Troubleshooting Surface Condenser Venting Systems

J.R. LINES, R.E. ATHEY AND L.L. FRENS GRAHAM MANUFACTURING COMPANY

ABSTRACT tem related problems designed into the system, or caused by a venting system malfunction. Steam condenser venting equipment In an ideal situation, the condensing pressure achievable in a steam is sometimes misdesigned, improperly installed, or required to surface condenser is determined by the exiting cooling water tem- operate beyond its capabilities. The recent trend to part load perature. However the failure of the venting system to properly operation has accentuated the problem. remove noncondensible gases from the steam condenser will result in elevated condenser pressures. Information is presented in this If the venting system is unable to remove the noncondensible paper relative to the most common venting systems available, as gases at the pressure which is achievable by the condenser the well as providing procedures for troubleshooting each type of sys- condenser back pressure will rise. In this case the condenser does tem. A description is given of the various operating characteristics, not control the back pressure. Rather the venting system is the along with narrative discussions of field problems and experiences. controlling factor. It is worth noting that any time the back pres- Simple visual, audible and physical guides to the analysis of vent- sure is higher than necessary due to air retained in the condenser ing system problems are discussed. Power plant operating the plant heat rate will reflect this condition. personnel will be able to utilize this information when investigating steam condenser performance problems. A checklist is Manufacturers that provide both condensers and vacuum equip- provided which can be used to isolate these performance prob- ment to the power industry are involved in the design, lems. fabrication, testing, and repair of condensers, ejectors, and liquid ring vacuum pumps. In this capacity, those manufacturers understand the relationship between the steam surface condenser and INTRODUCTION the venting system. The authors of this paper have accumulated a great deal of experience in field service troubleshooting having The condensing pressure achieved in a steam surface condenser is various types of venting problems. The methods outlined in the determined by the exiting cooling water temperature if the condi- following analysis should prove to be a valuable tool in the assess- tions are ideal. All other factors steam condenser design operate ment of steam condenser performance and in solving venting to limit this optimum condition and to raise the condensing pres- related problems. sure, which results in increasing the plant heat rate. Some of the factors which have a negative impact on the condenser pressure include inadequate tube surface area, both vapor and cooling CHARACTERISTICS OF water maldistribution, air inleakage, resistance due to tube bundle SURFACE CONDENSER VENTING EQUIPMENT layout and/or baffle placement, and an inadequate venting system capability. These factors may act independently, or concurrently. Before examining the various methods of identifying and dealing with condenser venting systems, it is essential to have an under- Although it would seem obvious that the failure of venting sys- standing of the tems to properly remove noncondensible gases from the steam several types of condenser must result in elevated condenser pressures, customer vacuum produc- requests for field service have often revealed a lack of understanding devices that ing of the relationship between the steam condenser and the are most com- venting system servicing it. Unfortunately this can lead to delays monly used in in remedying many problems, and in a costly waste of effort in conjunction attempts to repair that which was not defective in the first place. with steam surface condensers. The operating characteristics of the venting system are often mis- These include understood, which can result in inherent limitations of steam ejector systems condenser performance. Sometimes condenser fouling or con- (usually com- denser design deficiencies are suspected and investigated, when prised of several failure to achieve the required vacuum level is due to venting sys- stages with

The American Society of Mechanical Engineers 1 intercondensers and aftercondensers), liquid ring vacuum pumps motive nozzle than is necessary for compression. If the actual (either single stage or two stage pumps), and the hybrid system motive steam pressure is below design, or if the steam tempera- (consisting of a combination of ejectors and a liquid ring vacuum ture is greater than intended, then, within limits, an ejector’s pump). nozzle can be rebored to a larger diameter. The larger nozzle diameter allows more steam to flow through and expand across Ejector Venting Systems the nozzle. This increases the energy available for compression. The ejector manufacturer should be consulted when considering Figure 1 is a schematic diagram of a typical two stage steam moti- reboring a motive nozzle. vated ejector system. Air and water vapor are removed from the main steam surface condenser, enter the first stage ejector and are Another potential ejector performance problem that is related to compressed to the intercondenser operating pressure by means of motive steam occurs if the supply pressure is greater than 20% the motive steam. After exiting the first stage ejector both the above design. When this happens, too much steam will expand load (noncondensible gases and associated water vapor) and the across the nozzle. This has a tendency to choke the diffuser. Whenever this occurs, less suction load can be handled by the ejector, and the suction pressure rises. If an increase in suction pressure is not acceptable, then the ejector nozzle must be replaced with one having a smaller throat diameter, or the steam pressure must be corrected.

