Orifice for dump line to liquid tank 1

[Whatsanolet]

Hi All,

I have a natural gas compressor site where the liquid dump lines from the sumps on natural gas scrubbers are plumbed into an atmospheric liquids holding tank. Some of the lines have restricting orifice plates and others don’t. Are the orifice plates needed/required? I was told they were installed so that if the automatic dump valves fail open and line pressure is sent to the tank, it won’t overpressure the tank. This sounds reasonable to me. If the orifice plates are needed, is there an ASME or API or other industry standard that’s used to size orifice plates?

 

[LittleInch]

Got a diagram or P & ID?

Yes they are normally needed to avoid destroying your tank.

Restriction orifices are strange things and you're best to get the vendor to design it.

 

[GBTorpenhow]

Yes, the orifice plate is sized in conjunction with the tank maximum working pressure and the capacity of the conservation vent on the tank such that with choked flow in the orifice at maximum upstream pressure it will not be possible to overpressure the tank.

Sizing the orifice is based on choked flow, you should be able to find these equations in any good engineering reference manual or handbook. API 520 critical flow relief valve capacity equations are a very close analog as well.

Discharge coefficients are where this can get tricky, but for a flow restriction application discharge coefficient can conservatively be taken as 1.

 

[Whatsanolet]

Got a diagram or P & ID?

Yes they are normally needed to avoid destroying your tank.

Restriction orifices are strange things and you're best to get the vendor to design it.

Thank you.

I’m not sure I can share a P&ID so let’s go with no on that. There is a flange at the spec break where piping design changes from 600 class to 150 class which is where I believe the orifice should go.

Ideally, the vendor would do it but this was done 11 years ago and URS is no longer in business so I don’t think that will be an option for me.

 

[Whatsanolet]

Yes, the orifice plate is sized in conjunction with the tank maximum working pressure and the capacity of the conservation vent on the tank such that with choked flow in the orifice at maximum upstream pressure it will not be possible to overpressure the tank.

Sizing the orifice is based on choked flow, you should be able to find these equations in any good engineering reference manual or handbook. API 520 critical flow relief valve capacity equations are a very close analog as well.

Discharge coefficients are where this can get tricky, but for a flow restriction application discharge coefficient can conservatively be taken as 1.

Thanks for your reply.

It was suggested to use the AGA-3 method to size the orifice to achieve a flow rate less than the tanks flame arrestor relieving capacity.

Is the AGA-3 equation a good approximation for flow rate in this case?

 

[LittleInch]

Most orifices are for much lower and non critical flow. AGA 3 is I think designed for non critical flow and not suitable.

Search on this site and you find things like this

https://www.eng-tips.com/threads/restriction-orifice-specifications.257163/ Basically says these things are created, not made to a specification or design code....

Or try this FAQ. https://www.eng-tips.com/forums/378/faqs/1201

Remember for gas the velocity might be fixed at the sonic velocity, but the mass flow rate changes with upstream pressure.

Also don't forget about Joule Thomson cooling, liquid drop out / hydrates and all sorts of other nasty things which happen in RO's. Plus using them regulary tends to increase the size of the orifice / hole....

 

[GBTorpenhow]

Most orifices are for much lower and non critical flow. AGA 3 is I think designed for non critical flow and not suitable.

^^^
AGA-3 is concerned with metering of flow, different application with a different flow regime and different equations.

 

[Snickster]

The flow through a gas flow restriction orifice at sonic flow conditions is determined by the following ideal gas equation for conditions in the throat of the restriction orifice during flow.

PQ=mRT

Choked sonic flow is the maximum velocity in the orifice throat. This is when the velocity of flow equal speed of sound in the gas. This occurs when P = CPo where C is constant based on ratio of specific heats of gas and is about 0.5 to 0.6 for most gases, Po is the upstream pressure and P is the pressure at the throat where sonic velocity exists. So when the downstream pressure of the orifice is about 1/2 the upstream pressure there is enough energy available for pressure energy to convert to kinetic energy to get the velocity up to sonic velocity in the throat of the orifice. Sonic velocity is the maximum velocity possible in a converging flow path/nozzle and is a phenomenon of nature. To get the flow to supersonic conditions a diverging nozzle/diverging flow path needs to attached to the converging nozzle.

Therefore for the throat of the orifice under sonic flow conditions the ideal gas equation can be written as follows and the mass flowrate at the throat "m" then can be solved for in the following equation:

(C)Po)(144)(Q)=mRT

C is constant determined by the gas ratio of specific heats = (2/k+1)^(k/k-1). For natural gas with k=1.27 then C = 0.527
Po is upstream orifice pressure in psia. This assumes that the pressure upstream is nearly the stagnation pressure and the velocity upstream very small relative to sonic velocity.
Q is actual volumetric flowrate in cubic ft/sec= Velocity ft/sec times Area of orifice bore in ft^2, where velocity is sonic= SQRT(gkRT)
m is mass flowrate in pounds per second (by weight)
R is universal gas constant = 1545/MW, where MW is molecular weight
T is temperature at sonic velocity = (2/(k+1))To where To is upstream temperature at Po pressure
k is ratio of specific heats
g is gravity constant ft/sec^2
d is diameter of port in feet (smallest flow area of the regulator valve)

Substituting the above values into the ideal gas equation:

0.527 Po (144) SQRT((32.2)(k)(1545/MW)(2/k+1)To)(Pi/4)d^2 = m (1545/MW)(2/(k+1))(To)

Solve for "m" mass flowrate in pounds per second and this is your relief flowrate. Note that this is the same equation for flow through a gas relief valve per API 520 when you solve for "m" and simplify a little and don't include all the correction "K" constants in the API equation, since the API equation is based on flow through an almost ideal converging flow of gas through an orifice/nozzle under adiabatic isentropic expansion of the gas from the upstream of the relief valve to the nozzle/orifice bore a sonic flow conditions in the orifice bore. In API 520 all the factors with ratio of specific heat "k" values in the above equation are combined into the "C" factor for a particular gas.

The vent on your tank must be designed to pass this flowrate without exceeding the design pressure of the tank.

An added complexity to your system is that the control dump valve also functions as a restriction orifice when failed wide open. Therefore you must calculate the maximum flowrate based on your dump valve failing wide open. If this is too much flow for your tank vent then you must install the restriction orifice.

To reduce the flow (with the orifice in the line) you must build up a pressure downstream of the dump valve greater than 0.527 times the upstream pressure. This is because the dump valve itself is under sonic critical flow conditions and the bore of the valve acts just like a restriction orifice. The only way to reduce flow is to increase the dump valve downstream pressure above 0.527 times the upstream pressure which will then reduce the flow through the dump valve to below sonic. Even if you induce a pressure across the orifice but still the pressure downstream of the dump valve is at or below 0.527 the upstream pressure, the flow rate through the dump valve will not be reduced. It can only be reduced if the flow through the dump valve is reduced below sonic flow and this can only happen if the downstream pressure is increased above 0.527 P upstream.

So you perform an iteration until a given size orifice will produce an upstream pressure (which is the dump valve downstream pressure) that reduces the flow to the desired value. At this point the pressure drop across the orifice will also likely be sonic and at critical flow with flow and throat conditions determined by the above equation. The above equation is the ideal equation. For a square edge orifice the mass flow calculated would be approximately 0.61 times the ideal mass flow calculated. Although not including the 0.61 factor would just allow for some conservatism.

Attached is a short paper on a simplified calculation I found on the internet for air. Do search for "restriction orifice under critical choked flow" to get more information.

I worked on some compressor stations for URS and installed such orifices at the Napoleonville La. site.