Boiler drum level control Drum level control is critical to good operation, as well as safe boiler operation

By Jerry Gilman he drum level must be controlled to the or operators having a trip bypass button. When Tlimits specified by the boiler manufacturer. the drum level gets too low, the boiler must have If the drum level does not stay within these a boiler trip interlock to prevent damage to the limits, there may be water carryover. If the level tubes and cracks in the tubes where they con- exceeds the limits, carryover into the nect to the boiler drum. The water tubes may or the turbine may cause damage re- crack or break where they connect to the drum, sulting in extensive maintenance costs or outages or the tubes may rupture resulting in an explo- of either the turbine or the boiler. If the level is low, sion. The water tube damage may also result overheating of the water wall tubes may cause in water leakage and create problems with the tube ruptures and serious accidents, resulting in drum level control. The water leakage will affect expensive repairs, downtime, and injury or death the drum level because not all the water going to personnel. A rupture or crack most commonly into the drum is producing steam. occurs where the tubes connect to the drum. Poor level control also has an effect on drum Damage may be a result of numerous or repeated pressure control. The feedwater going into the low drum level conditions where the water level is drum is not as hot as the water in the drum. below the tube entry into the drum. Adding feedwater too fast will result in a cooling Some companies have had cracked or dam- effect in the boiler drum reducing drum pres- aged water tubes as a result of time delayed trips sure and causing boiler level shrinkage. This can be demonstrated by pouring tap water into a pan of boiling water. FasT Forward ● learn why drum level control is critical for boiler safety. Shrink and swell ● Boiler damage due to low water level is Shrink and swell must be considered in deter- one of the largest claims to insurance mining the control strategy of a boiler. During companies. a rapid increase in load, a severe increase in ● Proper drum level control minimizes level may occur. Shrink and swell is a result of downtime and cost. pressure changes in the drum changing water density. The water in the drum contains steam

36 INTECH july/augusT 2010 WWW.Isa.ORg Special Section: Flow/level

bubbles similar to when water is boiled in our Saturated The water in the steam homes. During a rapid increase in load, a severe drum contains steam bubbles similar to when rise in level may occur because of an increase in water is boiled in our volume of the bubbles. This increased volume is homes. the result of a drop in steam pressure from the load increase and the increase in steam genera- Steam tion from the greater firing rate to match the load increase (i.e., bubbles expand). If the level in the drum is too high at this time, it may result Steam in water carryover into the superheater or the turbine. The firing rate cycle can result in drum pressure cycles. The drum pressure cycles will Feedwater cause a change in drum level. Water The firing rate change has an effect on drum level, but the most significant cause of shrink and swell is rapid changes in drum pressure expanding or shrinking the steam bubbles due to load changes. When there is a decrease in demand, the drum pressure increases and the Water firing rate changes, thus reducing the volume of the bubbles (i.e., bubbles get smaller). A sud- den loss in load could result in high drum pres- sure causing shrinkage severe enough to trip the boiler on low level. A boiler trip at high firing Water-mixed steam bubbles rates creates a furnace implosion. If the implo- sion is severe enough, the boiler walls will be Boiler drums/level damaged due to high vacuum in the furnace. measurement. Note Typically, for redundancy, there are three dif- that the connections ferent methods used to measure drum level. In shown here are not the “Boiler drums/level measurement” exam- realistic. ple, the bull’s eye technology is a direct reading level measurement. The differential pressure transmitter represents the level control mea- surement, and the probe type sensor is a com- mon method for level alarms and low and high LT Water tubes level shutdown. Note the connections in the second illustration are not realistic. The chamber with the probes is for drum level alarms and boiler trips. The longest probe is the

Mud drum

INTECH july/augusT 2010 37 Special Section: Flow/level

Sight glass drum level indication common one. The one above it is low water trip. The one above that is the Gage glass low water alarm. The short probe can Steam be a high level alarm or a boiler trip. The length of the probes is determined Mirror by the boiler manufacturer. My experi- h(1) h(2) t(2) t(1) ence is the low water shutdown probe Water is 1½ to 2½ inches above the water tube boiler connections. The basic indication of the drum wa- ter level is commonly shown in a sight h(1) < h(2) because t(1) < t(2) gage glass (bull’s eye) connected to the boiler drum. The American Society of t = Water temperature Mechanical Engineers requires a direct h = Height of water column reading of the drum level. Due to the configuration of the boiler, and the dis- tance the boiler drum is from the oper- ator, a line-of-sight indication may not be practical. The gage glass image can be projected with a periscope arrange- ment of mirrors. There are a number of methods for drum level measurement. Other methods are a closed circuit tele- vision and the use of fiber optics. Operator The sight glass reading is affected by Mirror the temperature/density of the water in the sight glass. The water in the sight glass is cooler than the water in the boiler drum.

