries and TAs and may be utilized to augment steam to the main steam range means for turbines through a suitable cross connection valve in case of an emergency. This too of SDNR type.
(d) Saturated steam stop valve Fitted on the steam drum to supply saturated steam.
(e) Feed check valve To give final control over the entry of feed water into the boiler and they must be SDNR valve so that in the event of a loss of feed pressure, the boiler water cannot be back into the feed line. Main and aux feed checks are fitted with extended spindles so that checks can be operated easily and quickly from the operator’s convenient positions.
(f) Feed water regulator. The water level in a boiler is critical. If it is too low, damage may result from overheating, too high and priming can occur with resultant carry-over of water and dissolved solids into superheaters steam lines. So automatic feed regulator are fitted to control the flow of water into the boiler and maintain the water level at its desired value. The regulator is fitted in the feed line (after the feed heater if provided) before the main feed check valve.
(g) Air cocks or Air vents. These are fitted to the upper parts of the boiler as required to release air from drums and headers either when filling the boiler, or raising steam. These ‘air cocks’ are fitted to purge out the air when the boiler is being topped up we have to release air while raising steam and also during ‘run down’ of a boiler.
(h) Water level indicators. The DOT demand that at least two water level gauges must be fitted on each boiler steam drum. In practice the usual arrangement consists of two direct reading water level gauges mounted on the steam drum, and a remote reading indicator placed at a convenient position.
(i) Pressure gauges Each boiler is fitted with two pressure gauge tappings on steam drum. One is for direct reading pressure gauge and the other for sensing pressure gauge. The sensing pressure gauge tapping after steam drum further branches to indicate drum pressure in boiler room and a remote position.
(j) Running down valves The purpose of these valves is to run down the water from the water drum when there is a need to lower the level of water in the steam drum before steaming or to drain the boiler when it has to be emptied.
(k) Blow down cocks There are mostly two blow down cocks fitted on each boiler. These cocks are fitted in series with two other valves i.e. the “intermediate blow down valve” and the “overboard blow down valve”. The main purpose of these cocks is to blow down the boiler water deposits when the boiler is steaming thereby reducing density.
(l) Superheater header drain valves These are fitted as required to boiler superheater header. These valves are fitted in connection with a steam trap to drain off water from the headers.
(m) Boiler water sampling cocks The main purpose of the ‘sampling cock’ is to take samples of the boiler water to calculate the alkalinity and salinity of the boiler water. Water drum is provided with one sampling valve which allows the sampling water to pass through a cooling coil which reduces its temperature.
(n) Soot blowers Soot tends to accumulate between the tubes of the boilers, superheaters and economisers. It requires to be cleared at frequent intervals while steaming to prevent subsequent blockage thereby reducing the boiler output. Soot blowers consist of a steam nozzle which when operated directs a jet of steam through the tube banks. The soot blowing routine is undertaken as required. (o) Chemical Dosing valve. These are fitted to the steam drum to enable feed treatment chemicals to be injected directly into the boiler
(p) Scum valves. These should be fitted when there is a possibility of oil contamination of the boiler. They are mounted on the steam drum, having an internal fitting in the form of a shallow pan situated just below the normal water level, with which to remove oil or scum from the surface of the water in the drum. These valves discharge into the blow down line.
6.2/18. Explain purpose and working of a a) reducing valve b) steam straps c) drains
Reducing valve: Reducing valve is used for the reduction of steam or air pressure. As steam passes through the valve no work is done since the reduction process is the throttling, hence the total heat before and after pressure reduction is nearly the same. The reducing valve would normally have a body of cast steel or iron. A valve, valve seat and spindle of steel or bronze. Choice of materials depends upon operating conditions. Fitted on the discharge side of the valve is a pressure gauge to record the reduced pressure and relief valve to prevent damage to the low pressure side of the system in the event of the reducing valve failing.
Steam traps: A steam trap is a special type of valve which prevents the passage of steam but allows condensate to pass. It works automatically and is put into drain lines so that these drain off condensate automatically without passing any steam. As its name implies and permits only the passage of condensed steam. Steam traps of three types, they are:- (a) Mechanical type (b) Thermostatic type (c) Thermodynamic type
Mechanical type Mechanical (operated by changes in fluid density) - This range of steam traps operates by sensing the difference in density between steam and condensate. These steam traps include 'ball float traps' and 'inverted bucket traps'. In the 'ball float trap', the ball rises in the presence of condensate, opening a valve which passes the denser condensate. With the 'inverted bucket trap', the inverted bucket floats when steam reaches the trap and rises to shut the valve. Both are essentially 'mechanical' in their method of operation Thermostatic type (Bimetallic steam trap)
As the name implies, bimetallic steam traps are constructed using two strips of dissimilar metals welded together into one element. The element deflects when heated. Deflection of the bimetallic strip with increasing temperature closes the valve. There are two important points to consider regarding this simple element:
Fig. Simple bimetallic element
• Operation of the steam trap takes place at a certain fixed temperature, which may not satisfy the requirements of a steam system possibly operating at varying pressures and temperatures • Because the power exerted by a single bimetal strip is small, a large mass would have be used which would be slow to react to temperature changes in the steam system.
The performance of any steam trap can be measured by its response to the steam saturation curve. The ideal response would closely follow the curve and be just below it. A simple bimetal element tends to react to temperature changes in a linear fashion.
Fig. Operation of a bimetal steam trap with two leaf element
Thermodynamic type
High pressure drain traps are chiefly of the thermodynamic type. Condensate and air raise the trap disc to permit the flow. When steam reaches the trap, the velocity under the disc is increased and recompression above the seat straps is shut. Heat loss from the control chamber causes pressure to decrease and this causes the trap disc to open again and discharge condensate. When the hot condensate reaches the chamber some of it flashes off into low pressure steam (saturated) which is taken away into the exhaust range. The remaining condensate drains at low pressure through a ball float trap to the unit drain cooler, or to a drain tank where there is a cooling element supplied with circulating water from the auxiliary circulating system. The drain tank is pumped out to the tank by a suitable pump. The thermodynamic trap is an extremely robust steam trap with a simple mode of operation. The trap operates by means of the dynamic effect of flash steam as it passes through the trap, as depicted in Figure. The only moving part is the disc above the flat face inside the control chamber or cap. On start-up, incoming pressure raises the disc, and cool condensate plus air is immediately discharged from the inner ring, under the disc, and out through three peripheral outlets [Fig (i)]. Hot condensate flowing through the inlet passage into the chamber under the disc drops in pressure and releases flash steam moving at high velocity. This high velocity creates a low pressure area under the disc, drawing it towards its seat (Figure ii). At the same time, the flash steam pressure builds up inside the chamber above the disc, forcing it down against the incoming condensate until it seats on the inner and outer rings. At this point, the flash steam is trapped in the upper chamber, and the pressure above the disc equals the pressure being applied to the underside of the disc from the inner ring subject to a greater force than the underside, as it has a greater surface area. Eventually the trapped pressure in the upper chamber falls as the flash steam condenses. The disc is raised by the now higher condensate pressure and the cycle repeats (Figure. iv). Fig. Thermodynamic Advantages of thermodynamic steam trap • Thermodynamic traps can operate across their entire working range without any adjustment or change of internals. • They are compact, simple, lightweight and have a large condensate capacity for their size. • Thermodynamic traps can be used on high pressure and superheated steam and are not affected by waterhammer or vibration. The all stainless steel construction offers a high degree of resistance to corrosive condensate. • Thermodynamic traps are not damaged by freezing and are unlikely to freeze if installed with the disc in a vertical plane and discharging freely to atmosphere. However, operation in this position may result in wear of the disc edge. • As the disc is the only moving part, maintenance can easily be carried out without removing the trap from the line. • The audible 'click' which occurs as the trap opens and closes makes trap testing very straight forward. Drains: The drains are fitted to auxiliary exhaust and low pressure saturated steam systems. In this system, initially the drains are put onto bilges in the boiler /engine room and thereafter may be led into suitable drain tank for use.