Steam quality is another important performance variable. Wet steam is generally damaging to an ejector. Moisture in the motive steam is noticeable when inspecting ejector nozzles, because the moisture droplets in the steam lines are accelerated to near sonic velocities. This causes erosion of the nozzle internals by etching a striated pattern on the diverging section of the nozzle, which may actually wear out the nozzle mouth, or the inlet diffuser taper(s) and throat will show signs of erosion. On larger ejectors, the exhaust elbow located at the discharge of the ejector can erode completely through the metal. Severe tube impingement in the motive steam are discharged to the intercondenser where a major intercondenser can also occur; but this is dependent upon the portion of the water vapor load and the motive steam are con- ejector orientation relative to the intercondenser. Finally, wet densed. Noncondensible gases (air) and the remaining water steam can cause performance problems. When water droplets pass vapor are then directed to the second stage ejector where further through an ejector nozzle, they decrease the energy available for compression to atmospheric pressure takes place. Finally, the gases compression. The effect is a decrease in load handling capability are discharged through the aftercondenser. and/or instability of the ejector. Furthermore, water droplets vaporize within the diffuser and then act as additional load, Two stage condensing ejector systems can be designed to operate which must also undergo compression. To solve wet steam prob- at any reasonable condenser pressure, and the design is not limit- lems, all lines leading to the ejector should be well insulated. In ed by the temperature of the available cooling water to the addition, a steam separator with a trap should be installed imme- intercondenser. These systems have no moving parts, are the most diately prior to the motive steam inlet connection. reliable, require the least maintenance of all venting systems, and are the least expensive in their initial cost. The ejector systems The maximum discharge pressure (MDP) is the highest pressure require a reliable motive steam source, generally in the range of that an ejector can attain while utilizing a given amount of 100-150 PSIG. One drawback to this type of system is that the motive steam having a specified amount of energy. If the actual motive steam pressure must be maintained at a relatively constant discharge pressure exceeds the MDP the ejector will become value in order to prevent instabilities (accompanied by a resulting unstable and break operation. When this occurs, a dramatic loss of vacuum). increase in suction pressure is common. As an example, when an ejector designed to produce 1 inch HgA of suction pressure Proper motive steam conditions are always essential to the satis- breaks operation, the suction pressure increases sharply to 2-3 factory operation of an ejector. The manufacturer will have inches HgA. Therefore, it is important to make certain that the designed the system to maintain stable operation with steam pres- ejectors do not exceed their MDP. sures at, or above, a minimum value. If the motive steam pressure falls below the minimum design value, then the motive nozzle Since increasing the discharge pressure above the MDP causes a will pass less steam than required to operate the ejector. When loss of performance, it seems logical that lowering the discharge this happens, the ejector is not provided with sufficient energy to pressure below the MDP should have the opposite effect. This, compress the design load to the design discharge pressure. The however is not the case. Ejectors with a compression ratio (dis- same problem occurs when the motive steam temperature rises charge pressure divided by suction pressure) higher than 2:1 are above the design value, resulting in a larger specific volume than termed “critical ejectors.” The performance of a critical ejector is acceptable. Again, this results in less steam passing through the