Drum level measurement The “Drum level connections” im- Drum level connections age is an example of the arrangement of a differential drum level measuring transmitter. The differential transmit- ter output signal increases as the dif- Constant head ferential pressure decreases. (Note the Drum chamber differential pressure connections. The connections may need to be reversed or calibrated so increasing level will go from 0 to 100%.) The differential pres- sure range will vary between 10 and 30 inches, depending on the size of the Correct Blowdown valve boiler drum, with a zero suppression of several inches. On the high pres- ∆ P sure side of the measuring device, the Reference effective pressure equals boiler drum Variable leg leg Transmitter pressure plus the weight of a water col- umn at ambient temperature having a length equal to the distance between Wrong the two drum pressure connections. ∆P On the low pressure side, the effective pressure equals boiler drum pressure, plus the weight of a column of satu- Transmitter rated steam having a length from the NOTE: Other drum level monitoring instrumentation may be applicable. upper drum pressure connection to the

38 INTECH july/augusT 2010 WWW.Isa.ORg Special Section: Flow/level

Uncompensated Drum level Measurement error Drum Saturated Saturated Saturated Saturated Saturated Actual Indicated Error (% of Error Pressure water liquid vapor liquid vapor Water Level Water Level 30” Span) (inches) (psia) temperature volume volume density density (inches) (inches) 15 213.0 0.0167 26.2900 59.88 0.04 0 0.02 0.061 0.02 100 327.8 0.0177 4.4310 56.50 0.23 0 0.11 0.362 0.11 600 486.2 0.0201 0.7697 49.75 1.30 0 0.62 2.083 0.62 1200 567.2 0.0223 0.3624 44.84 2.76 0 1.33 4.423 1.33 2000 635.8 0.0256 0.1883 39.06 5.31 0 2.55 8.513 2.55 2600 673.9 0.0294 0.1211 34.01 8.26 0 3.97 13.237 3.97 15 213.0 0.0167 26.2900 59.88 0.04 30 28.80 -4.009 -1.20 100 327.8 0.0177 4.4310 56.50 0.23 30 27.17 -9.432 -2.83 600 486.2 0.0201 0.7697 49.75 1.30 30 23.93 -20.246 -6.07 1200 567.2 0.0223 0.3624 44.84 2.76 30 21.57 -28.114 -8.43 2000 635.8 0.0256 0.1883 39.06 5.31 30 18.79 -37.381 -11.21 2600 673.9 0.0294 0.1211 34.01 8.26 30 16.36 -45.474 -13.64

Assumptions: Reference column temperature is assumed to be 100 degF. Reference temperature for inches of water is 39.2 degF. DP transmitter is calibrated for 0-30 inches of water.

Uncompensated Drum Level Measurement Error 35

30

25

20 Actual Level

ches Indicated Level

n 15 I 10

5

0 1 2 3 4 5 6

1=15, 2=100, 3=600, 4=1200, 5=2000, 6=2600 PSIA water level, and the weight of a column that lines be sloped at least a half inch ing zero level when the drum level is at of water at saturation temperature hav- per foot from the boiler drum to the 100% (“Sight glass drum level indication” ing a length from the water level to the transmitter. If not properly sloped, air image). By reversing the connections at lower drum pressure connection. pockets may form in the lines creating the transmitter, the drum level signal is On high pressure , a conden- improper drum level measurement. reversed. The reading may also be cor- sate pot is connected on the top water When a differential pressure trans- rected with transmitter calibration. leg to keep the leg full of condensate. If mitter is used to measure drum level The drum level control indicator scale the condensate level varies in the top and the instruments used are sensitive for a 30-inch span, the distance between connected leg, the drum level mea- to density variation, density compen- the upper and lower drum connections, surement will not be accurate. On low sation techniques must be employed. A would be -15 to +15 inches with zero as pressure boilers, a condensate pot may mass steam flow and water flow signal the controller set point. On higher pres- not be required. The “Drum level con- is required for two and three element sure boilers, typically above 1000 psi, a nections” image is an example of the control systems. (For more informa- considerable error in level measurement correct method of installing a differ- tion, refer to ANSI/ISA-77.42.01-1999 at other than the operating pressures ex- ential pressure transmitter. The correct (R2006) – Fossil Fuel Power Plant Feed- ist when a differential pressure is used installation allows the sediment to re- water Control System – Drum Type.) to measure level due to water density main in the blowdown line without get- Observe the error due to density in the changes in the drum. ting into the transmitter. “Uncompensated drum level measure- Problems with drum level measure- ment error” chart. The top boiler connec- ment can be a result of improper instal- tion to the transmitter will be filled with SOURCE: Boiler Control Systems Engineering, 2nd Edition, by Jerry Gilman, http://www.isa.org/boilereng. lation of the sensing legs from the boil- condensate. As the drum level increases, er drum to the transmitter. It is critical the two signals become equal, thus read- View the online version at www.isa.org/intech/20100806.

40 INTECH july/augusT 2010 WWW.Isa.ORg