By S. Zaheer Akhtar and Magdy Mahmoud, Power Generation Engineering and Services Co.
In a typical thermal power plant, the boiler feed water (BFW) pump takes suction from the deaerator and discharges to the boiler through the feedwater heaters. During normal operation, the deaerator is supplied with extraction steam from the steam turbine and serves to provide a) net positive suction head (NPSH) for the BFW pump and b) a continuous supply of feed water to cope with surges in boiler demand.
The intent of this article is to provide the feedwater system designer with the methodology required to evaluate adequacy of the feed water system and determine whether the BFW pump is capable of handling a deaerator pressure transient or not. In this context, a deaerator pressure transient refers to a rapid loss of deaerator pressure as experienced during a steam turbine generator trip (boiler remaining in service) or sudden load reduction on the steam turbine generator.
If the results obtained from this methodology indicate that the BFW pump is not capable of handling the pressure transient, then it is probable that the pump internals will suffer cavitation damage during the transient event. Alternatively, the pump could shut down on a trip and lead to a total plant shutdown. In any case, some system-design changes would be required to ensure trouble-free operation during pressure transients.
The methodology presented in this article is useful for checking the adequacy of new system-design under development as well as those designs already installed and in-service.
NPSH and NPSH Margin
The deaerator is installed at an elevation to provide the NPSH to the BFW pump. By definition, the NPSH is the total suction head over and above the vapor pressure of the liquid pumped.
The deaerator elevation minus the dynamic losses in the BFW suction piping provides the net positive suction head available (NPSHa) to the pump. The difference between the value of the NPSHa and the net positive suction head required (NPSHr) by the pump gives the NPSH margin.
The NPSH margin or the NPSH margin ratio (NPSHa/NPSHr) is an important factor in ensuring adequate service life of the pump and minimizing noise, vibration, cavitation and seal damage. The NPSH margin requirements increase as the suction energy level (for example, high suction specific speed, high peripheral velocity of impeller and so on) of the pump increases. In case of the BFW pump, this ratio could be in the range of 1.8 to 2.5.
Additionally, the NPSH margin enhances the capability of the BFW pump in handling a deaerator pressure transient. This aspect of the system design is the main topic of this article and the methodology presented facilitates efforts to determine if the NPSH margin is adequate to handle the transient.
Deaerator Pressure Decay and Effect
Immediately after a steam turbine generator trip, turbine extraction steam is no longer available to the deaerator resulting in pressure decay in the deaerator. Also during a sudden steam turbine generator load reduction, the extraction steam pressure decreases and a point is reached when the extraction stage supplying the deaerator has insufficient pressure to feed the deaerator. This also results in deaerator pressure decay as the condensate continues to enter the deaerator and provide a cooling effect. The decrease in deaerator pressure causes some of the water in the storage tank to flash to steam until saturation is attained at the new pressure.
The water in the BFW pump suction line has a static head exerted on it by the level in the storage tank, preventing it from flashing immediately. Therefore, the water in the suction line can be considered as a slug of hot fluid which is trapped and has to be moved through the pump. In other words, the pump will not perceive a decrease in vapor pressure (or a decrease in water temperature) until the entire slug of hot water has passed through the pump.
During the passage of the hot-water slug, the combination of high vapor pressure at the pump suction along with a decrease in pump suction pressure (due to deaerator pressure decay) leads to a critical point at which the suction pressure may drop below the minimum required pressure (that is, the vapor pressure of the hot-water slug plus the pressure equivalent of the NPSHr). This low suction pressure could result in cavitation damage to the pump internals due to insufficient NPSHr.
The time required for passage of the hot-water slug through the pump suction line is known as the residence time. It can be expressed as the suction line volume divided by the volumetric flow rate (or alternatively as the mass of liquid in the suction line divided by the mass flow rate). Note that since the vapor pressure at pump suction is considered to decay only after the residence time has elapsed, the critical point occurs at the end of the residence time interval.
A simplified expression for the deaerator pressure decay can be expressed by the exponential decay equation for corresponding enthalpy as follows:
hd = enthalpy of deaerator water storage tank at any time t, Btu/lb
hhc = enthalpy of condensate in condenser hotwell
h1 = initial enthalpy of deaerator water storage tank, Btu/lb
Wc = condensate flow after steam cut-off, lbs/min
M = mass of water in deaerator storage tank, lbs
Eqn (1) is given in terms of enthalpy. The corresponding deaerator pressure is the saturation pressure at the enthalpy established by Eqn (1).
This equation is a simplification as it does not consider the warm condensate contained in the low pressure heaters and the condensate piping. However, it is adequate for our use as it is conservative. If the use of Eqn (1) indicates a problem with the BFW pumps’ capability in handling the deaerator transient, the system designer can redesign the system or re-check the calculations using the more exact equations available in published literature (Ref. 1). Note that the condensate flow to the deaerator (Wc) after steam cut-off needs to be established correctly, based on subsequent boiler load and spray water consumption in the steam attemperaters. The boiler load at this stage is expected to be limited to the capacity of the turbine bypass system.
