February 1, 2010

Can Your Boiler Feed Pump Handle a Deaerator Pressure Transient?

S. Zaheer Akhtar, PE, Magdy Mahmoud

In a typical steam power plant, the boiler feedwater (BFW) pump takes suction from the deaerator (DA) and discharges high-pressure water to the boiler through the feedwater heaters. During normal operation, the DA is supplied with steam turbine extraction steam to mix with and heat the feedwater. Other purposes for the DA are to provide the required net positive suction head (NPSH) for the BFW pump and to serve as a storage tank to ensure a continuous supply of feedwater during rapid changes in BFW demand (Figure 1).


1. Circumventing cavitation. The available net positive suction head provided to a boiler feedwater pump can drop enough during a pressure excursion to cause cavitation and damage to the pump’s internal parts. A careful analysis of various operating profiles can ensure that the pump operates safely during the pressure fluctuations that occur after a steam turbine trip or large load change. Courtesy: PGESCo., Egypt


How does the plant designer or operator determine the adequacy of the BFW pump selection or the DA and feedwater system design? It’s not uncommon to find that the BFW pump was originally specified based on steady-state conditions and did not consider the DA pressure transients that occur during a steam turbine trip (with the boiler remaining in service) or a sudden steam turbine load reduction. If the NPSH available to the BFW pump during the pressure transient drops below that required by the pump for only a short period of time, cavitation and damage to the pump internals often result.

An NPSH deficit in an existing system or a new system under development can be avoided by using some very simple analytic tools.