6.2/19. Explain a) how lengths of steam pipes are joined b) how the pipes are supported c) how expansion is allowed for
Pipe Installation Pipe connections should be as direct as possible, sharp bends and loops must be avoided. The loop could increase turbulence and be the location of an air pocket. Vibration is the frequent cause of eventual pipe failure but supports and clips to prevent this problem must permit free expansion and contraction. A pipe, which has to be twisted or bowed when being connected, has inbuilt stress which can lead to ultimate failure. Pipes should be accurately made and installed with simple supports before being permanently clipped. The pipework is assembled cold with a spacer piece, of length equal to half the expansion, between two flanges. When the pipework is fully installed and anchored at both ends, the spacer is removed and the joint pulled up tight (see Figure). The pipework system must be sufficiently flexible to accommodate the movements of the components as they expand. In many cases the flexibility of the pipework system, due to the length of the pipe and number of bends and supports, means that no undue stresses are imposed. In other installations, however, it will be necessary to incorporate some means of achieving this required flexibility.
Expansion arrangements The expansion fitting is one method of accommodating expansion. These fittings are placed within a line, and are designed to accommodate the expansion, without the total length of the line changing. They are commonly called expansion bellows. Other expansion fittings can be made from the pipework itself. This can be a cheaper way to solve the problem, but more space is needed to accommodate the pipe. Provision must be made in pipe systems to accommodate changes in length due to change of temperature, and so prevent undue stress or distortion as pipes expand or contract. One type of expansion joint has an anchored sleeve with a stuffing box and gland in which an extension of joining pipe can slide freely within imposed limits. Simpler schemes allow for change of length with a right angle bend arrangement or a loop. For high pressures and temperatures with associated greater pipe diameter and thickness other methods may be more appropriate. Stainless steel bellows expansion joints are commonly used since they will absorb some movement or vibration in several planes, eliminate maintenance, reduce friction and heat losses. Maximum and minimum working temperatures must be considered when choosing a bellows piece, which must be son installed that it is neither over- compressed nor over-extended. Its length must be correct for the temperature change. Stainless steel is the usual material for temperatures up to 5000 C. Beyond that and for severe corrosive conditions, other materials are required. Normally the bellows has an internal sleeve, to give smooth flow, to act as a heat shield and to prevent erosion. If exposed to the possibility of external damage, it should have cover. In usual marine application, bellows joints are designed and fitted to accommodate straight line axial movement only and the associated piping requires anchors and guides to prevent misalignment. It will be apparent that, in certain cases, the end connection will act adequately as anchors and that well designed hangers will be effective guides.
Horseshoe or lyre loop When space is available this type is sometimes used. It is best fitted horizontally so that the loop and the main are on the same plane. Pressure does not tend to blow the ends of the loop apart, but there is a very slight straightening out effect. This is due to the design but causes no misalignment of the flanges. If any of these arrangements are fitted with the loop vertically above the pipe then a drain point must be provided on the upstream side as depicted in Figure 10.4.8. Fig. 10.4.8 Horseshoe or lyre loop Expansion loops The expansion loop can be fabricated from lengths of straight pipes and elbows welded at the joints (Fig). An indication of the expansion of pipe that can be accommodated by these assemblies is shown in Fig. It can be seen from Fig that the depth of the loop should be twice the width, and the width is determined from Fig, knowing the total amount of expansion expected from the pipes either side of the loop.
Fig. 10.4.9 Expansion loop
6.2/20. Describe correct procedure of raising steam boilers and coupling them to steam system 6.3/7. Describe operation and control
Boiler operation The procedure adopted for raising steam will vary from boiler to boiler and the manufacturer’s instructions should always be followed. A number of aspects are common to all boilers and general procedure might be as follows:-
Preparations The uptakes should be checked to ensure a clear path for the exhaust gases through the boiler; any dampers should be operated and then correctly positioned. All vents, alarm, water and pressure gauge connections should be opened. The super heater circulating valves or drains should be opened to ensure a flow of steam through the superheater. All the other boiler drains and blow-down valves should be checked to ensure that they are closed. The boiler should then be filled to slightly below the working level with hot de-aerated water. The various header vents should be closed as water is seen to flow from them. The economizer should be checked to ensure that it is full of water and all air vented off. The operation of the forced draught fan should be checked and where exhaust gas air heaters are fitted they should be bypassed. The fuel oil system should be checked for the correct positioning of valves, etc. The fuel oil should then be circulated and heated.
Raising Steam The forced draught fan should be started and air passed through the furnace for several minutes to ‘purge’ it of any exhaust gas or oil vapours. The air slides (checks) at every register, except the ‘lighting up’ burner, should then be closed. The operating burner can now be lit an adjusted to provide a low firing rate with good combustion. The fuel oil pressure and forced draught pressure should be matched to ensure good combustion with a full steady flame. Initially, the boiler is to be flashed up for minimum 24 hrs to ensure the settlement of furnace and correct combustion process. The superheater header vents may be closed once steam issues from them. When a drum pressure of about 2.1 bar has been reached the drum air vent may be closed. The boiler must be brought slowly up to working pressure in order to ensure gradual expansion and to avoid overheating the superheater elements and damaging any refractory material. Boiler manufacturers usually provide a steam raising diagram in the form of a graph of drum pressure against hours after flashing up. The main and auxiliary steam lines should now be warmed through and then the drains closed. In addition the water level gauges should be blown though and checked for correct reading. When the steam pressure is about 3 bar below the normal operating value the safety valves should be lifted and released using the easing gear. Once at operating pressure the boiler may be put on load and the su- 0perheater circulating valves closed. All other vents, drains and bypasses should then be closed. The water level in the boiler should be carefully checked and the automatic water regulating arrangements observed for correct operation.
6.2/21. Describe how to check correctly the water level in steaming boiler, the dangers of low level and high level and corrective actions required in either place.
Checking correct water level in boiler:
Following is to be strictly adopted to ensure the correct functioning of gauge glass to check water level in boiler. (a) Shut the steam and water cocks. Open the drain cock. If the drain is clear, the pressure remaining in the water level glass will be blown to bilge. (b) Open and close the steam cock. If the steam cock is clear it will blow to bilge. (c) Open and close water cock. If the water cock is clear it will blow to bilge. (d) Close the drain cock. (e) Open the water cock slowly. (f) Open the steam cock slowly. After ensuring the correct functioning of gauge glass watch keeper has to monitor closely the boiler water level and any variations in water level to be attended promptly.
High water level in boiler (Priming of Boiler) Causes (a) High water level in boiler (b) Feed regulator non-operational (c) Auxiliary feed check valve opened accidentally (d) Rapid increase in power (e) High salinity of boiler water (f) Addition of excess boiler compound (g) Underwater explosion close to the ship.
Symptoms
(a) Dimming of lights. (b) Sudden drop in steam temperature. (c) Drop in speed of main machinery. (d) High water level in boilers. (e) Vibrations, noise and leaks in steam system due to water hammer
Actions
(a) Upon symptoms boiler controls are taken over to servo manual and appropriate valve pertaining to control the feed to be operated. Burners are taken off the affected boiler and all steam stops are shut. Drains of FD blowers and TAs are opened. The unaffected boiler water level is maintained using the main feed check valve.
(b) In the engine room, the main engine throttles are shut, but the affected shaft can be expected to trail. Put all drains to bilges. Monitor and check feed water tanks for contamination.
Low water level in boiler (Tube leak / tube failure) Causes
(a) Flame impingement due to badly aligned burners. (b) Lack of circulation in boiler tubes. (c) Low water level in boiler tubes by means of valve stuck/partial open . (d) External corrosion of tubes. (e) Impurities in feed water. (f) Very high firing rate in the beginning that causes steam blanketing and ultimately tube burst.
