Natural gas line sizing with regulator

[Bribyk]

I'm working on sizing a natural gas line for engine fuel supply but am unsure how to include the line pressure regulator (Fisher 299H) in my flow/p-drop calcs. The regulator manufacturer doesn't supply droop (or pressure drop) numbers for the regulator, only rated capacity figures. The rated capacity is sufficient for the required flow rate and my line size (with fittings, etc...) is large enough (using Q =K1*D^2.623*((P1^2-P2^2)*Y/(Cr*L*F))^.541). I know the regulator has a set pressure drop (which is why it's there) but I need to know the flow-related pressure drop so I can include it with my other inline equipment. I can't find any info relating to gas pipeline calculations with a regulator.
The inline sequence is: OBV->OBV->filter->OBV->regulator->OBV->mass flowmeter->OBV->hose->engine integral regulator->carb
where: OBV = open ball valve, and
tees and elbows are interspersed throughout.

 

[Bribyk]

I did manage to find flow coefficients for this regulator. I think I've got all I need now.

 

[Bribyk]

Or not... This regulator has internal registration and, thus, dowstream pressure increases with a flowrate increase. The manufacturer says that the flow coefficient formulas can not be used. Can I just assume negligible pressure drop then?

 

[Ashereng]

The outlet pressure of the pressure regulator is your set pressure, regardless of inlet pressure (within the capaility of the pressure regulator).

Use that as the pressure out of the regulator.



"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[Bribyk]

We have 80 psi supply pressure and some of the engines require 60 psi to their regulator. I have no intermediate values for the regulator flow capacity. At 80 psi inlet and 60 psi outlet the regulator is rated for 42,160 scfh (w/ a 3/4" orifice) and we need 32,000. The actual regulator inlet pressure will be less than 80 psi due to the pipe and equipment losses before it, same for the engine inlet. If I have 78 psi to the regulator inlet I don't know my capacity. I'm probably going to have to get ahold of the manufacturer.

 

[Ashereng]

I thought you are looking for the pressure drop across your regulator for your line loss calcs. If this is the case, the outlet pressure at your regulator equals the setpoint of the regulator, regardless of flow (the pressure regulator controls only pressure).

If you are looking to control flow, you need a flow control valve.

Hope this answers some of your question.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[BigInch]

Modeling all types of control valves (a regulator being hydraulicly equivalent to a control valve with a "settable" outlet pressure) have a Q to dP relationship relating flow across the valve to the current differential pressure. Flow to differential pressure is related by the valve's Cv coefficient. Furthermore, Cv changes with the percent open of the valve, so to obtain Q at a certain dP, you must additionally relate Cv to the regulator's position (typ stem to seat distance as a %Open or % of full actuaor travel). Now when you give this valve a set downstream pressure, the program will calculate the flow at the current dP and a new flowrate. If that dP gives a pressure different than the outlet pressure setting, a new position will be calculated, a new flowrate and checked again. The program stops the iteration when the valve's position gives a Cv that in turn gives a flowrate at the pressure drop that matches the set outlet pressure. The valve manufacturer must be able to furnish you the valve's curve showing Cv vs %Open to properly model its pressure regulating function.

http://virtualpipeline.spaces.msn.com

 

[vzeos]

You initially indicated you were unsure how to include the line pressure regulator in your flow calculations. The answer is that regulator capacity calculations and pipe capacity calculations are done using different equations. An easy way to handle your problem is to start at the appliance. Once you determine your required delivery pressure and required flow rate at the appliance, use your pipe equation to find out the required up stream pressure in the pipe coming from the regulator. This would be your regulator outlet pressure, P2. Now, from the supply line use your pipe equation to find out the pressure you can deliver to your regulator. This would be your regulator inlet pressure, P1. Among other things, the flow through the regulator depends on the pressure difference across the regulator (P1-P2), which, by the way, is not a set pressure drop. Calculate the flow through the regulator and compare it to the flow rate required by the appliance. Repeat the process until you get a unique flow rate through all elements. You should make sure that the regulator operates in the regulating range of its capacity and you should check out the droop characteristic with respect to your application. By all means, before you do anything, contact your regulator representative.

Attached is a link that will help you understand regulator sizing.

http://www.fisherregulators.com/technical/sizingcalculations/#SizingforGasorSteamService