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How to design pump minimum flow line by Relief valve?

How to design pump minimum flow line by Relief valve?

Sorry, that my English language is not good but I will try.
In combine cycle power plant system, the condensate make-up system shall be provided to condenser to accommodate for HRSG(Boiler)blowdown and others cycle loss.

Please see the attached file for system diagram.

Conceptual design
Normal make-up: Flow 10 m3/h
The make-up water during normal make-up shall be provided to condenser from condensate storage tank using difference between condenser vacuum (0.1 bara) and water level of condensate storage tank by gravity.
Condensate transfer pump (CTP) will be stand-by in normal make-up operation.

Emergency make-up: Flow 290 m3/h (approximate plant start-up 1 time/year)
I designed CTP: Capacity = 320 m3/h, TDH = 50 m3/h.
In this stage, CTP shall be started-up to supply water to condenser for replacing the much blowdown in power plant start-up.

Condensate storage tank capacity: 500 m3
Total make-up quantity during start-up: 400 m3/start-up

Sequence of power plant operation

Start-up operation --> Normal operation --> Shut down

After CTP supply make-up water (400m3) in plant start-up via condenser emergency makeup line by flow 290 m3/h. CTP would need to continue operation after start-up to supply condenser normal make-up water, until condensate storage tank water level requires for gravity make-up is reached. So CTP discharge flow excess to condenser make-up requirement and may be operate lower than CTP’s minimum flow need to be recirculated. This would require a minimum flow line.
The condensate storage tank make-up system is not enough capacity to supply water for keeping water level instantly.

In my opinion, I would like to design this line by using the relief valve and multi-stage orifice. Base on the consideration below.

Continuous bypass flow: CTP capacity will be increased to always keep the minimum Flow and CTP may be operate 1 time /year.

Automatic bypass with control valve: High cost (Electrical & Control instrument)and CTP may be operate 1 time /year.

Automatic recirculation valve(Yarway): High cost when we compare with relief valve.

I do not have experience about the pump minimum flow design by the relief valve and muti-stage orifice because I  just graduated for 1 year ago.

My Question
1. Please advise that how to design the pump minimum flow line
(relief valve and multi-stage orifice).                                                               

CTP Shut of Pressure = 70 meter of water
Operating pressure = 50(TDH) + 8 (Static head of tank) = 58 meter of water
Pressure @200 m3/h (60% of BEP) = 75 meter of water (include tank static head)
Pressure @150 m3/h (manufacturer minimum flow) = 76 meter of water (include tank static head)

My design concept.
I can ensure the constant flow of normal make-up by control valve operation and emergency make-up flow is greater than the CTP minimum flow.

I will design that CTP operate at 200 m3/h (normal make-up 10 m3/h, reicir. flow 190 m3/h) and set the relief valve open at pressure = 75 meter of water.

To adjust the recirculation flow=190 m3/h, I will increase the pressure loss of recirculation line correspond with the inlet pressure = 75 meter of water by using the fix multi-stage orifice.

CTP will be operated with bypass flow via recirculation line for 50 hour because the make-up water flow very small (approx. 21 m3/h)

My Question
Base on your experience
2. Can you accept my design concept?

3. What’s the acceptation velocity in recirculation line for pipe diameter design?(with consider cavitaiton)

4. How to calculation/selection about the orifice and multi-stage orifice?

5. Base on my design concept, Is the variable orifice necessary to control constant flow of this line? In case, the inlet pressure is not 75 meter of water.

6. Please advise to design this line by using Automatic recirculation valve (Yarway).

RE: How to design pump minimum flow line by Relief valve?

For the minimum amount of time that the pump is operating, it would probably be a better to design to omit the relief valve entirely and use a constant recirculation line with an appropriately sized orifice.  This arrangement would require the pump to be sized accordingly which may be a problem if you have already purchased the pump.  

Also, I'm surprised that you would need a multistage orifice block in conjunction with the relief valve.  I would expect high built-up backpressure at the relief valve would reduce the capacity quite a bit requiring a larger valve.  If a constant recirc cannot be accomodated and you wish to control based on a pressure signal, I think that a differential pressure switch and an on-off actuated gate/ball/butterfly valve w/ orifice or modulating control valve would be a better option.


RE: How to design pump minimum flow line by Relief valve?

Thank you for your comment.
I’m considering that this minimum line design by continuous bypass line with a fixed orifice.

Please refer to the attached file. (Attachment-1)

Continuous bypass (not require air or electricity) may be lower cost method of protecting a pump when I compare with the differential pressure switch and an on-off actuated gate valve.

Then, I design the capacity of CTP = 430 m3/h and TDH=50 meter of water
The maximum process flow: 290 m3/h
I estimate the minimum flow required of CTP(430m3/h)= 100 m3/h (~20 % of BEP)
CTP capacity design: 290m3/h + 100m3/h = 390 m3/h
Specific margin: 10%
CTP capacity design (include margin) = 390 m3/h x 1.1 = 429m3/h -> 430 m3/h

Base on my understanding below:
Continuous bypass system provide continuous flow whenever the pump in running, regardless the process demand. And I designed the flow rate of recirculation line is 100 m3/h. (equal to minimum flow requirement)

Recirculation line pipe diameter design
Nominal diameter = DN100 (4 inch)
Velocity in pipe (@100m3/h) = approx. 3.72 m/s

From above calculation, this conceptual design affect to the increment of CTP capacity.
I think, there is the possibility of decrease the above design flow (100m3/h) in recirculation line for reducing the capacity of CTP.

In my opinion, the way to decrease capacity of CTP
1.At CTP flow = 0 m3/h, I will design flow to orifice = 100 m3/h. The orifice  is designed by inlet pressure = 78 meter @ (CTP flow=100m3)
2.At CTP normal operation, the operating pressure is ~58 meter of water. And water flow in recir. line will be less than 100 m3/h (= x m3/h)
3.I will design the capacity of CTP = 290 m3/h + x m3/h.

My question
1.How to design the flow rate of recirculation line (continuous bypass) base on your experience?


RE: How to design pump minimum flow line by Relief valve?

The orifice should be designed to pass the minimum stable flow from the pump at the corresponding pump head.  When normally operating and transferring demin to the condenser hotwell, the actual flow through the orifice will be a little less as you noted.


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