Symptoms
(a)Leakage will result in high rushing noise of water. (b)Heavy loss of feed water. Gauge glass showing dropping water level. Actions (a) Boiler controls are taken on servo manual and boiler is flamed out immediately. The auxiliary feed pump is started on cold suction and the affected boiler is flooded with cold water. (b) All clear evaporators are changed over to MUF. All steam stops on the affected boiler are shut. Boiler is fed with water till the furnace becomes black. (c) Shaft restrictions may be imposed.
6.2/22. Explain how water treatment is provided and why is it necessary
Purity of Boiler Water Most ‘pure’ water will contain some dissolved salts which come out of solution on boiling. These salts then adhere to the heating surfaces as a scale and reduce heat transfer, which can result in local overheating and failure of the tubes. Other salts remain solution and may produce acids which will attack the metal of the boiler. An excess of alkaline salts in a boiler, together with the effects of operating stresses, will produce a condition known as ‘caustic cracking’. This is actual cracking of the metal which may lead to serious failure. The presence of dissolved oxygen and carbon dioxide in a boiler feedwater can cause considerable corrosion of the boiler and feed systems. When boiler water is contaminated by suspended matter, an excess of salts or oil, then ’foaming’ may occur. This is a foam or froth which collects on the water surface in the boiler drum. Foaming leads to ‘priming’ which is the carry over of water with the steam leaving the boiler drum. Any water present in the steam entering a turbine will do considerable damage. It has been estimated that a 3mm thickness scale increases the fuel consumption by 16% and 6mm by 50%. This proves that the effect is not a straight line gradient but is exponential. Salts whose solubility decreases with increase in temperature are those that form scale upon heating surfaces . Salts whose solubility increases with increase in temperature do not normally form scale upon heating surfaces. Common impurities found in the boiler water are chlorides, sulphates and bicarbonates of calcium, magnesium and, to some extent sulphur. These dissolved salts in water make up what is called the ‘hardness’ of the water. Calcium and magnesium salts are the main causes of hardness. The bicarbonates of calcium and magnesium are decomposed by heat and come out of solution as scale-forming carbonates. These alkaline salts are known as temporary hardness. The chlorides, sulphates and nitrates are not decomposed by boiling and are known as permanent hardness. Total hardness is the sum of temporary and permanent hardness and gives a measure of the scale-forming salts present in the feed water
Necessity of boiler water treatment
(a) To prevent scale formation in the boiler (b) To give alkalinity and minimize corrosion (c) To condition sludge (by sodium aluminate). (d) To remove oxygen from water. (e) To reduce risk of caustic cracking. (f) To reduce risk of carry over of foam (by antifoam) (g) To minimize feed and condensate system from corrosion and filming amines
Feed water treatment: Feed water treatment is achieved by adding suitable chemicals with the boiler water in appropriate quantities and is as below:- (a) Prevention of scale formation in the boiler and feed system by (i) using distilled water or (ii) precipitating all scale forming salts into the form of a non-adherent sludge. (b) Prevention of corrosion in the boiler and feed system by maintaining the boiler water in an alkaline condition and free from dissolved gases. (c) Control of the sludge formation and prevention of carry over with the steam (d) Prevention of entry into, the boiler of foreign matter such as oil, waste, mill-scale, iron oxides copper particles, sand, weld spatter, etc. By careful use of oil heating arrangements (close watch on steam drains), effective pre-commission cleaning and maintaining the steam and condensate systems in a non-corrosive condition.
De-aeration
It has been stated for corrosion to take place O2 must be present to accomplish the formation of metal oxides. Hence de-aeration of the feed water is essential. De- aeration can be accomplished either mechanically or chemically, or a combination of both. It is usual to carry a reserve of chemicals in the boiler water in order to deal with any ingress of dissolved oxygen that may result due to mal-operation of the de- aerating equipment, or some other circumstances. The chemicals used for this purpose are usually sodium sulphate or hydrazine. Hydrazine should be stored in a cool, well ventilated place since it is a fire hazard. When handling, protective clothing should be worn- treat in the same way as caustic soda. 6.2/21. Describe the construction of steam plants as fitted on board the ship 6.3/6. Describe the steam plant as fitted on board the ship including the design
Steam Plants
Water in the form of steam has the ability to store great amounts of energy. With its ease of control and delivery, steam brought the advent of power to the shipping world. There are still some steam powered vessels such as ULCC ( Ultra Large Crude Carrier ) where steam turbines can provide the necessary, high power shaft requirements to propel the ship. However it's time as passed; most ships nowadays use the more economical diesel or heavy fuel engines.
Although boilers may no longer be commonplace for ship propulsion they are almost guaranteed to be one boiler for various duties on board a ship. Duties like heating cargo, fuel, and accommodations. Some ships also use boilers for auxiliary power. Such as deck winches and pumps, where electrical machines would prove to be a hazard as in the oil industry.
Steam Theory
Within the boiler, fuel and air are force into the furnace by the burner. There, it burns to produce heat. From there, the heat (flue gases) travel throughout the boiler. The water absorbs the heat, and eventually absorb enough to change into a gaseous state - steam. To the left is the basic theoretical design of a modern boiler. Boiler makers have developed various designs to squeeze the most energy out of fuel and to maximized its transfer to the water. But it all boils down; pardon the pun, to the basic design shown here.
The water tube boiler
As you can see, the Babcock Marine Water Tube Boiler (below) looks very complicated. Thousands of tubes are placed in strategic location to optimize the exchange of energy from the heat to the water in the tubes. These types of boilers are most common because of their ability to deliver large quantities of steam. The large tube like structure at the top of the boiler is called the steam drum. You could call it the heart of the boiler. That's where the steam collects before being discharged from the boiler. The hundreds of tube start and eventually end up at the steam drum. Water enters the boiler, preheated, at the top. The ht water naturally circulates through the tubes down to the lower area where it is hot. The water heats up and flows back to the steam drum where the steam collects. Not all the water gets turn to steam, so the process starts again. Water keeps on circulating until it becomes steam. Meanwhile, the control system is taking the temperature of the steam drum, along with numerous other readings, to determine if it should keep the burner burning, or shut it down. As well, sensors control the amount of water entering the boiler, this water is know as feedwater. Feedwater is not your regular drinking water. It is treated with chemicals to neutralize various minerals in the water, which untreated, would cling to the tubes clogging or worst, rusting them. This would make the boiler expensive to operate because it would not be very efficient.
Steam Rear/Front Water wall drum headers Flue gas
Super Water wall heater tubes Super heaters tubes
Burner
Water drum
Header
DRUM TYPE BOILER. FIG.11 On the fire side of the boiler, carbon deposit resulting from improper combustion or impurities in the fuel can accumulate on the outer surface of the water tube. This creates an insulation which quickly decreases the energy transfer from the heat to the water. To remedy this problem the engineer will carry out soot blowing. At a specified time the engineer uses a long tool and inserts it into the fire side of the boiler. This device, which looks like a lance, has a tip at the end which "blows" steam. This blowing action of the steam "scrubs" the outside of the water tubes, cleaning the carbon build up. Water tube boilers can have pressures from 7 bar (one bar = ~15 psi) to as high as 250 bar. The steam temperature's can vary between saturated steam, 100 degrees Celsius steam with particle of water, or be as high as 600 - 650 degrees Celsius, know as superheated steam or dry steam (all water particle have been turn to a gaseous state). The performance of boiler is generally referred to as tons of steam produced in one hour. In water tube boilers that could be as low as 1.5 t/hr to as high as 2500 t/hr. The fire tube boiler
This type of boilers started it all. This is the original design of boiler which brought the tide of power to the marine world. On a modern ship, the fire tube boiler meet the ship's heating needs and is generally not used for deck machinery. The steam produced will circulate through coils in the cargo tanks, fuel tanks, and accommodation heating system. They are generally supplied as a complete package, such as the one pictured above. This is a single furnace, three pass type fire tube boiler. Heat - flue gases - travels through three different sets of tubes. All the tubes are surrounded by water which absorbs the heat. As the water turns to steam, pressure builds up within the boiler, once enough pressure has built up the engineer will open main steam outlet valve slowly, supplying steam for service. Fire tube boilers are also known as "smoke tube" and "donkey boiler". Auxiliary boiler
On smaller ships the auxiliary boiler can be a stand alone unit and would most likely be of the fire tube boiler arrangement as described above. But on a larger vessel it is more efficient for the auxiliary boiler to take advantage of the main engine's flue gases to heat the water. Basically this means that the hot gases from the main engine must pass through a heat exchanger (the auxiliary fire tube boiler) before exiting to the atmosphere. It is called the "cargo heating boiler". As you can imagine if the ship's main engine was not running, there would be no hot flue gases to make steam. The auxiliary boiler also has a burner assembly which can be operated while the ship is in port or when the flue gases are not hot enough to provide the necessary steam. With this Cochran type boiler, the flow of flue gases from the engine is controlled by a damper. Should the damper not allow engine flue gases through, the burner would automatically come on and provide heat for the water to absorb. It would do so until the controls of the damper allowed the flue gases to flow through the boiler providing the necessary heat for the water, the burner would then shut down.
