February 1, 2010

Can Your Boiler Feed Pump Handle a Deaerator Pressure Transient?

S. Zaheer Akhtar, PE, Magdy Mahmoud

Example Illustrates Calculations

A solved example explaining the methodology for evaluating the effect of the DA pressure transient on the BFW pump NPSH is presented below. Note that the hot restart transient is not part of this calculation. Calculation of the residence time requires dimensional information of DA and the BFW pump suction piping.


Keep your margin. Deaerator pressure decay and boiler feedwater pump net positive suction head margin during a deaerator pressure fluctuation can be calculated using the methods described in this article. Source: PGESCo., Egypt

Step 1: Collect data. Collect the following data from plant instrumentation/design documents:

• Deaerator pressure at instant of steam cut-off, p₁ = 73.2 psia

• Saturated water enthalpy at instant of steam cut-off, h₁ = 275.86 Btu/lb

• Final deaerator pressure, p₂ = 18.5 psia

• Saturated water enthalpy at end of transient, h₂ = 192.11 Btu/lb

• Condensate flow after steam cut-off to deaerator, W_c = 29,265 lb/min

• Warm condensate enthalpy at time of load reduction, h₄ = 202.0 Btu/lb

• Hotwell condensate enthalpy, h₅ = 69.80 Btu/lb

• Mass of water in deaerator storage tank, M = 20,3958 lb

• Mass of warm condensate in LP heater and connecting piping, M = 32,849 lb

• Time required to replace warm condensate, t_w = M/W_c = 1.12 minutes

• Residence time (time required to replace warm feedwater in BFW pump suction line) = 0.5 minutes (estimate based on plant-specific piping configuration)

Step 2: Perform the calculations. Use the data above with the equation provided earlier to develop the results shown in the table.

For the present case, the residence time is assumed to be 0.5 min and therefore becomes the critical point. At this point, the table shows a reduced DA pressure of 59.45 psi (or 66.1 psi, depending upon the correlation used). The actual pressure at pump suction then corresponds to DA 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).

Step 3: Evaluate the results. 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. This means the design is inadequate and should be revised. If the NPSHa is larger than the NPSHr provided by the pump vendor, then the system should work satisfactorily during a similar pressure transient.

As a closing note, the NPSHr provided by the pump vendor is usually based on either an assumed 3% head loss or 1% head loss. It is more conservative to have the vendor provide the NPSHr based on 1% head loss.

— Contributed by S. Zaheer Akhtar, PE (szakhtar@bechtel.com), assistant chief – mechanical on assignment from Bechtel Power to Power Generation Engineering and Services Co. (PGESCo) in Cairo, Egypt. Magdy Mahmoud is manager of engineering for PGESCo., Egypt.