The American Society of Mechanical Engineers 2 will not improve even if the discharge pressure is reduced. This is the con- primarily due to the presence of the shock wave in the diffuser denser is throat of the ejector. operated under part Ejector designers summarize the critical data on a performance load con- curve. Figure 2 shows a typical performance curve for a single ditions. stage ejector. The information displayed on this performance curve gives the suction pressure a function of the water vapor The equivalent load. (Equivalent load is used to express a non conden- amount of sible gas and water vapor process stream in terms of an equivalent seal liquid amount of water vapor load.) The performance curve can be used also has a in two ways. First, if the suction pressure is known for an ejector, significant the equivalent vapor load it handles may be determined. impact on Secondly, if the loading to an ejector is known, the suction pres- the per- sure can be found. If field measurements differ significantly from formance of a liquid ring vacuum pump. A seal water flow rate the performance curve, this indicates that there may be a problem that is too low will result in the temperature rise across the pump with either the process utilities or the ejector. being excessive. This causes the previously described effect where the seal liquid flashes and limits the pressure level obtainable by Liquid Ring Vacuum Pump Venting Systems the pump. Alternately, if the seal liquid flow rate is too high, the performance of the unit will probably be adversely affected due to Liquid ring vacuum pumps are often used as condenser high horsepower requirements and/or a loss of capacity. exhausters. The primary components of a liquid ring vacuum pump are the impeller, the pump casing and the seal liquid. The The performance of a liquid ring vacuum pump may also be impeller is mounted eccentrically in a round casing, which is par- adversely affected as a consequence of operating at a speed that is tially filled with the seal liquid. As the impeller rotates, the seal not at the design condition. This will either result in poor per- liquid is acted on by centripetal forces to produce a liquid ring formance, or in a breakdown of the gas compression process. which is concentric with the casing. It is the eccentricity of the When the pump is rotating at a speed greater than optimum, the impeller with respect to the liquid ring and casing that provides efficiency is decreased due to high velocities of the gas and vapor. compression of the gases inside the pump. At a speed that is below the optimum it is possible that the liquid ring will “collapse” and reduce the compression of the gas. The Noncondensible gas (along with accompanying water vapor) effect of attempting to compress to a discharge pressure that is enters the vacuum pump where compression takes place. greater than design is identical to the case where the pump speed Condensation of a portion of the water vapor also occurs inside is too low. Again, the liquid ring may potentially collapse. the pump. The remaining water vapor and noncondensible gas is segregated from the seal liquid in a separator, where the gas is dis- Hybrid Venting Systems charged to the atmosphere. The seal liquid is usually cooled and (Ejector intercondenser / Liquid Ring Vacuum Pump) returned to the pump, although once-through coolant systems are sometimes used. A “hybrid” venting system is a combination of an ejector and a liquid ring vacuum pump. Figure 4 illustrates a typical hybrid sys- In addition to being the compressing medium, the liquid ring tem consisting of an ejector, an intercondenser, and a vacuum absorbs the heat generated by gas compression, condensation of pump. The vacuum pump operates at a higher interstage pressure the water vapor and friction. Figure 3 illustrates a typical liquid ring vacuum pump system having a recirculating seal liquid arrangement. Under normal operating conditions the seal liquid temperature will be 3-5 °F warmer than the inlet cooling water the pump heat exchanger.

The vacuum attainable by a liquid ring vacuum pump is limited by the vapor pressure of the seal liquid. As the suction pressure approaches the vapor pressure of the seal liquid, it will “flash” into vapor. This reduces the capacity of the liquid ring pump, as more of the impeller space is occupied by vapor from the seal liquid, leaving less space available to accept the incoming load gas. If this condition is allowed to persist, cavitation will occur, resulting in possible severe damage to the internal surfaces, and preventing the pump from achieving design vacuum levels. Even if a pump with greater capacity is used, it will not be possible to obtain lower pressure levels than permitted by the vapor pressure of the seal water. This limitation can be a significant disadvantage when

