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# Line Sizing for Gas with Control Valve

## Line Sizing for Gas with Control Valve

(OP)
Gas is being sent from Equipment 1 (Separator) to Equipment 2 (Compressor)
I have a pressure control valve in b/w Eqpment 1 and 2
For line sizing, I have straight line length as 50 feet
I have two ball valves and a control valve (globe valve)
Eqment 1 pressure is 50 psig and control valve is sized for 5 psi pressure drop
Now for line sizing based on equivalent length should I consider the pressure control valve as a globe valve and take equivalent length? OR should I consider equivalent length and also add 5 psi to the overall pressure drop for calculation? What is the suitable approach for line sizing?
For calculating pr. Drop per 100 ft should the pr. Drop across control valve be considered (5 psi)
Sketch Attached

### RE: Line Sizing for Gas with Control Valve

It's a bit confusion as described. The hydraulic pressure drop of the piping is simply:
1. the equivalent length of piping, including pipe, fittings and ball valves, etc.
2. control valve

### RE: Line Sizing for Gas with Control Valve

Where's the control valve?

Anyway, look at the control valve as a piece of equipment.

Do whatever sizing you want from ept1 to the valve and then from the valve to eqpt 2.

When you say sized for 5 psi, do you mean 5 psi is the pressure drop when flowing?

Seems quite a low pressure drop, but that's a different story.

I guess you line sizing is worse at the downstream side as the density is lower and hence velocities higher, but 5 psi ins 50 psi is nothing.

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

### RE: Line Sizing for Gas with Control Valve

It is very unusual to size a control valve for a fixed pressure drop. A control valve by definition is a modulating device and the pressure drop will change in accordance with the current process requirements. You need to look at the range of flow rates and downstream pressures required and investigate how the line size and valve size interact with each other. You cannot arbitrarily size the control valve and then independently size the line.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

### RE: Line Sizing for Gas with Control Valve

I've sized control valves off separators for fixed DP which varies due to changes in flow.

This was for a test unit where the liquid and gas was measured then the liquid injected back into the flow d/s the control valve. I needed a dP to drive the level control valve and give some reason for the liquid to go back into the combined line.

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

### RE: Line Sizing for Gas with Control Valve

Zamakaze,

What is the max./design flow rate, gas/MW/density, and CV pressure set point? And, where is the CV in that 50 feet of pipe length? 5/45, 25/25, 45/5? Then everyone will be on the same page as you.

Good Luck,
Latexman

### RE: Line Sizing for Gas with Control Valve

In this type of line dont size for dP - you can e.g. size for velocity (noise/erosion)

The bulk of your dP will be over the control valve so sizing the piping for dP does not make any sense - velocity (either to avoid erosion or noise) makes more sense.

Best regards, Morten

### RE: Line Sizing for Gas with Control Valve

Just calculate the pressure drop in the line with a control valve...

horacio

### RE: Line Sizing for Gas with Control Valve

I dont agree.

You should allow for pressure drops that include the drops across the control valves when they are at their operating position.

Control valves typically operate most effectively between 40 to 70% open. Thus your maximum pressure drop across each control valves during normal operation should be when they are 40% open.

If you had a max allowable pressure drop between equip#1 and #2 of 30 bars and you sized your pipe only for that pressure drop, then when you put your valves in operating position at 40-70% open, you would have a pressure drop between Equip#1 and #2 of much more than your max 30 bars. Make your pipe big enough to handle the flow for a pressure drop of the pipe + all control valves at 40% open.

That will ensure that you can operate equipment#2 at low pressure and that your control valves will function accurately and still within their best operating range at hi pressure, when they will be 70% open.

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

### RE: Line Sizing for Gas with Control Valve

(OP)
@latexman - THe control valve is set to maintain 45 psig at its downstream.

Would it be suitable to carry out line sizing up to the inlet of the control valve, then just take the control valve pr. drop and carry out line sizing at the outlet of control valve?

The flow is ~ 30 MMSCFD for gas and MW is around 20 lb/lbmol

The sizing of control valve to be done for Max = 30 * 1.1 = 33 MMSCFD, normal = 30 MMSCFD and turndown case / min flow of 18 MMSCFD

control valve is located at 40 ft from equipment -1

### RE: Line Sizing for Gas with Control Valve

(OP)
@mortena - what is the general criteria for erosional velocity for single-phase gas? I have not come across any
I know API 14E has max gas velocity of 60 ft/s
NORSOK standard has max gas velocity of v = 175 x (1 / density)^0.43
where,
v = max. velocity of gas to avoid noise (m/s)
ρ = density of gas (kg/m3

### RE: Line Sizing for Gas with Control Valve

(OP)
@katmar - in this case the control valve is supposed to maintain 45 psig at its downstream
since equipment - 1 pressure is 50 psig, it is assumed that for the sizing of control valve 50 psig is upstream pressure, pr drop = 5 psi, downstream pressure = psig
The sizing of control valve to be done for Max = 30 * 1.1 = 33 MMSCFD, normal = 30 MMSCFD and turndown case / min flow of 18 MMSCFD

### RE: Line Sizing for Gas with Control Valve

To size a control valve you need the maximum flow rate of the pipe, the maximum operating pressure upstream of the valve and the lowest operating pressure downstream of the valve.

You may have several cases, each with their own different flow, up/s pressure and dn/s pressure conditions. In that case you need to ensure that the valve works for all cases.

Statements above are the result of works performed solely by my AI providers.
I take no responsibility for any damages or injuries of any kind that may result.

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