pump min. flow

[Ahmedlng]

what are the basis on which the engineer to decide to select flow control or pressure control for the pump circulation valve (minimum flow) ?

 

[georgeverghese]

Pressure control can be used only when both upstream and downstream pressure at the min flow PCV is constant for the entire operating envelope. Else, select a flow control loop.
The frictional pressure drop leading up to the PCV or FCV and the frictional drop in the downstream line must be only a small fraction of the max pressure drop at the PCV or FCV. In either case, check if the fluid flashes at the vena contracta in the control valve, and compensate as required. Avoid oversizing to enable high turndown.

 

[shvet]

what are the basis on which the engineer to decide to select flow control or pressure control for the pump circulation valve (minimum flow) ?

1/ Pressure control is not suitable for pump having flat performance curve at min flow region as discharge pressure is not affected by change of flowrate.
2/ Most of pumps have continuously declined performance curve.

But some pumps have that bulged.

Pressure control is not suitable for bulged performance curve as 1 pressure corresponds with 2 flowrates.
3/ Pressure control for min flow is not suitable when pump capacity is controlled by a variable speed drive.
4/ Pressure control for min flow should be used with precautions when there is a variety of densities as discharge pressure is affected by density.
5/ Pressure control should be used with precautions in case of impeller deterioration, e.g. when regular inspections of pump are not planned.
6/ In case of a pump has NPSHA<NPSHR in min flow region excessive cavitation leads to fluctuations of discharge pressure and disturbs pressure measuring.

Pressure control can be used only when both upstream and downstream pressure at the min flow PCV is constant for the entire operating envelope.

1/ "Upstream and downstream" - did you mean suction vessel and discharge vessel?
2/ Why not use a pressure drop sensor for "pressure control"? Pressure rise across a pump is affected only by density and wear of impeller / wear ring.

P.S.
1/ See link

https://www.cheresources.com/invisi...e-process-flow-schemes-for-centrifugal-pumps/

2/ Note that "flow control" is referred to 2 options:
- flow control by a flow sensor, e.g. by a vortex flowmeter
- flow control by a self-controlling device like in links below

http://www.schroedahl.de/en/products/pump-protection-valves/

https://www.agivalves.com/automatic-recirculation-valves/

Mentioned options are different but sometimes confused. Make sure you have distinct understanding what FCV means in the particular case.

 

[georgeverghese]

Upstream and downstream with reference to the control valve.

Agreed, PCV cannot be used for flat profile Q-h pump curves, and when there is significant density variation in the pumped fluid also.

 

[LittleInch]

Also consider Automatic Recirculation Valves (ARV).

E.g.

https://blackhall.co.uk/arv.html

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

 

[Ahmedlng]

thanks to all for the valuable information.

 

[georgeverghese]

Correction to my first post:
"Pressure control can be used only when both upstream and downstream pressure at the min flow PCV is constant for the entire operating envelope."

Amendment:
Pressure control can be used only when downstream pressure at the min flow PCV is constant for the entire operating envelope, and where the pressure measurement upstream of the PCV uniquely indicates a single operating point when the pump is at min flow. Cases where it is not possible to identify a unique operating point at min flow are (a) pump Q-h curve is relatively flat in the region of pump min flow (b) source vessel pressure fluctuates (c) source vessel static head variation is a significant contributor to pump developed head

 

[shvet]

Pressure control can be used only when downstream pressure at the min flow PCV is constant for the entire operating envelope, and where the pressure measurement upstream of the PCV uniquely indicates a single operating point when the pump is at min flow. Cases where it is not possible to identify a unique operating point at min flow are (a) pump Q-h curve is relatively flat in the region of pump min flow (b) source vessel pressure fluctuates (c) source vessel static head variation is a significant contributor to pump developed head

If "pressure control" is realized by a pressure sensor at pump discharge it will be affected by fluctuations of upstream pressure, e.g. at outlet nozzle of suction vessel. No doubts.
Why "pressure control" refers only to control by a pressure sensor? Lets look in a handbook.

(Source: Karassik's Pump Handbook 2008)

You can see that author does not mention a pressure sensor. Only a dif pressure sensor is considered because it is affected by (a) actual head of pump and (b) liquid density and is not affected by (c) pump upstream pressure. The same for spillback control valve - it is affected only by pump head and fluid properties.

(Sure I know that cavitation of spillback control valve is affected by suction pressure but let me neglect this fact as cavitaion can be compensated by valve design.)

 

[georgeverghese]

There are only limited situations where a PIC - PCV will suffice, and these do exist in operating plants. Agreed, in most cases, an FIC - FCV or dPIC-dPCV is the only way to get a min flow loop to work as intended. But I wouldnt recommend an FT on pump suction, better for the FE to be on pump discharge, unless the fluid is well removed ( ie. subcooled) from bubble point conditions.