The American Society of Mechanical Engineers 3 than would otherwise be obtained through the use of only a vacu- equipment pressure remains approximately the same, the air leak um pump). Three advantages are found when utilizing a hybrid is probably in the venting equipment. While there are numerous vacuum system: ways to detect air leakage, the most commonly used methods include: (1) hydrotest; (2) a shaving cream test; (3) a smoke test; • The pump suction pressure is higher than the vapor pressure (4) a gas sniffer test; or (5) a soap bubble test. of the seal water. This means that the seal water temperature does not limit the suction pressure achievable; II. The second most common problem is the accuracy of the pressure gauge. Absolute pressure type instruments are strongly • The major portion of the motive steam used by the ejector is recommended. Pressure gauges that are open to the atmosphere condensed in the intercondenser leaving only a relatively are subject to changes in barometric pressure, which can be +/- 2 small amount which enters the liquid ring vacuum pump; inches Hg. The best way to verify the accuracy of the pressure • The volume of noncondensible gas and water vapor to be gauge reading for the condenser is to compare the value of the compressed by the vacuum pump will be reduced at a fairly gauge against the condensate temperature. The temperature of the high interstage pressure, which results in a smaller pump condensate can never be higher than the saturation temperature being used. corresponding to the condenser pressure, but it can sometimes be colder. Cold condensate can occur with severe tube leaks, partial flooding of the tubes, cold makeup or dump water returns, and TROUBLESHOOTING sometimes under light condenser loads. THE CONDENSER VENTING SYSTEM III. A determination of whether the venting equipment or the Troubleshooting a surface condenser venting system requires a condenser is setting the operating pressure is important. Once systematic procedure in order to insure that the problem is both that has been established, a systematic approach to troubleshoot- identified and corrected. The following procedure is used by engi- ing is possible. One technique of determining which component neers in field situations to isolate venting problems: is controlling can be accomplished by increasing the venting equipment capacity. This can be done by adding a redundant vac- I. The first point that should be checked when poor performance uum element, or by turning on the hogging unit (if available). occurs is air leakage. This is the most common cause of poor per- When this is done, if the operating pressure in the condenser formance, and is probably the easiest one to identify, but often decreases, then the venting equipment is the controlling factor. If the hardest to find. The most common sources of air leakage in a the condenser operating pressure remains unchanged, then the condenser/venting system are at: (1) the turbine gland; (2) large condenser is limiting the vacuum level. diameter flanges, such as the steam inlet or turbine exhaust; (3) open valves; or (4) a loose steam chest on the ejectors. 1. If the condenser is limiting the vacuum level, the following items need to be checked: To identify air leakage as the problem, check the vent of the vacu- a. Cooling water inlet and outlet temperatures - If either um equipment, because any air leak in the system must exit at this the temperature of the inlet cooling water, or the cooling point. In the case of an ejector system, the vent is the vapor outlet water temperature rise, is greater than design, the con- connection of the aftercondenser. If the venting equipment com- denser pressure may be higher than design. A high prises vacuum pumps, the vent is located on the discharge of the cooling water temperature rise means that the condenser separator. In both cases, the air leakage rate should be checked is either over loaded, or that the cooling water flow rate when only the normal venting equipment is operating, i.e., without is below design. a hogger unit in operation. The majority of venting equipment is supplied with some type of air leakage flow device, but if one is not b. Cooling water inlet and outlet pressures - If the cool- available, or if it is suspect, a simple plastic bag and stopwatch will ing water pressure drop is below the design value, the accomplish the same results. The average air leakage rate, regardless cooling water flow rate is probably too low. if the cool- of which type of venting equipment design is employed, should not ing water pressure drop is above the design value, either exceed 15 pounds per hour for most commercial systems. If the the cooling water flow rate is above the design value measured air flow is in this range, air leakage is probably not the (this can be checked by observing a low temperature rise problem. If the measured air flow is above this rate, even if it is across the tubes), or tubeside fouling could be present. below the specified venting equipment design flow rate, it is recom- (Note: A high cooling water flow rate rarely poses a per- mended that a search be made for air leaks. formance problem.) 2. If the venting equipment is limiting the vacuum level, and If air leakage has been determined to be a problem, it is impor- the venting system utilizes ejectors, the following items tant to locate and isolate the problem. It is essential to know should be checked: whether the air leak path is present in the condenser or in the venting equipment. The easiest way to determine this is to close a. Motive steam pressure - If the motive steam pressure is the isolating valve between the venting equipment and the con- below design by more than 5%, or above design by denser. If the condenser pressure rises, and the venting equipment 20%, poor performance may occur with a resulting pressure decreases, the problem is in the condenser. If the con- increase in the condenser pressure. denser pressure remains relatively unchanged and the venting

