Pressure reduction valve between economiser and a steam drum in HRSG

[Pederator]

Hello. My question is about control / reducing valve between economiser and a drum in HRSG, which is used in every HRSG to avoid steaming of water entering the drum. I need an information if this valve:
- is "open" at nominal conditions (design performance), so there is no pressure reduction, and water enters the drum at the temperature a little bit lower than the saturation temperature for the pressure in the drum as a result of proper dimensioning the economiser, or
- there is a pressure reduction even at nominal conditions

Thanks for reply.

 

[crshears]

Hello Pederator,

'...used in every HRSG...'

I'll be the first to admit I'm still learning, but is this in fact so? It would seem that it depends on how far off-design-case the operating conditions in a given plant design and configuration are expected to get; would such a valve ALWAYS be needed? And if it IS required, is the feedwater admission valve ever used to serve this purpose?

If I'm wrong, someone please tell me, but it would make sense to me [at least in larger plants] to place the feedwater flow control valve between the economizer outlet and the inlet to the steam drum. In plants where the boiler feed pump prime movers can operate at variable speed using, for example, steam turbines, the ST throttle could be used to regulate HRSG drum level in the upper flow ranges with the feedwater control valve wide open so as to pose minimal flow restriction, whereas at lower flows when the turbine-driven BFP has reached its minimum speed the flow control valve would be used to regulate drum level.

The up side of this would be that as the low-flow approach point is reduced, the valve would admirably serve the duty of precluding economizer steaming by maintaining the pressure near the tail end [relative to water flow] of the economizer; the down side would/could be that there might be enuogh flashing of the feedwater to steam as it passes through the valve to cause problems such as vibration or possibly disturbing the smooth flow of water into the downcomers and/or reducing the density difference upon which natural boiler circulation depends.

Just my CDN $0.02 worth...

CR

 

[davefitz]

On most HRSG's, the LP drum is configured with the LP feedwater control valve at the economizer outlet , as described. maybe 50% of IP boiler economizers have this configuration, and maybe 5% of HP economizers. This configuration requires an increase in the design pressure of the economizer tubes and adding releif valves on the economizer inlet header, but it has the advantage of preventing steaming ( and thus prevent 2 phase FAC) in the economizers and also allows use of hotter economizer outlet fluid for use as superheater spray or bypass system spray.

If you use a constant speed boiler feedpump, then the feedwater control valve will throttle from time to time, and thus the pressure in the econmizer can reach pump "dead head" pressure during periods of zero fedwater flow ( as occurs with hi-hi drum water level events).

"Nobody expects the Spanish Inquisition! "

 

[Pederator]

Thanks for explanation, very valuable comments.