Flash Tank for Blowdown Recovery Desgn 1

[Gibby53]

First time for me!
Need some help or advice from the groups experience here.

We are installing a blowdown recovery system. A maximum of 60 gpm of saturated liquid at 1000 psia (boiler conditions) will be piped to a new flash tank (sized appropriately by flask tank manufacturer). The flash tank will be vented to a power plant feedwater heater (shell side), which operates at 70 psia. The flash tank drains will be piped to the same feedwater heater (bottom connection), which will be in the sub-cooled drains section outside the integral drains cooler. The design engineer intends for the flash tank drain line to have a manual globe valve the operators will adjust periodically as needed to maintain flash tank liquid level (maybe once per shift, per day, per week?). He intends to have the bottom of the flash tank at the same elevation as the bottom of the FW heater shell bottom. The flash tank vent is to be sized so that the FW heater will essentially establish the pressure in the flash tank (70 psia). Doing the math, approximately 30% of the blowdown entering the flash tank will flash to steam, therefore 70% will be saturated liquid to be drained.

My questions are;
1) Will there be sufficient driving force to send the drains flow from the flash tank to the heater.
2) Shouldn’t the flash tank utilize an automatic level control system to operate the drain control valve? (The flash tank is relatively small. I see our operators going a little crazy constantly tweaking the drain valve while trying to maintain flash tank level in a system that isn’t exactly steady state.)
3) What would happen if the flash tank level isn’t maintained and the drains blows out and flash steam enters the FW heater below the heater liquid level? Might this result in a water hammer event … seen primarily by the flash tank drain piping?

 

[s0eebuch]

Gibby53,

I did some work with a boiler manfacturer. Is the boiler blowdown coming from the intermittent blow (mud drum) or the continuous blow? Usually, that and the end use of the 'vented' steam from the blowdown tank normally determine weither you need a blowdown separator or a flash tank. It sounds like this tank has no cooling water line at the discharge. And, it also sounds like the design engineer is trying to use the heat exchange in lieu of the cooling water line, while at the same time recovering some heat to up his efficiency. Go to Penn Separator's website and call their number. The engineers that work there are very helpful in answering questions.

As far as your questions, don't know offhand about (1), but I think for (2), there should be an overflow drain on that tank or it may fill up and send saturated water to the steam side of your heat exchanger. You are right in that a globe valve for water level control is not easy. I bet the feedwater control to your boiler is probably a 2- or 3-element feedwater control with level transmitters and a pneumatic feedwater control valve. Ideally, this is the setup you'd want on the tank, but overkill. An overflow drain should be there. For question (3), it depends on the piping configuration, velocity, and wheither this piping terminates at an open drain (ie atmospheric conditions). Hope some of this helps!

 

[Gibby53]

Thanks sOeebuch!

Just to clarify! The steam generator blowdown is continuous. No cooling water line at discharge. Both the flash steam and flash tank condensate drains will be piped to the same heat exchanger (feedwater heater). The flash tank steam introduced to the heat exchanger through the flash tank vent line (no valves) and the drains introduced through a connection at the bottom of the heat exchanger (below water) while controlling flash tank level via a manual globe valve. Just as you stated, the design engineer thinks a pneumatic level control system and its cost for the flash tank is overkill. I think it will be necessary.
Thanks for the tip on Penn Separator’s.