Establishing the Curve
The actual pressure at BFW pump suction is simply the deaerator vapor pressure as computed by using Eqn (1) plus the deaerator static head less the frictional pressure drop in the BFW pump suction piping.
Figure 1 shows a plot of the actual pressure at BFW pump suction and the deaerator vapor pressure. The difference is the available net positive suction head (NPSHa) at pump inlet.
During a deaerator pressure transient, the actual pressure at pump suction decreases as the deaerator vapor pressure decays. However, due to hold-up of the hot-water slug in the pump suction line, the pump suction vapor pressure does not decrease until after the residence time has elapsed. This results in a critical point corresponding to the BFW pump suction pipe residence time where the NPSHa is at a minimum. If the value of NPSHa at this point (critical point margin) falls below the pump NPSHr, the system design is inadequate and the pump could be damaged due to cavitation during the transient. In this case, one of the solutions would be to install a low NPSH booster pump upstream of the BFW pump.
Booster Pump Upstream
The main BFW pumps are generally large, high-energy pumps needing large values of NPSHr. This requires raising the elevation of the deaerator which is costly and sometimes not practical. In such cases, low-speed, low-NPSH booster pumps are used upstream of the BFW pump. The booster pump discharge head then provides the necessary NPSH to the BFW pump.
Although the above discussion on critical point and critical point margin is presented here with reference to the majority of installations where the main BFW pump takes suction from the deaerator, it is equally applicable to the case where a booster pump is installed upstream of the main BFW pump. The only difference is that in case of the booster pump arrangement, the critical point and the critical point margin needs to be evaluated at the booster pump suction as well as the BFW pump suction.
Additional Transient Condition
An additional transient condition which the system designer should be aware of is that which could occur during a hot start. In this case also, steam flash (water-steam mixture) can occur at pump suction and cause cavitation damage to the pump internals. However, the mechanism causing steam flash is slightly different.
On a plant trip, the deaerator pressure and the water temperature both drop. However, the pump and suction piping near the pump remain at a higher temperature due to mass of the metal. As a result, when the pump is operated on a hot re-start of the plant, steam flash and cavitation is likely to occur at the pump suction.
A solved example explaining the methodology for evaluating the effect of the deaerator pressure transient on the BFW pump NPSH is presented below.
(Note that the hot re-start transient is not a part of this calculation).
A. Input Data:
- – Deaerator pressure @ instant of steam cut-off, p1: 73.20 psia
- – Saturated water enthalpy @ instant of steam cut-off, h1: 275.86 Btu/lb
- – Final deaerator pressure, p2: 18.50 psia
- – Saturated water enthalpy @ end of transient, H2: 192.11 Btu/lb
- – Condensate flow after steam cut-off to deaerator, Wc: 29,265.00 lbs/min
- – Warm condensate enthalpy @ time of load reduction, h4: 202.00 Btu/lb
- – Hotwell condensate enthalpy, h5: 69.80 Btu/lb
- – Mass of water in deaerator storage tank, M: 20,3958.00 lbs
- – Mass of warm condensate in LP htr & conn. piping, Mw: 32,849.00 lbs
- – Time required to replace warm condensate, tw=Mw/Wc 1.12 mins
- – Residence Time (time required to replace warm feed water in BFW pump suction line): say 0.5 mins
- – Using the above data along with Eqn (1), Table 1 can be developed.
For the case under consideration, the residence time is estimated at 0.5 min and this becomes the critical point. At this point, Table 1 shows a reduced deaerator pressure of 59.45 psi (or 66.1 psi depending upon the correlation used). The actual pressure at pump suction then corresponds to deaerator pressure of 59.45 psi + 34 psi (where 34 psi equals the static head minus friction loss) = 93.45 psi. However, the pump suction vapor pressure during the residence time remains unchanged at 73.2 psi. This provides a margin above the pump suction vapor pressure of 93.45-73.2=20.25 psi (equivalent to 52 ft of NPSHa).
If this calculated value of NPSHa = 52 ft. is less than the NPSHr provided by the pump vendor, the pump is expected to cavitate during the transient and such a design would therefore be considered as inadequate.
The NPSHr provided by the vendor can be based on 3 percent head loss or 1 percent head loss. It is more conservative to have the vendor provide the NPSHr based on 1 percent head loss.
1. Liao, C.S., and Leung, P “Analysis of Feedwater Pump Suction Pressure Decay”, ASME J. Eng. Power, April 1972
2. Karassik, I.J., Messina, J.P., Cooper, P., and Heald, C.C. “Pump Handbook” 4th Edition, Chapter 12.