Special Section: Flow/Level
Boiler drum level control
Drum level control is critical to good boiler operation, as well as safe boiler operation
By Jerry Gilman
The drum level must be controlled to the limits specified by the boiler manufacturer. If the drum level does not stay within these limits, there may be water carryover. If the level exceeds the limits, boiler water carryover into the superheater or the turbine may cause damage resulting in extensive maintenance costs or outages of either the turbine or the boiler. If the level is low, overheating of the water wall tubes may cause tube ruptures and serious accidents, resulting in expensive repairs, downtime, and injury or death to personnel. A rupture or crack most commonly occurs where the tubes connect to the drum. Damage may be a result of numerous or repeated low drum level conditions where the water level is below the tube entry into the drum.
Some companies have had cracked or damaged water tubes as a result of time delayed trips or operators having a trip bypass button. When the drum level gets too low, the boiler must have a boiler trip interlock to prevent damage to the tubes and cracks in the tubes where they connect to the boiler drum. The water tubes may crack or break where they connect to the drum, or the tubes may rupture resulting in an explosion. The water tube damage may also result in water leakage and create problems with the drum level control. The water leakage will affect the drum level because not all the water going into the drum is producing steam.
Poor level control also has an effect on drum pressure control. The feedwater going into the drum is not as hot as the water in the drum. Adding feedwater too fast will result in a cooling effect in the boiler drum reducing drum pressure and causing boiler level shrinkage. This can be demonstrated by pouring tap water into a pan of boiling water.
Shrink and swell
Shrink and swell must be considered in determining the control strategy of a boiler. During a rapid increase in load, a severe increase in level may occur. Shrink and swell is a result of pressure changes in the drum changing water density. The water in the drum contains steam bubbles similar to when water is boiled in our homes. During a rapid increase in load, a severe rise in level may occur because of an increase in volume of the bubbles. This increased volume is the result of a drop in steam pressure from the load increase and the increase in steam generation 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 in water carryover into the superheater or the turbine. The firing rate cycle can result in drum pressure cycles. The drum pressure cycles will cause a change in drum level.
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 firing rate changes, thus reducing the volume of the bubbles (i.e., bubbles get smaller). A sudden loss in load could result in high drum pressure causing shrinkage severe enough to trip the boiler on low level. A boiler trip at high firing rates creates a furnace implosion. If the implosion is severe enough, the boiler walls will be damaged due to high vacuum in the furnace.
Typically, for redundancy, there are three different methods used to measure drum level. In the “Boiler drums/level measurement” example, the bull’s eye technology is a direct reading level measurement. The differential pressure transmitter represents the level control measurement, and the probe type sensor is a common method for level alarms and low and high 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 common one. The one above it is low water trip. The one above that is the low water alarm. The short probe can be a high level alarm or a boiler trip. The length of the probes is determined by the boiler manufacturer. My experience is the low water shutdown probe is 1½ to 2½ inches above the water tube boiler connections.
The basic indication of the drum water level is commonly shown in a sight gage glass (bull’s eye) connected to the boiler drum. The American Society of Mechanical Engineers requires a direct reading of the drum level. Due to the configuration of the boiler, and the distance the boiler drum is from the operator, a line-of-sight indication may not be practical. The gage glass image can be projected with a periscope arrangement of mirrors. There are a number of methods for drum level measurement. Other methods are a closed circuit television and the use of fiber optics.
The sight glass reading is affected by 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” image is an example of the arrangement of a differential drum level measuring transmitter. The differential transmitter output signal increases as the differential pressure decreases. (Note the differential pressure connections. The connections may need to be reversed or calibrated so increasing level will go from 0 to 100%.) The differential pressure range will vary between 10 and 30 inches, depending on the size of the boiler drum, with a zero suppression of several inches. On the high pressure side of the measuring device, the effective pressure equals boiler drum pressure plus the weight of a water column at ambient temperature having a length equal to the distance between the two drum pressure connections. On the low pressure side, the effective pressure equals boiler drum pressure, plus the weight of a column of saturated steam having a length from the upper drum pressure connection to the water level, and the weight of a column of water at saturation temperature having a length from the water level to the lower drum pressure connection.
On high pressure boilers, a condensate pot is connected on the top water leg to keep the leg full of condensate. If the condensate level varies in the top connected leg, the drum level measurement will not be accurate. On low pressure boilers, a condensate pot may not be required. The “Drum level connections” image is an example of the correct method of installing a differential pressure transmitter. The correct installation allows the sediment to remain in the blowdown line without getting into the transmitter.
Problems with drum level measurement can be a result of improper installation of the sensing legs from the boiler drum to the transmitter. It is critical that lines be sloped at least a half inch per foot from the boiler drum to the transmitter. If not properly sloped, air pockets may form in the lines creating improper drum level measurement.
When a differential pressure transmitter is used to measure drum level and the instruments used are sensitive to density variation, density compensation techniques must be employed. A mass steam flow and water flow signal is required for two and three element control systems. (For more information, refer to ANSI/ISA-77.42.01-1999 (R2006) – Fossil Fuel Power Plant Feedwater Control System – Drum Type.)
Observe the error due to density in the “Uncompensated drum level measurement error” chart. The top boiler connection to the transmitter will be filled with condensate. As the drum level increases, the two signals become equal, thus reading zero level when the drum level is at 100% (“Sight glass drum level indication” image). By reversing the connections at the transmitter, the drum level signal is reversed. The reading may also be corrected with transmitter calibration.
The drum level control indicator scale for a 30-inch span, the distance between the upper and lower drum connections, would be -15 to +15 inches with zero as the controller set point. On higher pressure boilers, typically above 1000 psi, a considerable error in level measurement at other than the operating pressures exist when a differential pressure is used to measure level due to water density changes in the drum.
SOURCE: Boiler Control Systems Engineering, 2nd Edition, by Jerry Gilman, http://www.isa.org/boilereng.
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