Relief load calculation 2

[sentrifice]

I am trying to determine the relief load required for overpressure protection of a natural gas system.

The system I am analyzing takes pipeline natural gas (800 psig), and uses a pressure reducing valve to lower the pressure to the equipment (400 psig). There is a safety valve on the downstream side of the pressure reducing valve to protect the equipment in the event that the pressure reducing valve fails. In this event it is assumed the equipment is off and therefore blocked.

However, I am running into trouble determining the mass/volume flow that the safety valve will need to be sized for. My understanding is that the safety valve will need to sized to discharge the maximum possible flow of the source (the pipeline), but there's no way to determine the maximum flow of the pipeline since the flow capacity could be as large as the entire pipeline network.

Thanks for any input.

 

[georgeverghese]

Pressure relief load will depend on what type of overpressure controls and protection devices and loops are installed in this location. Post us a PID of this pressure reduction station. Also state what the maximum possible normal operating pressure could be upstream of this pressure reducing valve.

Is this an offsite located pressure regulating station ( ie well away from main plant and plant operators) that is designed to B31.8 Code?

 

[Snickster]

Typically the relief flow would be based on pressure regulator failing open. A conservative approach would consider the bore (smallest diameter of regulator valve) an ideal orifice at critical sonic flow conditions. This would be the case if the pressure downstream is about 0.55 times the pressure upstream for natural gas so that flow reaches sonic in the throat, which is basically the conditions of your system.

In this case the flowrate can be determined considering there is sonic pressure at the bore of the valve at sonic velocity and sonic temperature using the ideal gas equation:

PQ=mRT

(0.55)Po)(144)(Q)=mRT

Po is upstream pressure in psia (use maximum possible if greater than 814.7 psia)
Q is actual volumetric flowrate in cubic ft/sec= Velocity ft/sec times Area of bore 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)

0.55 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.

 

[sentrifice]

Typically the relief flow would be based on pressure regulator failing open. A conservative approach would consider the bore (smallest diameter of regulator valve) an ideal orifice at critical sonic flow conditions. This would be the case if the pressure downstream is about 0.55 times the pressure upstream for natural gas so that flow reaches sonic in the throat, which is basically the conditions of your system.

In this case the flowrate can be determined considering there is sonic pressure at the bore of the valve at sonic velocity and sonic temperature using the ideal gas equation:

PQ=mRT

(0.55)Po)(144)(Q)=mRT

Po is upstream pressure in psia (use maximum possible if greater than 814.7 psia)
Q is actual volumetric flowrate in cubic ft/sec= Velocity ft/sec times Area of bore 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)

0.55 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.

So basically choked flow at the open regulator? I hadn't even thought of that! Makes sense though. Thanks for the input.

 

[pierreick]

Hi,
Consider this resource.
The first thing to acknowledge is the scenario to size the valve.

Pierre

 

[LittleInch]

I am trying to determine the relief load required for overpressure protection of a natural gas system.

The system I am analyzing takes pipeline natural gas (800 psig), and uses a pressure reducing valve to lower the pressure to the equipment (400 psig). There is a safety valve on the downstream side of the pressure reducing valve to protect the equipment in the event that the pressure reducing valve fails. In this event it is assumed the equipment is off and therefore blocked.

However, I am running into trouble determining the mass/volume flow that the safety valve will need to be sized for. My understanding is that the safety valve will need to sized to discharge the maximum possible flow of the source (the pipeline), but there's no way to determine the maximum flow of the pipeline since the flow capacity could be as large as the entire pipeline network.

Thanks for any input.

Sentrifice,

To be honest here, this is very basic process safety design stuff here and it is rather worrying that you're designing something without knowing this or having others around you who do.

But the answers above are correct. You take max flow when the valve is fully open (you can normally find CV ont he valve data sheet for 100%) and use the downstream pressure as the design pressure of your downstream system or whatever your relief valve is set at if lower, plus maximum upstream pressure from the pipeline.

This may need more than one valve to get the venting rate right and it can be very large flow.

 

[sentrifice]

Sentrifice,

To be honest here, this is very basic process safety design stuff here and it is rather worrying that you're designing something without knowing this or having others around you who do.

But the answers above are correct. You take max flow when the valve is fully open (you can normally find CV ont he valve data sheet for 100%) and use the downstream pressure as the design pressure of your downstream system or whatever your relief valve is set at if lower, plus maximum upstream pressure from the pipeline.

This may need more than one valve to get the venting rate right and it can be very large flow.

Compressible flow is something that I am admittedly new at.

But everyone was new at some point. Senior engineers love to wave the red flag when young engineers are given responsibility but also roll their eyes and curse this generation when they don't want to dive in and figure things out.

To ease your mind, there will be a comprehensive review before it's all done.

 

[LittleInch]

I have no problem in people finding things out, but we do get some rather worrying questions here sometimes and asking for and getting support locally is much better than random old engineers like me.... We all needed to pick things up as we went along, but knowing when you don't know something is more important than thinking you do so fair enough for asking.

you really should have been given some of this information by the process engineer though.