Fig. 9
6.3/8. Describe circuit of generated steam
B
O
I T
N
I
U
L
E
R
E
B
R
EXTN PP
CONDENSER
ECONOMISER FEED HEATER MAIN FD PP Fig. 10 SCHEMATIC ARRANGEMENT OF STEAM PLANTS IN BOILER ROOM
6.3/9. Describe preparations to be made for putting the steam plant in operation
Before closing up the boiler inspect the internal surfaces to ensure they are clean all openings to the boiler mountings clear obstruction buy means of search balls, flexible wires, air or water jets. Replace any internal fittings which have been removed, checking to ensure they are correctly positioned and secured. The header handhole plugs and lower manhole doors are now replaced. Operate all boiler mountings to ensure they work freely, leaving all the valves in a closed position. Check the gas side of the boiler is clean and in good order. Make sure the soot blowers are correctly fitted, and operate over their correct traverse. Operate any gas or air control dampers fitted to ensure they move freely for their full travel. Leave them closed or in mid-position as necessary. The boiler casing doors are now replaced. Open the direct reading water level gauge isolating cocks, together with all boilers, alarm and pressure gauge connections. The super heater drains are also opened. Check that all other drains and blow down valves are closed. Commence to fill the boiler with hot deaerated water. At this stage the initial dose of chemical treatment can be added through the top manhole doors, which are then replaced. Continue to fill until water just shows in the water level gauges. Close any header vents as water issues. Remove the funnel cover, and ensure that all air checks operate correctly and that the forced draught fans are in working order. If gas air heaters are fitted that should be by-passed. Check the fuel oil system to ascertain it is in good order. Start up the fuel oil service pumps and check for leaks. The boiler is now ready to commence raising steam. Heat the fuel oil to the required temperature, using the recirculating line to get the heated oil through the system. If no heat is available for this, use gas oil until sufficient steam is available to heat the residual fuel oil normally used. Start the forced draught fan, and with all the air checks full open purge the boiler, making sure any gas control dampers are in mid position so giving a clear air passage. Carry out a final check to make sure water level gauge cocks are open, water is showing in the glass, and that steam drum and superheater vents are open. Now close all the air checks except for the burner to be flashed up, this being done byu means of ignition equipment or a paraffin torch. Use the lowest possible firing rate. Adjust about one hour steam should show at the drum and superheater vents and, when issuing strongly, open the superheater circulating valve and close the air vents. When the steam pressure reached a value of about 300 kN/m2 blow through the water level gauges to ensure they are working correctly. The isolating valves on the remote reading water level indicator can now be opened, and the indicator placed in service. With the steam pressure at about 1000 kN/m2 follow up the nuts on all new boiler joints. At a pressure of bout 1400 kN/m2 open the drains on the auxiliary steam lines, crack open the auxiliary stop valve and warm the auxiliary line through. Now close the drains and fully open the auxiliary stop valve. Various auxiliary equipment such as fuel oil heaters, turbo-feed pumps, can be put into service and, provided this entails a flow of steam through the superheater, the superheater circulating and drain valves are closed. Bring the boiler up to working pressure, keeping the firing rate as steady as possible, and avoiding intermittent flashing up. Check the water level alarms. Open the main steam line drains, and crack open the main stop valve and warm through the main steam line. Then close the drains and fully open the main stop valve. The procedure from flashing up to coupling up at full working pressure should take about four to six hours. Only in emergency should it be carried out more rapidly. If new refractory material has been installed carry out the procedure more slowly. At all times during the raising of steam the superheaters must be circulated with steam to prevent them overheating. If the temperature of the superheater goes above the permitted value for the boiler reduce the rate of firing. It must be noted that, due to the great variety of water tube boiler designs in use, the foregoing procedure is only to be taken as a guide, for example, header boilers with their greater amount of refractory material will require about eight hours to reach full pressure. Thus the engineer should always follow the procedure laid down for his particular boiler, which may vary in detail from the basic principles previously stated.
6.3/10. Describe checks to be made during firing up of boilers
. Inspect the internal surface to ensure they are clean e.g. no tools, rags or other things left inside. . All opening of the mountings are clean properly. . Mountings to be fixed back with new set of gaskets or joints. . Ensure all tools are accounted for. . The header hand hole cover and bottom manhole door now replaced. . Operate all mounting valves to ensure they work freely and leave all valves in closed position. . Top manhole door is replaced. . Check gas side of boiler and ensure they are clean e.g. no tools, rags, or other thing left inside. . Soot blowers are correctly fitted and air control dampers move freely for their full travel. . Furnace door is replaced with new joint. . Open the gauge glass steam, water cocks and shut drain cock. . Open vent, alarm and pressure agauge connection valves. . Ensure all other drain valves are shut. . Switch on the power for the combustion control panel, feed water pump, forced draft fan and fuel oil pump . Now commence to fill boiler with hot distilled water until water level below normal water level. . Initial chemical dosage can be added. . Check all control air is available to the combustion control, controllers & control valves. . Ready for raising steam. . Start the forced draught fan and purge the boiler 3-5 minutes. . Start fuel pump, ensure re-circulation is operating. . Bring the air damper to firing position. . Fire the boiler, at lowest possible firing rate, ensuring air supply is adjusted for best combustion. . As boiler heats up water level rises to about normal level. . When steam c0omes out the vent, then shut the vent. . When steam pressure is about 3 bar then blow though the gauge glass to ensure they are working correctly.
6.3/11. Describe automatic control for starting up & shutting down exhaust gas boiler and oil fired boiler
Marine boiler plants require adequate control systems to raise steam, maintain design conditions for steady steaming, secure the boiler units and detect promptly malfunctions and failures. The automatic control arrangement on a shipboard boiler is divided into two parts:
(a) Safety system that controls that all values are within the predetermined limits and give automatic alarm if some of them are not, and also initiate an automatic burner trip in case of a dangerous situation.
