Gas Separator Liquid Dump Orifice 1

[1969grad]

It is common practice to use a flow limiting orifice downstream of the level control valve on gas separators.

The purpose is to limit the gas flow rate in the event that the level control valve sticks open and blows gas.

What is the rational for sizing this orifice as far as flow rate.

 

[mbeychok]

1969grad:

I worked in petroleum refinery operations and in refinery process design for almost 40 years ... and I never heard of providing restriction orifices downstream of the liquid level controller on a gas/liquid separator drum. Therefore, I find it difficult to agree with your statement that it is "common practice".

If the level control valve sticks open and your low level alarms and/or low level shutdowns all fail to function, then I don't see how such a restriction orifice could do any good other than slightly prolonging the inevitable blowing of gas.

Milton Beychok
(Contact me at www.air-dispersion.com)

 

[MortenA]

mbeychok

I have seen it - it can be used if the PSV on your downstream equipment has a limited capacity and you want to limit gas blow-by.

In this case the sizing case should be the capacity of the PSV at relieving conditions (note this may be higher than the relief case used for design of teh PSV since a larger orifice is chosen that what is calcuated).

Best regards

Morten

 

[1969grad]

Additional information:

The application is in natural gas compressor and metering stations. The liquid off of the separator goes to an atmospheric storage tank good for less than 1 psig so the fear is that the level control valve will stick open because of dirt, etc. and you will hit the tank with high pressure gas.

Low level switches are not used in these types of applications.

"Common" may not be the correct word but it is often found in this application.

 

[jay165]

1969:

MortenA is correct. I've also seen this situation on a blowdown line when the limiting factor is the flare capacity.

 

[hiebs]

1969grad,

I have seen restriction orifices used in this manner to prevent overpressure of the atmospheric storage tank in a gas blow through case, where the dump valve fails wide open. A restriction orifice should only be used if you have a storage tank where your vent, thief hatch or relief valve cannot be changed to match the fail open case on the dump valve.

However, I have also seen these same restriction orifices removed by operators (without an engineering review) because they also restrict the liquid flow and can cause problems with liquid dumping from the separator (this will depend somewhat on the service your separator is in).

Regards,
Hiebs

 

[tickle]

There will be blowing of gas, but the orifice will reduce the flowrate of the blowing gas, as opposed to the low pressure liquid storage tank being the limiting device.

 

[Samiran]

The downstream equipment is obviously designed for lower pressure say typically 150#. Once the blowby occurs the downstream pressure would increase which may exceed the design pressures. The orifice would restrict the flow of gas to the low pressure system such that downstream pressure do not excurse beyond limit during relieving condition. However, the ESD system MUST close within a certain time. The loss in level must actuate an ESD. The orifice should only cover during the time the ESD is closing typically 90-150 secs.