In this two-part series, how transient events in process equipment affect other system components will be discussed. Part 1 covers how deaerators (DAs) may be affected by generator trips and how these deaerator transients may also affect system pumps.
A power plant includes a complex array of machinery, pipes, tanks, fuel, controls and auxiliary equipment. Figure 1 illustrates a simplified sketch of this system.
The Steam Generation Process
Fuel heats the boiler to convert liquid to steam. High temperature and pressure feed the hot steam from the boiler to a turbine, which typically has several stages (high, intermediate and low pressures). The system includes additional components for better efficiency, control and reliability, but these are omitted to simplify the article.
Steam turbine shaft rotation is transmitted to a generator shaft, and electric current is delivered to a net. Exhausted steam is dumped into a condenser where it is further cooled to complete its transformation back to cold water, which a condensate pump sends to a DA.
The DA's size depends on the design and the plant's requirements, but a 20,000-gallon tank with roughly half its capacity filled is typical for a relatively small power plant.
The DA's Function
The main purpose of a DA is to remove entrained air, which is accomplished by extracting auxiliary (pegging) steam from the low-pressure steam turbine.
This steam heats the water and strips it of most of the air and keeps the fluid in the DA at saturation temperature. The deaerated water flows to the boiler feedwater pump's (BFP) inlet, which moves it to the boiler, and the cycle repeats.
Problems that may interrupt this process include a sudden reduction or a complete loss of generator load (a load rejection condition).
When the generator trips, steam is bypassed through the diverting valve around the steam turbine and is typically directed into the condenser. A less common method is to release the steam directly into a DA or to the atmosphere.
During roughly the same time, fuel is diverted from the boiler, and the boiler begins to cool. Condensate pumps continue to operate to keep up with the cooling steam arriving at the condenser. Because the pegging steam has been diverted to the condenser, it no longer pressurizes the DA. The continuous arrival of the condensate into the DA cools the liquid. As a result, the temperature and pressure in the DA drop. The temperature falls as the condensate cools it, and the pressure decreases because of the loss of pressurizing pegging steam.
A basic understanding of thermodynamics provides the foundation for what happens next. Steam tables and graphs contain information about pressure, temperature, specific volume, enthalpy (energy), entropy and quality of the water in its different states—liquid, vapor and mixed. Figure 2 depicts a temperature-specific volume (T-v) diagram, with tabulated data and examples.
(Source: Cameron Hydraulic Data Book, 19th Edition, 2002)
For example, tabulated data within Figure 2 show that at 302 degrees F, 69.07 psia of pressure is required to keep water at saturation point A. If the water were heated further at a constant pressure of 69.07 psia (shown as a horizontal line moving to the right), it would eventually boil out into vapor, which would significantly increase its volume. The volume would be much greater than the DA tank could hold. Conversely, if it were to be cooled at a constant pressure, 69.07 psia in this case, it would become subcooled, meaning it would be pure liquid with no vapor. Point A is a border line at which the vapor is saturated.
The DA is, however, a fixed-volume tank. For any vapor to form within it, the pressure would need to change. Reduction in temperature is shown as a vertical movement downward on the diagram from Point A to Point B, at which the liquid reaches the assumed final temperature of 266 degrees F.
The combined specific volume (v) is a ratio of the tank volume (liquid and vapor) to a total mass of the fluid in the tank. A half full 20,000-gallon tank (approximately 2,600 cubic feet) contains 10,000 gallons of water, or 1,300 cubic feet. Liquid water weighs 57.27 pounds per cubic foot (lb/ft3) at 302 F. The inverse is 0.01746 cubic feet per pound (ft3/lb), which is the specific volume. The liquid in the tank weighs:
1,300 x 57.27 = 74,450 pounds
Point B is in the mixed phase region. Figure 2 shows the pressure at 39.20 psia, with fluid in a two-phase state—some liquid and some vapor.
The specific volume at that temperature/pressure is 0.1714 ft3/lb for liquid, shown at the left intersection with the saturation curve. The volume at that temperature/pressure for steam is 10.70 ft3/lb, shown at the right intersection. The water quality (x) is a proportion of gas's mass (Mg), in this example the vapor's mass, to the total mass (M) of the mixture, which is shown in Equation 1.
Equation 2 shows the formula for the average total specific volume.