(b) Continuously control of the different parameters for water level control, steam pressure control, fuel oil pressure control, fuel oil temperature control, blowdown control, superheat temperature control etc. Pressure
switch
Feed box boiler Master high and initiatin g relay Fixed Water level . low
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limb level a . n
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alarm i
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o Variable fuel t
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a limb i
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fuel initiator i Photo. cell a . signal non Solvent valve conducting Electrode electrode initiator relay signal charging fluid traps dc … fan . . . dv . supply ...... Level feed not – . . . .. /\/\/\/\/\/\. . controller ...... feedback . . . . . and amp ...... signal lines shown dotted mercury Variable Centrifugat Electroform level speed feed pump motor transmitter
AUTOMATIC BOILER CONTROL SYSTEM START
AUT MANUA O L
FLAME FAILURE A LOCK FAIL TO IGNITE A MAUAL OUT FAILURE HIGH HIGH WATER RESET A CLEAR LEVEL HIGH WATER A PRE- LEVEL POST LOW WATER A PURGE LEVEL PURGE 180 LOW LOW WATER A SECS LEVEL LOW FO TEMP A FO V/V LOW PILOT FO SHUT A PILOT PRES A BURNER LOW FO PRES ON LOW STM PRES A LOCK FD FAN NON A OUT START COMBUST AIR PRES A LOW FLAME ON BURNER NOT IN 5 SECS A NO POSITION HIGH STEAM A YES PRESSURE FEED WATER PRES A LOW ALARM / CONTROL PANEL MAIN BURNER FAIL TO IGNITE
FLAME FLAME NO ON 5 YES FAILURE SECS NO
FO BLR BOILER PILOT FLAME V/V SET MODULATIN BURNER YES YES ON SHUT PRES G OFF NO POST- STM CUT- PURG BOILE IN E R S/BY PRESSUR ON E Automatic boiler control system: (a) The pressure switch initiates the start of the cycle., The switch is often arranged to cut in at about 1 bar below the working pressure and cut out at about 1/5 bar above the working pressure (this differential is adjustable) (b) The master initiating relay now allows ‘air on’. The air feedback confirms ‘air on’ and allows 30 second time delay to proceed. (c) The master now allows the arc to be struck by the electrode relay. The ‘arc made’ feed back signal allows a 3 second time delay to proceed. (d) The master now allows the fuel initiating signal to proceed. The solenoid valve allows fuel on to the burner. The ‘fuel on’ feed back signal allows a 5 second time delay to proceed (this may be preceded by a fuel heating sequence for boiler oils). (e) The master now examines the photo electric cell. If in order the cycle is complete, if not then fuel is shut off, an alarm bell rings and the cycle is repeated. Refer to fig. 7 for emergency devices. Obviously failure of any item in the above cycle causes shut down and alarm operation. In addition the following apply; (i) High or low water levels initiate alarms and allow the master to interrupt and shut down the sequential system. (ii) Water level is controlled by an Electroflo type of feed regulator and controller. Sequential level resistors are immersed in conducting mercury or non-conducting fluid, so deciding pump speed by variable limbs level. The fixed limb level passes over a weir in the feed box. The combustion control system maintains constant steam pressure by controlling the flow of air and oil to the burner. The more advanced combustion controls transmit the air and oil loading simultaneously but with a slight lag between air and oil, so that with an increased boiler load, the air will lead the oil, and on a decrease in the boiler load the oil will lead the air. Such an arrangement makes it possible to minimize the emission of smoke during maneuvering. All the classification societies have special requirement for marine applications due to the environment and the fact that one can't escape from an accident nor get service when the ship is sailing at sea. Things just have to work. Shutting down procedures of oil fired boiler
Following routines are to be carried out assiduously for shutting down of boilers:- . Inform Chief Engineer and inform duty officer in bridge.
. Reduce boiler pressure as required.
. Change over M/E, A/E and boiler to diesel oil
. Top up diesel oil service tank, stop HO and LO purifiers
. Stop all tanks and tracing steam heating and carry out soot blow
. Change over from automatic combustion control to manual firing of boiler.
. Stop firing of boiler. As burners are taken off, steam purge, retract and park hoses.
. Switch off power and off the circuit breaker for forced draft fan, FO pump, feed pump and combustion control panel. Hang necessary notices.
. Shut main steam stop valves and shut all fuel valves to boilers.
. Let the boiler cool down, do not blow down.
. Shut down boiler. Stop lighting up pump (if provided).
. Pump up boilers for 30 sec over the top by opening appropriate feed check valve. . Take care to establish a controlled rate of increase before the level leaves the top of the gauge glass
. Check header drains shut.
. Check all auxiliary master, throttle and exhaust valves shut on shut down machinery.
. Ensure boiler pressures are dropping.
. Clean boiler room for inspection by Chief Engineer
6.3/12. Describe safety devices of plant mandatory safety requirements. Workers that use, maintain, and service boilers know that they can be potentially dangerous. Though boilers are usually equipped with a pressure relief valve, if the boiler fails to contain the expansion pressure, the steam energy is released instantly. This combination of exploding metal and superheated steam can be extremely dangerous. Only trained and authorized workers should operate a boiler. Workers should be familiar with the boiler manufacturer's operating manual and instructions. Boiler operators should frequently inspect boilers for leakage, proper combustion, operation of safety devices and gauges, and other functions. Many older boilers and hot water and steam piping may have asbestos insulation coatings, wraps, or “lagging.” Workers should periodically inspect these areas to make sure that the materials are not damaged, flaking, or deteriorating. Damaged materials should be reported and repaired or removed immediately by a certified asbestos contractor. Signs of cracked surfaces, bulges, corrosion or other deformities should be repaired by an authorized technician immediately. Detailed logs of boiler operation and maintenance can help ensure boiler safety.
Boilers should always be brought on line slowly and cold water should never be injected into a hot system. Sudden changes in temperature can warp or rupture the boiler. Because many boilers are fire-operated by natural gas, diesel or fuel oil, special precautions need to be taken. Boiler operators should ensure that the fuel system, including valves, lines, and tanks, is operating properly with no leaks. To prevent furnace explosions, it is imperative that boiler operators purge the boiler before ignition of the burner. Workers should check the fuel to air ratio, the condition of the draft, and the flame to make sure that it is not too high and not smoky. Ventilation systems should also be inspected and maintained to make sure that combustion gases do not build up in the boiler room.
The area around the boiler should be kept clean of dust and debris, and no flammable materials should be stored near any boiler. Floors are often sealed concrete and can be very slippery when wet. Spills should be mopped or cleaned up immediately. Make sure that adequate lighting is provided and that malfunctioning light fixtures are repaired immediately. Because boilers have hot surface areas, there should be plenty of clearance for workers to move around the room. Boiler rooms can be noisy, so the area should be posted and workers should wear hearing protection when working inside the boiler room.
Boiler repairs are allowed only by authorized boiler repair technicians. Repair workers should wear personal protective equipment such as hard hats, heavy-duty work gloves, eye protection and coveralls. When entering a boiler for service or repair, authorized boiler repair workers should treat the vessel as a permit-required confined space. When the boiler is shut down for repair, all sources of energy should be isolated using approved Lock-out / Tag-out procedures and residual pressure in steam, water, and fuel lines should be relieved by following proper bleed and block or capping procedures.
(a) Every steam boiler and every unfired steam generator shall be provided with not less than two safety valves of adequate capacity. (b) Each oil fired boiler which is intended to operate without manual supervision shall have safety arrangements which shut off the fuel supply and give an alarm in the case of low-water level, air supply failure or flame failure. (c) Water tube boilers serving turbine propulsion machinery shall be fitted with a high water level alarm. (d) Every steam generating system which provides services essential for the safety of the ship, or which could be rendered dangerous by the failure of its feed water supply, shall be provided with not less than tow separate feed systems from and including the feed pumps, noting that a single penetration of the steam drum is acceptable. Unless overpressure is prevented by the pump characteristics, means shall be provided, which will prevent over-pressure in any part of the system. (e) Boilers shall be provided with means to supervise and control the quality of the feed water. Suitable arrangements shall be provided to preclude, as far as practicable, the entry of the oil or other contaminants, which may adversely affect the boiler. (f) Every boiler essential for the safety of the ship and designed to contain water at a specified level shall be provided with at least tow means for indicating its water level, at least one of which shall be a direct reading gauge glass. Safety valves These are fitted to protect the boiler form the effects of overpressure. As per international regulations, at least two safety valves are fitted to each boiler, but in practice it is usual to fit three safety valves – two on the steam drum, and one on the super heater outlet header. This is fitted to release excess steam pressure from the boiler before a dangerous pressure can be built up. Pressure setting of one steam drum safety valve should be same as the design pressure of the boiler and the other should be 2-3% more than the design pressure of the boiler. Fusible plugs
A fusible plug is a threaded metal plug, usually of bronze, brass or gun metal, with a tapered hole drilled completely through its length. This hole is sealed with a metal of low melting point, usually lead or tin. It is screwed into the crown sheet (the top plate) of a steam engine firebox, typically extending about an inch into the water space above. Its purpose is to act as a last-resort safety device in the event of the water level falling dangerously low: when the top of the plug is out of the water it overheats, the lead core melts away and the resulting noisy release of steam into the firebox serves to warn the operators of the danger before the top of the firebox itself runs completely dry. The temperature of the flue gases in a steam engine firebox can reach 1000 °F (550 °C), at which temperature copper, from which historically most fireboxes were made, softens to a state which can no longer sustain the boiler pressure and a severe explosion will result if water is not put into the boiler quickly and the fire thrown out. The hole is too small to have any great effect in reducing the steam pressure and, as little or no water passes through, it is not expected to have any great impact in quenching the fire.
