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Equal Percentage Control Valve for Level Control

Equal Percentage Control Valve for Level Control

Equal Percentage Control Valve for Level Control

Dear All,
In our process plant, there is a horizontal drum containing water. Water level is controlled by a Level Control Valve (LV) on the discharge line of a vertical pump installed in the drum.
Pump data:
Max. Capacity=4.6 m3/h
Head @max. capacity=45 m
Max. operating temp.=145 C

Control valve data:
Valve type and size: 1 1/2" Globe, Equal Percentage
Inlet pressure=5.4 Bar (a)
Outlet pressure=4.7 Bar (a) [constant deltaP]
Liquid flow (kg/h) vs. Cv:
1000-------- 4200-------- 5880
1.383------- 5.817------- 8.154

trim# : Cv 0.81 Cv: 12.9

The problem is:
when the valve opens at 52% or more, pump trips on high flow (red mark ampere). Does it means that the valve is not sized properly or the valve type (equal percentage) is not proper and should we change to a linear type valve? any recommendation?

RE: Equal Percentage Control Valve for Level Control

You've answered you own question I think

Valve CV is too big or pump is too small.

You're not controlling on flow so the valve is responding as it should to maintain level, but your pump can't do what it is being asked to do.

you could restrict the valve opening to less than 52%?

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

RE: Equal Percentage Control Valve for Level Control

Do you have a flowmeter on the pump discharge? If so- set up a cascade loop. Valve controls flow, flow controls level. Limit the output of the flow control loop to the maximum the pump can sustain without tripping.

Otherwise- limit the output of the existing level control loop to 50% as per comments from Littleinch above

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"

RE: Equal Percentage Control Valve for Level Control

The problem you are facing could be either a control problem or a design problem - based on the description provided.

If this is a control problem, you would still be able to operate the system in manual mode without causing the pump to trip. You can verify this by decoupling the controls and operate the control valve in manual mode. If you are able to maintain the required level in the drum without tripping the pump, then you have a control problem. There is a possibility that the controller is not tuned properly, and it results in excessive opening of the control valve based on the signal from the controller. This means that the amount of incremental valve opening greatly overshoots the incremental flow required by the system.

Secondly, it could be a design problem. The way I see it, there is a demand for the pump to deliver certain flow (at a certain point in time), and this amount of flow causes the pump to overload and trip. Now, whether you use EQ% or Linear valve, it does not matter - the pump will always trip once when it reaches that flow. This can easily be confirmed if you run the system in manual mode, as explained in the previous paragraph. If the pump keeps tripping while you are attempting to maintain the level in the drum, you have a design problem (i.e. undersized pump).

Some good reading on the subject:

Process Engineer, MSChE

RE: Equal Percentage Control Valve for Level Control

Assuming your liquid has a SG ~1, your flow rate at some point between Cv of 5.817 and 8.l54 ends up exceeding your pump's stated max capacity (4.6 m3/hr = 4600 L/hr ~ 4600 kg/hr at SG 1), and you should trip. If the response is linear, this should be ~ 6.4 Cv value. Check flow rate at max Cv and resize pump accordingly, or follow some of the above advice and see if you really need that much capacity. Have you done a check on inflow into the horizontal drum to see if your total influx is => 4.6 m3/hr? If so, then you definitely need a larger pump (you will never be able to keep up).


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