Boiler Feedwater Pressure Requirements 1

[dave2109]

In ASME Section I, PG-61.1 it states that the feedwater supply shall be capable of feeding the boiler at a pressure 3% higher than the highest relief valve setpoint on the boiler. It also lists a few exceptions where this requirement does not need to be met. Does anybody know specifically if this requirement applies to HRSG's that use exhaust gas from combustion turbines? Does it apply to gas/oil burning auxiliary boilers w/ steam drums?

I see nothing that would exempt these items from the requirement but I've talked with people who've sized these pumps in the past and they never took the ASME requirement into consideration.

Applying the ASME requirement results in pump and control valve sizing that doesn't match the normal operating conditions very well so I'm reluctant to do it if it's not required.

Any help would be appreciated.

 

[TBP]

How much difference is there between the operating pressure of the boiler at full load, and the safety valve setting?

 

[dave2109]

The relief valve setpoints are 775 and 790 psig. The full load operating pressure is 700 psig.

 

[TBP]

That's pretty close. I was thinking perhaps you had a situation where the safety valves were set at double the operating pressure, or something.

I'd ask the inspection authority for the jurisdiction where the boiler is to be installed what they require. As someone who spent a number of years as a chief engineer in powerhouses, I always thought that being able to push feedwater into a boiler maintain the water level, even with the drum pressure high enough that the safeties were lifting, was a good thing.

 

[davefitz]

The code requirement needs to be met for all power boilers, but it is not neccesary to supply MCR flowrate at the overpressure condition. The pump's deadhead pressure almost certainly meets the requirement to supply at least a small amount of flow at the case of the drum at safety pressure + 3%.

The code requires a power boiler to have the firing system ( burners, combustors, gas turbine combustor) reduce fuel firing rate automatically if the main steam outlet pressure esceeds the design pressure of the main steam line . It may be prudent to include in this control logic a runback in firing rate if the drum pressure exceed relief valve lift pressure - 3% ( for accuracy in set pressure)