Stays Stays, or ties, physically link the firebox and boiler casing, preventing them from warping. Since any corrosion is hidden, the stays may have longitudinal holes, called tell-tales, drilled in them which leak before they become unsafe.
6.3/13. Describe the testing and treatment of boiler feed water
What is the purpose of boiler water treatment? (a) To prevent scale formation in the boiler (b) To give alkalinity and minimize corrosion (c) To condition sludge (by sodium aluminate). (d) To remove oxygen from water. (e) To reduce risk of caustic cracking. (f) To reduce risk of carry over of foam (by antifoam) (g) To minimize feed and condensate system from corrosion and filming amines
The principal objects of boiler feed water treatment should be:- (a) Prevention of scale formation in the boiler and feed system by (i) Using distilled water or (ii) Precipitating all scale forming salts into the form of a non- adherent sludge. (b) Prevention of corrosion in the boiler and feed system by maintaining the boiler water in and alkaline condition and free from dissolved gases. (c) Control of sludge formation and prevention of carry over with the system. (d) Prevention of entry into the boiler of foreign matter such as oil, waste, mill-scale, iron oxide, copper particles, sand weld spatter etc. By careful use of coil heating arrangements, effective pre-commission cleaning and maintaining the steam & condensate system in a non-corrosive condition. Boiler water should be regularly tested and the treatment of the boiler water should be conducted according to the results obtained from the results. For low pressure boilers salinometers and litmus paper s are still frequently used as testing equipment. For accurate testing of the boiler water, above said tests are inadequate. Refined tests are being practiced to ascertain the exact quantity of alkalinity and salinity concentrations.
TOTAL HARDNESS TEST
Apparatus Reagents
1-Burette, automatic, 10 ml 16 oz, bottle Versenate solution Ethylenedimaine Tetraacetate (1ml equals 1 ml as CaCO3) 1-Evaporating Dish 4 oz. bottle Drew Dry total Hardness Buffer Reagent with plastic scoop 1-Cylinder, graduated,100ml capacity 4 oz. bottle Drew Dry Total Hardness Indicator 1-Strirring Rod 1-Brass measuring scoop
Procedure
1. Transfer 50 ml feedwater sample to the evaporating dish. 2. Add one plastic scoop of Drew Dry Total Hardness indicator. Stir until dissolved. 3. Add one brass scoop of Drew Dry Total Hardness indicator. Stir until dissolved. 4. If a pure blue color develops, the hardness is zero. Any reddish color indicates hardness is present. 5. Titrate with standard versenate solution, adding the reagent drop by drop with continuous stirring as the red color fades. The end point is a pure sky blue color without any reddish tinge.
Calculations:
Total hardness (PPM as CaCO3) equals ml versenate solution X 20. If test result is in excess of ______add ______B according to dosing instructions and investigate source of contamination.
ALKALINITY TEST Apparatus
10 ml automatic burette White porcelain evaporating dish 100ml graduated cylinder Stirring rod
Reagents
Sulfuring acid N/50 16 oz. bottle with burette Phenolphthalein Indicator1 oz dropping bottle Total Alkalinity Indiacator 1 oz. dropping bottle
Procedure
A. PHENOLPHTHALEIN ALKALINITY TEST
1. Fill burette to 0.0 mark with N/50 sulfuric acid\ 2. Using graduated cylinder, measure 50ml of boiler water to be tested. 3. Add 4 drops of phenolphthalein Indicator. Stir. 4. If no pink or red color develops, record Zero phenolphthalein alkalinity. Proceed to Part B (Total Alkalinity test) 5. If, however, sample turns pink or red with Phenolphthalein, add N/50 sulfuric acid drop by drop while stirring continuously. Continue until pink color disappears (sample is back to its original color.) Do not discard sample do not refill burette 6. Calculations of results (Ml of N/50 sulfuric acid) X 20 = (ppm phenolphthalein alkalinity). For convenience, use the titration chart to get result. 7. Record the Phenolphthalein Alkalinity in the daily log and proceed with Part B
B. TOTAL ALKALINITY TEST 1. Do not refill burette. Use the same sample that was used for the Phenolphthalein Alkalinity test and add exactly 4 drops of Total Alkalinity Indicator. Sample will turn a green color. 2. Add sulfuric acid, drop by drop, stirring continuously. A purple color will soon begin to form where the drops fall into the sample. When a permanent, pale purple color develops throughout the sample, the test is ended. Color change will go from green to slate gray to purple. The purple color is the end point. 3. Calculation of result: (Total ml of N/50 sulfuric acid – 0.6) X 20 = ppm Total Alkalinity. For convenience use titration chart to get results. 4. Record the total alkalinity in the daily log. Discard sample. CHLORIDE TEST Apparatus
10 ml automatic burette White porcelain evaporating dish 100ml graduated cylinder Stirring rod
Reagents
Mixed Chloride Indicator – Make up fresh every 4 weeks. Discard any indicator over 4 weeks old Nitric Acid N/50 1 oz. dropping bottle Mercuric Nitrate, 0.0141 N 16 0z.bottle with burette
Preparation of mixed chloride indicator Apparatus
100 ml graduated cylinder 4 oz. Amber glass dropping bottle
Reagents
1 capsule of mixed chloride indicator (Diphenyl Carbazone Indicator) Methyl alcohol (anhydrous)
Procedure 1. Empty capsule of indicator powder into 4 oz. amber glass dropping bottle. 2. Measure 100ml alcohol and add to bottle. 3. Cap bottle, Dissolve powder by swirling or shaking. 4. Make up fresh every 4 weeks. Discard any indicator that is 4 weeks old.
CAUTION! METHYL ALCOHOL IS POISONOUS. DO NOT SWALLOW. AVOID CONTACT WITH EYES Test procedure 1. Do not use th4e sample that was used for the Alkalinity tests. Fill burette to 0.0 mark with 0.0141 N Mercuric Nitrate. 2. Using graduated cylinder, measure 50 ml of boiler water and transfer into the evaporating dish. 3. Add 10 drops of Mixed Chloride Indicator. Stir. 4. Add N/5 Nitric Acid drop by drop, while stirring. Continue until sample just turns yellow. Then add another 5 drops of the acid. 5. Add 0.0141 N Mercuric Nitrate drop by drop while stirring until the sample shows the first permanent violet color. Read the burette. 6. Calculation of results: (Ml of 0.0141 N Mercuric Nitrate) X 10 = ppm Chloride. For convenience, use the titration chart to get results. Compare test result with limit marked on the control chart. If too high, start continuous blowdown and investigate source. Repeat test in 30 minutes. 7. Place the comparator base slide in its slot the base. Move the slide form side to side, while comparing the color of the sample with those of the standards in the slide. Continue until the color of the sample color appears to be between two standards. In the latter case, take the average of both readings. NOTE that a comparison can be made only when one of the white lines on the slide is opposite the middle (sample) tube. 8. When a color match is obtained, read the test result in ppm phosphate from the numbers on the slide. Compare phosphate readings with limit marked on the control chart. Readings in excess of limits require blowdown. Readings below recommended limits require proportionate dosing. Refer to dosing instructions. If the results of the phosphate test show a reading above the upper limit of 25 ppm, it will be necessary to repeat the testing using a diluted sample.
