What is a "pegging steam" in the HRSG deaerator ? 1

[komecha]

Hi, Everyone.

Our HRSG have a " pegging steam piping" from HP Drum to Deaerator.
I want to know What it play an important role ?

Thanks,

 

[davefitz]

The dearator not only dearates the feedwater, but it supplies liquid to the boiler feed pump BFP.

If the dearator pressure drops faster than about 1 psi/sec, then the liquid to the BFP will boil in the downcomer, and cause cavitation in the BFP suction.

Several startup events can cause the dearator pressure to drop , so to prevent that, there should be a backup supply of steam to peg the dearator . The logic for the pegging steam should be based on 2 limits: do not let the dearator pressure drop below 3 psig .AND. do not let the rate of decrease of derator pressure exceed 0.5 psi / sec.

 

[gmorin]

Pardon the repetition with the previous, but I think I have something to add:

You need to consider the HRSG deaerator like a regular boiler's deaerator. It's an open cycle feedwater heater which brings the feedwater up to saturation before it goes through the feedwater pump. It needs a certain amount of heat to do so.

During startup or other off-design operating conditions (even heavy supplemental firing in some cases) the LP drum will not pick up enough heat from the gases to maintain saturated steam. In that case, the enthalpy in the LP drum will start to drop off. That may be OK, as long as the rate of change isn't too high or the pressure inside the drum doesn't approach atmospheric pressure. You need to have pressure above atmospheric to allow all of the noncondensible gases to be vented. 3 psi is an absolute minimum, but it depends boiler to boiler. You also can't have rapid changes in the deaerator pressure. Flashing or inconsistent oxygen removal are the result.

To make up for any shortfalls in heat to the LP system you bring in a more reliable source of steam - i.e. from the HP steam system (or IP, if you have an IP drum). That's pegging steam. It's not limited to HRSG's. A conventional rankine cycle running off turbine extractions will have pegging steam for startup.

Good luck

Greg