Steam Power Plant- Heater drip pump recirculate to Low Pressure Feedwater heater - increasing flow

[vaavas]

Hello,
Do you guys know if there are any concerns/issues that I should look into in regard to this:

Increasing the amount of water that's being recirculated back to a LP Feedwater heater?

This is for an existing heater drip pump (HDP). The existing HDP has a 1" recirculation line tapped from the discharge piping that drains back to a low pressure feedwater heater (lp fwh). The existing 1" recirc line connects to the lp fwh with an existing nozzle that's on the FWH.

The HDP is having to be continually rebuilt due to times where the HDP goes past its Min flow rating. The plan is to
-connect a flow meter on the discharge piping side of the HDP
-resize the recirculation line with a control valve.
-connect the flow meter with the control valve so that the HDP will always be able to discharge its rated minimum flow.
-Resize the existing FWH nozzle to match the new recirculation line pipe size.

Thanks for taking the time to read this.

 

[mk3223]

To be clear, your existing HDP couldn't flow above the min flow rate due to the restriction of the existing 1" recirculation pipe. Is it correct?
It seems your proposed plan should work. The flow meter at the pump discharge can be used to control the FCV at the recirculation line. And, the flow meter at the recirculation line is a redundant meter.

 

[LittleInch]

Depending on how these units are connected (a sketch would help) in the event of no or very little forward flow, the temperature of the water circulating around could increase quite quickly if the water from the HDP re-circulation line finds its way back into the inlet of the HDP.

If your re-circulation flow is low then this might not happen at the moment, but I can't follow the flow path from your description.

Also what impact will a higher return flow have on the LP FWH?

You need to work out the anticipated pressures and temperatures for a variety of cases.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[vaavas]

I have details of the system. Direct components are involved are FWH73, Drip Pot, Htr No 3 drip pump, condenser.

 

[EdStainless]

I have been in that plant, condenser re-tube job I believe.

So #3 is the first of the LP heaters, and this line is recirc.
What happens to the heat balance if you recirc less? Should this pump be smaller?It sounds like you have been recircing less water for a long time. If you make the pump smaller to match you actual flows would that work?


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[vaavas]

Thanks everyone for the responses. I appreciate the time taken for all the feedback.

mk3223: you are correct, the existing 1" recirculation pipe is not providing enough flow for the pump to reach its minimum flow requirement.

LittleInch: I attached a sketch for your reference. The recirculation water is being discharged to FWH 73. The water is mixed with FWH 73 condensate, FWH 74 drain, condensate from steam traps and reboiler drains. I am not sure where to look up the impacts of changing the return flow on the LP FWH. Im thinking the FWH data sheet that lists the steam side conditions of the drips and drains out lbs/hr numbers.

EdStainless: FWH 73 is the 3rd low pressure fwh before it goes to the boiler feed pumps (hotwell- condensate pumps - fwh 71 - 72 - 73 - boiler feed pump - 74 - 75 - boiler). I need to look at the heatbalance, thanks, that's something I need to consider. I think the problem is the unit is starting to be run at very low loads compared to 10 years ago (not originally designed to operate like this). this is due to other energy sources emerging (photovoltaic, wind, biodiesel, diesel) instead of burning low sulfur fuel oil (these units). But since photovoltaic and wind aren't reliable, sometimes these units are required to operate at the designed capacity.

I hope understood and answered everyone.

 

[LittleInch]

I'm glad you can follow that P&ID....

Anyway I think you've picked up the point which is just check to see what happens if your re-circ line starts emitting more water...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.