The American Society of Mechanical Engineers 4 b. Motive steam quality - Wet motive steam will cause 3. If the venting equipment is limiting the vacuum level, and poor performance as well as ejector wear. Superheated the venting system utilizes a liquid ring vacuum pump, the steam having a temperature greater than 50° F above the following items should be checked: saturation temperature will also cause poor performance a. Seal water inlet and outlet temperature - Higher than if not considered in the design. design inlet or outlet temperatures will cause poor per- c. Intercondenser shellside pressure drop - If the shell- formance. When the outlet seal water temperature is side pressure drop is greater than 5% of the absolute high, this indicates either low seal water flow a high operating pressure, then either shellside fouling or flood- inlet temperature, or a malfunction of the seal cooler. ing of the condenser could be present. Check the trap or b. Operating pressure of the pump - When this pressure loop seal on the condensate outlet for proper drainage. is too low, the problem may be due to low noncondensi- d. Ejector internals - Check for internal wear, as well as ble gas loading or a cooling water temperature that is checking the critical dimensions. Both the steam nozzle lower than design. The concern is for cavitation of the and ejector diffuser throat dimensions should be meas- pump performance. ured. If the cross section area at those locations is greater than 7% above the design values, performance problems IV. If the problem does not appear to be items I, II or III, are likely. Examine the motive steam nozzle for steam check all vapor lines between the condenser and the venting leaks around the threads. equipment for any low point areas where condensation could e. Cooling water parameters on the inter- and aftercon- occur and create a large pressure drop. densers - Use the same procedure as described for the main condenser.

TROUBLESHOOTING ASSISTANCE CHARTS

The following charts are useful in identifying the causes of common venting system problems, and in helping to define the corrective action that is necessary to remedy the problems.

The American Society of Mechanical Engineers 5 The American Society of Mechanical Engineers 6 The American Society of Mechanical Engineers 7 Selected Case Histories for several feet and then went vertically downward. After a drop of Of Venting System Problems approximately 6 ft., the piping traveled horizontally for another 6 ft. and then went vertically upward to the venting system. The following case histories are drawn from among the field problems which have been experienced by the operators of con- The loop in the noncondensible gas extraction piping was imme- denser vacuum systems. The approach previously described was diately suspected. When systems are shut down, vapor within the utilized to identify and correct the problem. piping condenses and liquid resides in low horizontal sections of the piping loop. Upon startup, a liquid slug restricts extraction flow to the venting system. If liquid entirely fills the horizontal CASE HISTORY I: run, a pressure differential must be established to overcome the IMPROPER PIPING LAYOUT difference between the operating pressure of the condenser and BETWEEN STEAM SURFACE CONDENSER AND that of the venting system in order to force the liquid slug to pass VENTING SYSTEM through to the venting system. When the horizontal portion of the loop is partially filled, a significant pressure within the con- Problem: denser increases to create a driving force of sufficient magnitude A combined cycle cogeneration facility could not maintain design to overcome any pressure resistance associated with the restric- vacuum at the turbine discharge. The turbine was supported by a tion. steam surface condenser with twin 100% liquid ring vacuum pump condenser exhausters. Even while operating under part To remedy the problem, it was determined that it was too costly load conditions, the vacuum level could not be reduced below 3.1 to modify the piping. The installation of a drain connection in inches HgA. This was the case even when the venting system was the horizontal section of the piping was all that was necessary. functioning with both 100% vacuum trains in operation. When This drain connection permitted removal of residual water from one of the pumps was shut down, an increase in operating pres- the loop prior to startup. Once the water was removed, the sys- sure of only 0.12 inch Hg was measured. tem was restarted and the condenser pressure was brought to design levels with only one liquid ring vacuum pump on opera- Solution: tion. Plant operators were given instructions to open the drain A 1.45 inch Hg pressure drop (3.1 inch HgA - 1.65 inch HgA) connection prior to each startup. between the steam surface condenser and the venting system seemed excessively high and quite uncustomary. Shutting down the second liquid ring vacuum pump and achieving only a slight increase in operating pressure suggested that air inleakage was not the problem.

As with any troubleshooting exercise, it is always appropriate to examine the equipment layout. This should include the motive steam piping to the ejector system, condensate drain to the hotwell, all miscella- neous connections entering the condenser, cooling water piping, and vent piping from the steam surface condenser to the venting equipment. While conducting the layout survey, it was noticed that piping between the steam surface condenser and venting equipment was not properly routed. Between the condenser and the venting system the noncondensible gas extraction piping exit- ed the condenser shell, traveled horizontally

