Level Control Valve 3-in linear or 4-in equal % on 4-in pipework

[w3285]

We currently have a 2-in level control valve installed on the test separator water outlet going to the Hydrocyclone unit. This valve currently can only flows about 8000 bwpd @ 1 bar dP. With another well coming on line and by lowering the separator pressure down to acceptable limit, we anticipate at peak production to flow between 10,000 to 14,000 bwpd.

Process Condition:
Fluid: Produced water
SG: 0.993
T degC: 45
Inlet Pressure: 9.5 bar
Pressure drop: 0.5 bar (should be constant for a long period)

3-in linear, max Cv 148
Flowrate m3/hr: 8 (min) 8 % open Cv 13; 60 (norm) 55% open Cv 98; 66 (max) 62% open Cv 108

4-in equal %, max Cv 236
Flowrate m3/hr: 8 (min) 5% open Cv 13; 60 (norm) 37% open Cv 98; 113 (max) 71% open Cv 184

My first thought is to use the 3-in linear due to constant pressure drop however, this limits us to max of 12,000 bwpd. Preferably I would like to use the 4-in =% valve but not too sure if this can lead to unstable control and sluggish controller performance. The pipework size is 4-in, ANSI 150#. From a control point of view, will it be advisable to use the 3-in linear or the 4-in equal pct for the above. Any suggestions will be very much appreciated.

 

[Fog1]

Where is the valve physically located, above the level or below the level. If below whats the difference in heights between the surface of the water and the valve.

Fog Jones

 

[TD2K]

If you have an inlet pressure of 9.5 bar and are only allowing 0.5 bar for the valve, I hope you have checked the hydraulics in the piping because the problem might be with your piping system. It's not clear if this is a design number or what you have calculated is remaining for the valve after doing your dydraulics.

At 14,000 BPD, your velocity in a 4" pipe is going to be in the order of 10 ft/sec, you are going to take a lot of pressure drop if your piping lengths are long.

The difference in operating pressuress between the source and receiver equipment, less the dP through your piping to and from the valve will set the inlet and outlet pressures for the control valve. If you get into a position where the piping is taking the majority of the pressure loss, a bigger valve won't help you as you don't have sufficient dP to move the flow through the piping.

Let me give you an example.

Source vessel at 10 barg

dP drop to control valve, insignficant

dP through line downstream of valve, 3 bar

Receiver vessel, 3 bar.

The dp for the control valve is therefore 4 bar. It's important to realize that a control valve in itself does not take a dP, the system imposes the dP the valve must take.

Let's say you want to double the flow through this system. You want the vessels' pressures to remain the same. The dP through the piping as a result of the flow increase increases by a factor of 4 to 12 bar.

BUT, you only have 7 bar available between the two vessels. No matter how big of a control valve you put in for this case, you can't achieve a 2x flow rate. Now, this may not be your case but I've seen this where people have said 'oh, we'll increase production by putting in a bigger control valve' because it is wide open when the hydraulics through the rest of the system is the restriction, increasing the control valve dP does nothing. This might not be your problem since you have a 2" valve in a 4" line but I would want to do the hydraulics, if you haven't already done them.

 

[TD2K]

increasing the control valve dP does nothing

should have been

increasing the control valve size does nothing