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# Calculating line pressure from required flow rate

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## Calculating line pressure from required flow rate

(OP)
Wondering what formula(s) are more suitable to calculate additional pressure required to drive increased gas flow rate in both incompressible and compressible cases.

Thanks!

### RE: Calculating line pressure from required flow rate

Friction loss and minor losses are common between the two - incompressible and compressible.

In case of compressible fluid, formulas for friction loss depends on the average pressure - from Weymouth formula to Harris formula - there is several in between. If temperature is a factor, or if sonic velocities are possible, it would become even more challenging.

For compressible fluid, it depends on the fluid properties - temperature, pumped media (viscosity), etc. If it is Water at room temperatures, then a simple estimation is Hazen-Williams. If academic precision is required, then I would follow Darcy-Weisbach, which is better for extreme temperature and viscous fluids.

Minor loss can be estimated fairly similarly between the two. fL/D first and then use that to convert fittings to equivalent length and thus using the same friction loss formula again for compressible. A bit simpler in incompressible - (fL/D) can be used as a minor loss coefficient directly.

### RE: Calculating line pressure from required flow rate

ghensky,

you're going to need to define your usage a bit more.

I've not come across incompressible gas flow before now, but assume you might mean low changes in pressure / density.??

The most suitable formula depends on the conditions you have in terms of pressure, flow rate, velocity, dry or multiphase, pressure drop in any one part of the system, what type of gas (methane?).

There are many formula, analysis programs available, each more suited to one set of conditions, but if you don't know what those conditions are you can't state it.

Also let us know what you use now, what your issues are and why you're asking the question.

At the moment this is like someone saying "what's the best car to buy?" with no other information supplied....

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

### RE: Calculating line pressure from required flow rate

(OP)
Thanks nondim and Little.
nondim, for compressible flow, my understanding is that friction losses depend on Reynolds number and wall roughness.
Little, indeed I was looking at 100% methane. The problem involved cutting the gas supply by half. I started with Weymouth formula (due to pipe size being smaller than 20" - Crane) to calculate the current flow rate (The reason of this thread to begin with). Next (possibly a new thread), I'm considering to put an orifice plate. But not sure how to size the plate diameter as the most orifice flow equations found come with a beta ratio - not helping much.

### RE: Calculating line pressure from required flow rate

See if you can get yours hands Handbook of Hydraulics by Idel'chik. Chapter four, example problem 4-10.

### RE: Calculating line pressure from required flow rate

If you're looking to increase flow by increasing pressure, I'm not sure how an orifice plate would help you - that's just going to restrict both flow and pressure.

What kind of distances are you looking at? Over short distances, estimating by treating gas as incompressible is simple and usually accurate enough. But over long enough distances, where pressure drop starts becoming significant, you need to use compressible flow formulas. But as mentioned by LittleInch, if you can supply some typical details for your operating conditions (pressure, temperature, distance, pipe size, flow rate), we can probably provide more valuable help. What exactly is this application? What are you trying to do, and where?

### RE: Calculating line pressure from required flow rate

Many ways of pressure reduction to a desired pressure downstream :
a) Thick plate restriction orifice / flow nozzle - works only for constant flow and constant pressure upstream - cheap
b) Self regulated forward sensing PCV -limited flow turndown - higher cost than (a)
c) Forward sensing pneumatic or electronic PIC - PCV loop - much wider flow / pressure ratio limits - highest cost, may most likely require maintenance isolation valves for PCV also.

If pressure reduction is high, low temps generated may cause freezing problems. Talk to your senior engineer and run a simulation on this pressure reduction to see if there are more problems. Beware of pressure / flow turndown limits in your selection.

### RE: Calculating line pressure from required flow rate

use the moody diagram with friction factor calculated from pipe roughness and reynolds number. also weymouth, AGA and panhandle equations can be used.

try : http://pipeng.com/index.php/ts/itdmotflow006c/ for the moody diagram

the moody diagram can be used with either the darcy or fanning friction factor. the equation is slightly different but gives the same result provided you use the right equation / factor combo.

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