Procedure
(1) Filter 5 ml of boiler water into the phosphate mixing tube. Dilute to 10 ml with distilled or demineralized* water. Proceed with steps 2 through 8 and, for results, double the ppm reading. (for example if slide shows 15 ppm with diluted sample, the actual reading is 30 ppm).
* To make demineralized water, simply fill plastic bottle with distillate and squeeze though demineralizer cartridge. The water discharged from the cartridge will be equivalent to distilled water. The cartridge is good until the demineralizer beads change color as indicated in the manufacturer’s instructions. (In the “Deem” cartridge, the color change is from blue to brown.) When this occurs, simply replace the cartridge.
Boiler water treatment using “BOILER WATER TEST KIT (FULL SERVICE) SPECTRAPAK 311
This test kit is for phosphate, P&M alkalinity chloride and pH. The hydrazine is an optional extra (Spectrapak 312).
Sampling A representative ware sample is required. Always take water sample from the same place. Allow the water to flow from the sample cock before taking the sample for testing to ensure the line is clear of sediment.
Phosphate PPM PO4
• Take the comparator with the 10 ml cells provided.
• Slide the phosphate disc into the comparator.
• Filter the water sample into both cells up to the 10ml mark.
• Place one cell in the left hand compartment
• To the other cell add one phosphate tablet, crush and mix until completely dissolved.
• After 10 min place this cell into the right hand compartment of the comparator.
• Hold the comparator towards a light.
• Rotate the disc until a colour match is obtained.
• Record the result obtained on the log sheet provided, against the date on which the test was taken.
P Alkalinity (PPM CaCO3)
• Take a 200ml water sample in the stopped bottle.
• Add one P alkalinity tablet and shake or rush to disintegrate.
• If alkalinity is present the sample will turn blue.
• Repeat the tablet addition, one at a time (giving time for the tablet to dissolve), until the blue colour turns to permanent yellow.
• Count the number of tablets used and carry out the following calculations:-
P Alkalinity, ppm CaCO3 = (Number of tablets x 20) – 10
e.g. 12 tablets = (12 x 20)-10 = 230 ppm CaCO3
• Record the result obtained on the log sheet provided, against the date on which the test was taken. • Retain the sample for the M Alkalinity test.
M Alkalinity (PPM CaCO3)
• To the P alkalinity sample add one M alkalinity tablet and shake or crush to disintegrate.
• Repeat tablet addition, one at a time (giving time for the tablet to dissolve), until the sample turns to permanent red/pink.
• Count the number of tablets used and carry out the following calculations:-
M Alkalinity, ppm CaCO3 = (Number of P & M tablets x 20) – 10
e.g. 12 P, and 5M. Alkalinity tablets are used
M alkalinity = [(12+5) x 20)]-10 = 330 ppm CaCO3
• Record the result obtained on the log sheet provided, against the date on which the test was taken.
Chloride (PPM) Cl
The range of chloride to be tested determines the size of water sample used. The higher the chloride level the smaller the size of water sample used – this saves tablets. e.g. for low chloride levels use 100ml water sample for higher chloride levels use 50ml water sample • Take the water sample in the stoppered bottle provided. • Add one chloride tablet and shake to disintegrate. Sample should turn yellow if chlorides are present. • Repeat the tablet addition, one at a time (giving time for the tablet to dissolve), until the yellow colour changes to permanent red/brown. • Count the number of tablets used and perform the following calculations:- For 100ml water sample – Chloride ppm = (Number of tablets x 10)-10 e.g. 4 tablets = (4 x 10) – 10 = 30 ppm For 50ml water sample – Chloride ppm = (Number of tablets x 20)-20 e.g. 4 tablets = (4 x 20) – 20 = 60 ppm For smaller steps of ppm chloride use a larger sample. For larger steps of ppm chloride use a smaller sample. • Record the result obtained on the log sheet provided, against the date on which the test was taken. pH Test 7.5 – 14.0 For boiler water 6.5 – 10.0 For condensate water • Take a 50ml sample of water to be tested in the plastic sample container provided. • Using the white 0.6grm scoop provided, add one measure of the pH reagent to the water sample, allow to dissolve. Stir if required. • Select the correct range of pH test strip and dip it into the water sample for approximately 10 seconds. • Withdraw the strip from the sample and compare the colour obtained with the colour scale on the pH indicator strips container. • Record the pH value obtained on the log sheet provided, against the date at which the test was taken. Hydrazine PPM Spectrapak 312 This is an optional extra (order Spectrapak 312). This test must be performed below 210C. A cooling coil should be fitted at the sampling point or the sample should be cooled immediately under cold running water. Cloudy samples should be filtered before testing. • Take the comparator with the 10ml cells provided. • Slide the hydrazine disc into the comparator. • Add the water sample to both cells up to the 10 ml mark. • Place one cell in the left hand compartment of the comparator. • To the other cell add one measure of hydrazine powder (using the black 1grm scoop provided) and mix until completely dissolved. • Wait 2 minutes and place the cell in the right hand compartment of the comparator. • Hold up to the light and rotate the disc until a colour match is obtained. • Record the reading shown as ppm hydrazine. Safety: These reagents are for chemical testing only. Not to be taken internally. Wash hands after use. Keep away from children.
FAQs on Marine Boilers
Question: Boiler drum level control goes haywire
We are having 100 tones FBC boilers. Frequent problem observed is that whenever there is sudden load change the boiler drum level control goes haywire leading to tripping of boiler and turbine on drum level low or high. Our drum level control is 3- element control in auto mode.
Answer: The most common fault with a three-point level controller is the steam flow transmitter. Loosen the impulse pipes and cleanse the holes into the measuring orifice.
Question: Firing light crude oil
Can a Main Boiler built to fire 380 cst HFO, be fired with light crude oil directly from the cargo-tanks?
Answer: A completely new fuel system is required, from deck to the burner rails of the boiler. To prevent any possibility of gasses leaking from flanges, there have to be ducting enclosing the entire fuel-system with forced draft fans that vent 30 times the volume of the trunking to the outside. Also there have to be a burner hood to be constructed all over the burner roof, equally vented. Naturally there have to be new burners and so is the burner management. There are two main contractors who are capable and willing to carry this out: HAMWORTHY-Combustion Engineering in UK and SAACKE in Germany.
Question: Seawater in the boiler If the boiler had to be operated with seawater what would be the result.
Answer: The salinity will rise rapidly since the salt remains in the boiler while the water boils off. Salt will son precipitate and accumulate on the bottom and also on the heating surface where it, just as boiler-scales, inhibit the heat transmission to the water and causes the metal to overheat and in worst case burst. You may also get foam in the boiler that will cause difficulties to maintain the water level and water droplets might follow with the steam, causing problems with turbines and engines. It is very dangerous to operate a boiler with salt in it, and you have to control the salt concentration by frequently blowing off from the bottom of the boiler and form the water surface to keep the salinity below 9.5% (boilermakers and classification societies may recommend other values). It would also be a good measure to reduce the capacity of the boiler. After this emergency operation it would be wise to open up the boiler for inspection since seawater further accumulation of scales. In the old days some ships sailing on lakes used the lakes water as make up water for their boilers, but even that water caused problem with salt in the boilers although it is supposed to be fresh-water.
Question: Heavy fuel oil viscosity Heavy fuel oil viscosity is defined in the standards as the viscosity at 100°C yet the oils are often described in terms of their nominal 40 or 50°C temperature viscosities. e.g. a G35 oil (35cst at 100°C) is often described as anything from a 350cst to 390cst oil. Refineries blend to control the viscosity at 100°C. Testing aboard ship or in boiler houses appears to test the viscosity at 50°C. Because of the variation in quality of the residual oil and distillate that make up a heavy fuel oil, it is difficult to make a good correlation between the 50°C and 100°C measurements. My question is, how is the blending of oil in the terminals or on fuel barges controlled? and how reliable is this in achieving the required viscosity?