The American Society of Mechanical Engineers 8 CASE HISTORY II A review of the plant operating conditions revealed that the sup- MOTIVE STEAM PRESSURE BELOW DESIGN PRESSURE plier of the motive steam used by the ejector system also supplied steam to other process equipment within the paper mill. When Problem: steam demand by other process equipment increased, the pressure A combined cycle cogeneration plant supplying both steam and of the steam was reduced. power for a paper mill experienced erratic pressure fluctuations in the steam surface condenser. The surface condenser had a two The motive steam pressure to the ejector was monitored over time, stage, twin-element ejector system for removal of the nonconden- and it was verified that the line pressure did vary. Steam pressure sible gas. Pressure measured in the condenser would be ranged from 40 PSIG to 33 PSIG. To insure stable operation of established at the design point of 2 inches HgA, when suddenly a the system it was necessary to design the nozzles based on the low- dramatic increase in pressure would occur. At this time the con- est expected operating pressure. In this case, 33 PSIG was selected denser pressure would sharply increase to 4.5 inches HgA. After as the lowest pressure. The ejector nozzles were rebored based on the sharp increase, the condenser would gradually return to the this minimum steam pressure. In doing this, the entire system was design operating pressure. maintained at the design point, even when other process equipment experienced high demand for low pressure steam. Solution: A review of the piping layout revealed that this was not the source of the problem. Recognizing that a variable operating pressure may be due to excessive air inleakage, an air leakage meter was installed. It was determined that the air leakage was 85 pounds per hour, while the design for this system was 45 pounds per hour. Both 100% second stage ejectors were then operated simul- taneously to overcome this excessive air load, because their combined capacity exceeded the leakage rate. The condenser still experienced pressure excursions ranging from 2 inches HgA to 4.5 inches HgA.

Another possible cause was the motive steam pressure. As previously discussed, when the motive steam pressure falls below the design value, an ejector may have insufficient energy to compress noncondensible gases and the associated water vapor. When this occurs, the ejector will break operation. One noticeable effect of an ejector in a broken condition is a sharp increase in the operating pressure.

The American Society of Mechanical Engineers 9 CASE HISTORY III was done, and an artificial air load was introduced into the sys- CAVITATION OF LIQUID RING VACUUM PUMP tem, the suction pressure of the vacuum pumps increased to CONDENSER EXHAUSTER approximately the same pressure as the condenser operating pressure. The noise level of the vacuum system also decreased to Problem: acceptable levels. The venting system for a central power plant consisted of twin 100% liquid ring vacuum pump condenser exhauster packages. A complaint was received that the vacuum pumps were operating CONCLUSION noisily. Also, the pressure at the pump suction was measured and found to be significantly lower than the operating pressure of the Steam surface condensers are directly affected by the performance surface condenser. of their venting systems which are used for the continuous removal of noncondensible gases. The failure of these venting sys- Solution: tems to properly remove the noncondensible gases from the Because the operator reported the vacuum pumps were running condenser results in elevated condenser pressures. Therefore, it is noisily, it was initially thought that the problem could be either essential to the operation of the overall power plant to be able to cavitation or alignment of the pump. In order to investigate the maintain the venting devices at their optimum performance level. possibility of pump cavitation, it was necessary to compare the operating pressure of the pump against the vapor pressure corre- Troubleshooting a surface condenser venting system requires a sys- sponding to the temperature of the seal water exiting the pump. tematic procedure in order to insure that the problem is both The vapor pressure of water at 75° F is 0.88 inch HgA. Since the identified and corrected. Information relative to the most common pump was operating with a suction pressure of 0.83 inch HgA, vacuum producing devices has been presented, along with a discus- the conclusion was that the pump was cavitating due to liquid in sion of some of the most common problems which occur. Utilization the ring “flashing” into vapor. This would also explain the fact of the troubleshooting guidelines presented in this paper, along with that the pump was excessively noisy during operation. an understanding of the various devices, should be helpful when investigating steam condenser performance problems. Based on measurements from an air leakage meter installed on the system, it was apparent that the air inleakage was very low. As previously discussed, a liquid ring vacuum pump requires some noncondensible gas load to prevent cavitation. Without this load, the pump will operate at a pressure that results in cavitation.

A vacuum relief valve was included as part of the condenser exhauster packages. However this valve was found to be closed. The solution to this particular problem was to open the relief valve to provide additional air inleakage to the pumps. Once this

The American Society of Mechanical Engineers 10