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# Determine pressure reduction in a pipe

## Determine pressure reduction in a pipe

(OP)
If a have a flow of natural gas going through 8 inch diameter pipe about 1500 PSIG and the pipe change the diameter to a half inch diameter (smaller one), How can I determine when the pressure in the 1500 line goes lower than 400 PSIG?

### RE: Determine pressure reduction in a pipe

Surely you don't mean you're taking whatever full flow in your 8" line through a 1/2" line?

### RE: Determine pressure reduction in a pipe

You calculate pressure loss in the small pipe knowing flow and end pressure, assuming inlet pressure is 1500 psig varying the length until you get to 400 psig.

But this is a very silly question as no one will do that.

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

### RE: Determine pressure reduction in a pipe

When (and if) the velocity in the smaller line goes beyond Mach 0.8 or so, pressure drop slope with distance declines steeply and is almost a vertical line down beyond Mach 0.9 or so. At these velocities, it is difficult to tell at what distance pressure will reach 400psig.

### RE: Determine pressure reduction in a pipe

(OP)
George: How I do determine these velocities? How can i meausure it. Any formula?

### RE: Determine pressure reduction in a pipe

A well dimensioned and labeled sketch may help us help you. Be sure to include all you know, including the downstream receiver conditions.

Good Luck,
Latexman

### RE: Determine pressure reduction in a pipe

This might be choked flow in which case the distance is about 10mm.

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

### RE: Determine pressure reduction in a pipe

Hi,
If you are a chemical engineer, you should be able to define much better your problem and submit a sketch with the relevant data to perform some calculation. We cannot read your mind.
Probably good for you to review the document attached.
Good luck
Pierre

### RE: Determine pressure reduction in a pipe

OK, so it's an offtake.

At anything less than abut 600 psig, this will be choked flow so the answer at 400 psig is as I said - about 10mm from the connection point at worst case.

If your tube is long enough then it could build up a back pressure so that the pressure just after the take off is >600 psig. Then it might be possible to calculate it, but you either need flow or you need length of the tub from the tap off tot he exit to atmosphere or exit into something other sort of vessel.

If you spent a few minutes actually explaining what this is then you might get some sensible responses.

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

### RE: Determine pressure reduction in a pipe

(OP)
It is a pretty long tube, we usually use those for the Gas reinjection for bullheading operations. I have to go through few isometric to find the actual lenght but my best guess it that , it could be an 3K feet approximate. The biggest dilema is to figure if this 1/2 inch could reduce the pressure in the line to about 400 PSIG . and how long will it take to do that.

### RE: Determine pressure reduction in a pipe

I have no dount it could do that, but just depends on how much flow.

Also no flow, the pressure would be 1500 psi.

Without flow rate you can't work this out unless you also have a fixed end pressure on your tubing, Then you might be able to work out the flow rate to get to your 400 psi.

what's so special about 400 psi?

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

### RE: Determine pressure reduction in a pipe

You can flow through this 1/2inch SS tubing if flowrate is small. If you say the line length is say 3300ft and required end pressure must be not lower than 400psig, and P1 = 1500psig, T1 = say 40degC, tube id = 12mm, you can get about 250kg/hr, with Mach no at 0.02 at inlet and 0.07 at exit.

### RE: Determine pressure reduction in a pipe

So, itâ€™s frictionally choked.

Good Luck,
Latexman

### RE: Determine pressure reduction in a pipe

Hi,
This is becoming ridiculous! On the drawing it's written 2100 PSIG. Please let us know where the pressure is measured and what is the value to take account.
What is the flowrate in the main pipe? If not known do you have a way to measure the pressure (P1 and P2) at two well identified location where L2-L1 is known, distance L?
So far only guesses.
Pierre

### RE: Determine pressure reduction in a pipe

Obviously, a throttle valve will be required at the start of this tubing run to keep flow low - a tubing needle valve will do.
Running HP gas through 3000ft of tubing may pose a safety issue - at least one Operating Company does not permit the use of threaded or compression fittings for flammable fluids - check with your Technical Safety engineers if they will permit this.

### RE: Determine pressure reduction in a pipe

Hi George,
The OP is talking about dropping the pressure from 1500 (2100?) PSIG to 400 PSIG on the main pipe (8") using 1/2" pipe! The main pipe is about 4Km long, I don't believe this is going to happen with a bleeding pipe located 1 km from the discharge point.

Pierre

### RE: Determine pressure reduction in a pipe

@pierrick, Think the OP wants some flow through this 1/2inch line, not the main 8/10inch line. Not sure if he wants to regulate it down to 400psig, and if so, where.
@smith, Why not install a forward self operated pressure regulator, set at 400psig, at the end of this tubing line? And allocate say 50psi drop through it at max flow, which, from my previous calc is approx 200-250kg/hr. This way, you have constant pressure of 400psig downstream of the pressure control valve.

### RE: Determine pressure reduction in a pipe

One thing to remember here is the JT cooling and possible water or condensate drop out and hydrate formation.

The long length and small size will help heat to get in from the outside air but it will still get colder than the inlet.

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

### RE: Determine pressure reduction in a pipe

200kg/hr at 400psig, at an approx density of 22kg/m3, converts to a displacement flow of 9 actual m3/hr.

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