Answer: The values you get from my Fuel Oil Calculation program are normally sufficient for firing a boilers heavy fuel oil burner. For a diesel engine on the other hand, I assume that an automatic viscosity-controller would be indispensable.
Question: Heating up a fire-tube boiler Is there a minimum temperature that a fire-tube boiler should reach before going to a high fire state to prevent tubes from leaking? Answer: The important thing is to heat up the boiler slowly so all parts of the boiler expand just as much. The leaks occur when some part expands more, or less, than the rest of the boiler. You will be on the safe side if you slowly heat up the boiler to, or almost to, normal operation pressure before you start high firing.
Question: Combustion air preheating
Please tell me how air inlet temperature affects boiler efficiency. What are the benefits of air preheating? Answer: The combustion air will be heated to the flame temperature. This heating cost money. If you have some waste heat to be used for preheating the combustion air it will pay.
Question: Water in the heavy fuel oil [ Is it possible to overheat heavy fuel oil thus causing any water in it to turn to steam and cause problems at the pump and burner?
Answer: Yes it is. The temperature of the heavy fuel oil is very often 130°C to 150°C and water introduced to that temperature would immediately evaporate into steam. When boiling, it expands about 1590 times. The situation might be dangerous since the safety valves not are designed for steam. This kind of problem is very likely to occur when you change fuel oil tank and some water from a poorly drained pipe mixes with the heavy fuel oil.
Question: Oil showing in the water level gauge glass Whilst on your engine room rounds, you discover oil showing in the water level gauge glass of an auxiliary boiler. Describe the remedial actions you would take, explaining why such actions must not be delayed.
Answer: Stop the burner immediately. Oil present, even small quantities, in boiler water will cause foaming and moisture carry-over. It also forms a heat insulating film, sometimes a carbonized layer, over tubes or shell surfaces. Even a very thin layer may result in tube or plate material failure due to overheating. The oil manifests itself by forming an oily ring inside the water gauge glasses, at the water level.
Question: Emergency low boiler water level
You are an officer on watch, & finds the boiler water-level gauge glass to be empty & the burner firing...What is your course of action? (Assuming the gauge glass to be clear & good working order)
Answer: Normally a boiler is provided with two independent sensors for emergency low water level burner cut-outs. So this would never happen. However, if it does, don’t take any chances! Shut off the burners immediately! Before you start raising the level in the boiler you have to find out if any part of the furnace walls has been overheated. If you raise the level over a glowing steel-wall then the boiler might produce more steam than the safety valves can handle and a nasty explosion would be the result. A quick test to see if it is safe to put water into the boiler is to temporarily close the steam cock on the gauge glass. If the level rises to the top of the glass, it means that there is still a water level in the water leg, which is also over the highest heat exchange surface in a firetube boiler. The water rises because of the vacuum caused in the glass with condensing steam.
Question: Differential pressure transmitters for the steam drum level Way is the high pressure leg of the transmitter connected to the water side and the low-pressure leg connected to the steam side?
Answer: The signal from the transmitter ought to increase when the water level raises and decreases when the level falls. Furthermore the signal shall be zero, and give impulse to stop the burner, in case of transmitter malfunction, power failure or cable breakdown. Both requirements will be fulfilled if the transmitter is mounted with the high pressure measuring point connected below the lowest water level and the low pressure measuring point connected above the highest water level. The output will increase when the level is raised. To compensate for the water column in the reference leg the output signal's zero-point has to be elevated. This is the common method. If the transmitter is swapped, with the low pressure side to lower end and the high pressure side to upper end, then the signal will decrease when the level is raised. This signal can be used to control the level as well, but the signal can not be used to stop the burner for emergency low level in case of power failure or cable breakdown. This system requires an extra sensor to trip burner at emergency low water level. One can of course use the emergency high water level alarm to stop the burner, but this is not correct. The emergency high water level shall stop the feed water pump and whenever applicable stop the steam turbine, but not the burner. Question: Fluctuating boiler water level The feed water control valve is fully open and the water levels fluctuate at normal boiler load.
Answer: Check if: • The control valve really is fully open by means of the hand-maneuver device. • All stop valves in the line are fully open. • The suction filter to the feed water pump is satisfactory clean. • The feed water pump discharge pressure is sufficient. • The feed water control valve pressure drop is normal. (>=2 bar or >=30 psi)
Question: The burner starts and stops very often The burner starts and stops very often, sometimes every second minute. An alarming temperature-raise has been observed in the combustion air fans electric motor.
Answer: Increasing the burners turn down ratio would be a nice solution, but it's not always possibly. • Run the burner in minimum load, i.e., prevent the burner from increasing the load just after the burner start. • Install a five to ten minutes' time-delay in the fan-motor stop function. Then the fan will continue to run during the shortest burner stops and the combustion air fan motor will get a little rest from the start current.
Question: Most likely source of errors In which part of a boiler control system is it most likely to get a failure.
Answer: When you have problem with a boiler control system you should keep in mind that most faults occur outside the control cubicle, but on the other hand, your problem might not be among the most common.
Statistically calculated faults in control systems. Transmitters and sensors 40 % Actuators 25 % Controllers 10 % Loss of electric power 5 % Others 20 %
Question: Open steam valves slowly Why has a steam valve, or at least a big steam valve, to be opened slowly?
Answer: If you have a one liter of water standing in the pipe just after the valve and open the valve too fast, then you will get a projectile of one kg rushing down the pipe. At next valve, bend or other obstacle the speed of the water mass will be converted into pressure. You can hardly imagine the damage this energy can cause. Thermal stress is another reason to be very cautious and drain out water carefully when you open a steam valve. A large steam valve ought to have a small by-pass valve to simplify preheating of the pipe. Question: Fuel Oil Ignition Quality, CCAI What is the CCAI of a fuel oil?
Answer: The CCAI, the Calculated Carbon Aromaticity Index, is a measure of the Fuel Oils Ignition Quality.
Question: Composite Boilers What is the inherent problem in Composite Boiler? Answer: There are different types of composite boilers. Normally one part of the boiler is heated by means of a fuel oil burner and the other part is heated by the exhaust gases from a diesel engine.
• Heating of one part of a boiler at the time often causes thermal stress that may lead to leakage. • One single composite boiler does not fulfill normal requirement of redundancy when the steam is used for essential service purpose.
Question: Steam valves open causing a sudden large load
I have two 82,000 lb/hr natural gas fired boiler that is designed for 300 psig and operating at 205 psig (saturated). The boilers serve a large campus with numerous buildings, each with an integrated building management control system. Due to an unresolvable characteristic of the building control system, occasionally all of the building steam valves open causing a sudden large load on the boilers that lasts for 20 to 30 minutes. The demand for steam is not real in that no heat is actually required by the buildings. When this condition occurs we have a serious water carryover problem. My question is how can we maintain boiler pressure and water level while either ignoring or controlling the sudden false load. Our combustion control system is a PLC based system, metered/cross limited air-fuel ratio , three element drum level and oxygen trim. Shall we try to correct thorugh the control algorithm or add backpressure control valves. I look forward to any advise.
Answer: The steam capacity doesn’t seem to be sufficient to supply all the fully open control valves. First of all recalculate the control valves. Over-sized control valves are very common cause of problems.
Question: Transport superheated steam Is it possible to transport superheated steam of the order of 45 t/h at 30 bar pressure and temp of 300 deg from a aux boiler to a distance of 1.5 Km?
Answer: Calculate with a velocity of 15 m/s (49 ft/s). To avoid water hammering the pipe-line should slop slightly downwards in the steam flowing direction. To start up the line you will need a drain valve on every 30 m (100 ft).