February 1, 2010

Can Your Boiler Feed Pump Handle a Deaerator Pressure Transient?

S. Zaheer Akhtar, PE, Magdy Mahmoud

Short Residence Time

The time required for passage of the hot-water slug through the pump suction line is the "residence time." Residence time 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 because the vapor pressure at pump suction is modeled to decay only after the residence time has elapsed, the critical point occurs at the end of the residence time interval. The challenge is to determine the DA pressure at this critical point and thereby the system NPSH margin.

Establishing the Deaerator Pressure Decay Curve

A simplified way to express the DA pressure decay is by the exponential decay equation for enthalpy. In addition, the corresponding DA pressure at any time is the saturation pressure at the calculated DA water enthalpy:

where,

h_d = enthalpy of deaerator water storage tank at any time t, Btu/lb

h_hc = enthalpy of condensate in condenser hotwell, Btu/lb

h₁ = initial enthalpy of deaerator water storage tank, Btu/lb

W_c = condensate flow after steam cut-off, lb/min

M = mass of water in deaerator storage tank, lb

The actual pressure at BFW pump suction is simply the DA vapor pressure as computed by the equation plus the DA static head less the frictional pressure drop in the BFW pump suction piping.

This equation conservatively models the system well but does not consider the warm condensate contained in the low-pressure (LP) heaters and the condensate piping.

If using the equation on your system indicates a problem with the BFW pumps’ capability in handling the DA transient, you may want to recheck the calculations using the more exact equations available in this reference source: Liao, C.S., and Leung, P., "Analysis of Feedwater Pump Suction Pressure Decay," ASME Journal of Engineering for Power (April 1972). However, the approach described in this article uses a more conservative approach to the calculations.

Note that the condensate flow to the deaerator (W_c) after steam cut-off must be established correctly based on subsequent boiler load and spray water consumption in the steam attemperators. The boiler load is also assumed to be limited to the capacity of the turbine bypass system.