PCV and FCV set points and work princilbe valve on cetrifugal pumps ?

[sign680]

PCV, FCV are added to centrifugal pumps for minimum flow incase of blockage in discharge side

The question is how PCV, FCV set points are determined and calculated on centrifugal pumps with fixed RPM and variable RPM

 

[LittleInch]

Not sure why you would have a pressure control but it's the same valve just different control inputs. Min flow comes from the vendor but a good ROT is 30% of the BEP flow.

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Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

For variable speed pumps, it would be better to set up an algorithm in DCS to compute the current value of the SP for min FIC based on the current speed. This would obviously be a must for the larger hp pumps.

Setting up a PIC in min flow service in a fixed speed application would only be possible when both of the following are true:
a) Suction pressure at the source vessel is more or less constant
b) Fluid density is relatively constant.

Else an FIC would be the way to go. Pls note that where (b) is not the case, a diff pressure based FE (orifice plate, venturi etc) is not a good choice either when density compensation is not done for the FIC loop. In such cases, use other FEs' such as vortex meters, ultrasonic etc.

 

[aliansari]

There are three methods used to ensure minimum flow protection in a centrifugal pump

1. Fixed are Restriction Orifice (RO)
2. Direct Acting Pressure Control Loop
2. Reverse Acting Flow Control Loop

1. Fixed Area RO

This is suitable for centrifugal pumps with relatively low flow and low discharge pressure, because high differential across the RO would lead to choked flow through the RO bore, and constant flow through the RO means continuous energy loss.

2. Direct Acting Pressure Control Loop

As the delivery through the pump decreases, pressure increases. Rate of increase of discharge pressure depends on the H-Q curve of the pump, provided by the vendor. Increasing pressure, thus means that the pump is shifting towards its shutoff. looking at the pump curve, minimum flow point with corresponding differential head may be determined. With appropriate safety margins, a setpoint can be given to the pressure controller to open the kickback valve when the pressure approaches towards the setpoint.

This method is suitable for the pumps with steep H-Q curves where drop/rise in pressure is more prominent in a given window of flow change

3. Reverse Acting Flow Control

In this control philosophy, in a situation of zero forward flow, control valve remains open to ensure delivery of minimum flow through the pump. As the flow is taken by the consumer, total flow at the discharge of the pump starts increasing. In response, the control valve start closing so that total flow (forward and recycle) remains equal of above the minimum flow setpoint. The kickback valve closes fully, eventually when forward flow exceeds the setpoint

This scheme is suitable for the pumps with curves where incremental rise in discharge pressure is not significant compared to the magnitude of the flow reduction

 

[georgeverghese]

A third condition is required for when a PIC in min flow servce would be suitable for a centrifugal pump

(c) the Q-h curve at around the location of the min flow should show a fair change in h for a small change in Q. A flat curve, or a near flat curve at around this location indicates the non viability of a min flow PIC.