33 replies to this topic

[VikingUK]

I know
flow rate
velocity
density
downstream pressure
pipe diameter/length

of a gas flowing in a pipe how do I calculate pressure drop ?

[breizh]

Hi VIKINGUK ,
Let you try Perry's chemical handbook or Crane technical paper N 410 .
Hope it helps
Breizh

[sheiko]

I know
flow rate
velocity
density
downstream pressure
pipe diameter/length

of a gas flowing in a pipe how do I calculate pressure drop ?

First, i assume you are a student. If so, you should have posted this question in the student forum.

I think you should go back to basics and, in particular, Darcy equation.

[latexman]

Crane Technical Paper No. 410 (TP-410) is the quintessential guide to understanding the flow of fluid through valves, pipes and fittings, enabling you to select the correct equipment for your piping system.

Originally developed in 1942, the latest edition of Crane TP-410 serves as an indispensable technical resource for specifying engineers, designers and engineering students.

TP-410 is published by Crane Valve Group (CVG), one of the world's leading suppliers of valve products and services.

It can be purchased for a nominal fee at http://www.flowoffluids.com/tp410.htm

It has many example problems with their solution.

[VikingUK]

Not a student just an engineer who has spent too long in management and forgot what he learnt at college (use it or lose it)
Let me expand on the problem... I want to do a pigging run on a gas line about 5 mile long 6 inch diamter.
I know pipe spec, velocity, flowrate and initial pressure at launcher. I want to calculate what the receiver pressure will be ( gas will be flowed to flare )

[mohtogh]

First of all, you need to "decide" if your case is adiabatic or isothermal. If it is isothermal, good for you because there are simplw formula for that. If your case is adiabatic, the case could be difficult if you have things with high dP on the line.

[VikingUK]

How does adiabatic or isothermal even come into this problem ?? It's simple fluid mechanics.

[sheiko]

VikingUK,

Mohtogh is on the right track. You need to go back to basics and follow latexman advice.

[latexman]

VikingUK,

It would be beneficial if you gave some data. What is the flow rate, velocity, density, downstream pressure, and pipe diameter/length that you know? Do you know what the gas is, molecular weight, compressibility (z) and Cp/Cv? This data will help us give better answers/advice.

Btw, what have you done/found in your effort to solve/research this problem?

[VikingUK]

I have calculated flow rate and velocity but finding it difficult to calculate downstream pressure

v = 2.5m/sec
q = 160 m3/hour
d = 0.5908
upstrm press = 40 barg
d = 150 mm
l = 7400 metres
mw = unknown
z = unknown
cp = unknown

gas is natural gas 95% methane

[VikingUK]

dear all,
it would be of great help(and the only one needed)if one of those who have the crane paper
#410 to offer it for download. some of us can't afford $45 plus p&p.
best regds
iceman

I think you will find that is copyright infringement and therefore illegal.
And if you are an industry professional and can't afford $45 you need to reconsider going back to macdonalds as you aint making much in this game.

[latexman]

Two values of d were given. Which one is correct and what are the units?

[VikingUK]

d = 150 mm (diamter of pipe)
d = 0.5908 (gas density)

sorry typo

[katmar]

What you have called the gas density is more likely the "specific gravity" or SG of the natural gas. For a gas the SG is defined to be the ratio of the molecular weight to that of dry air - your value for SG of 0.5908 would make your MW about 17.1

For a good article that discusses the calculation of pressure drops for NG download http://www.psig.org/...s/2000/0112.pdf

Edited by katmar, 11 September 2009 - 11:09 AM.

[ashetty]

Reciever pressure during pigging cannot be calculated by usual pressure drop calcs.Remember that the pig would be pushing liquid in front of it as it moves,hence pressure dowsteam of the pig would gradually reduce.Your Sales gas compressores would possibly experience "starvation" of gas.Normally It would be a good idea to operate the compressors at reduced load.

Genrally what you need to do before every pig run is to calculated the pigging rates, with inputs from the gas sales nominations (or the planned rate of flow).Calculate the expected ∆P across the line (Using gas-flow DP equations), compare it with the actual ∆P (Your current reading from the well to your terminal or reciever) and approximate the quantum of liquid expected from the pipeline (by 2 phase flow calculation/precedence).Then plan the pigging accordingly (Liquid draining rates...monitoring etc). Individual flow rates from wells were determined/altered accordingly.
The cricial thing here is to know how much liquid can be expected....if you have sand or muck in the line its another issue.

[katmar]

Excellent advice from ashetty. I had not read all the posts by the OP sufficiently carefully.

[VikingUK]

Thanks for this but a couple of points.

There is no liquid it is dry gas.
There is no sales gas compressor. The gas flows direct from wellhead to flare (7.4 Km away). We bypass the plant during pigging

What I am looking to calculate is the pressure I should have at the receiever so I can adjust my contriol valve to that pressure.

[latexman]

VikingUK - A volumetric flow rate of a gas without specifying the flowing conditions (T & P) is pretty meaningless. What is the reference T & P for 160 m³/hr? If it's "standard T & P", what standard T & P?

katmar - that link didn't work for me this morning. It may be a temporary problem though.

[katmar]

katmar - that link didn't work for me this morning. It may be a temporary problem though.

latexman - thanks for the feedback. I had put the file type as .pd instead of .pdf. I have edited it now and it should work.

[ashetty]

Hello Viking,

No liquids expected during pigging? OK...

Can you elaborate on the purpose of this proposed pig run. I had assumed it was a cleaning run with the intent of removing liquid hold up and debris.

Is it a pipeline inspection with an intelligent pig?? In any case, if you can send us a brief sketch/schematic incl properties, conditions, linelength etc , maybe we can help you better.

Thx

[VikingUK]

Hello Viking,

No liquids expected during pigging? OK...

Can you elaborate on the purpose of this proposed pig run. I had assumed it was a cleaning run with the intent of removing liquid hold up and debris.

Is it a pipeline inspection with an intelligent pig?? In any case, if you can send us a brief sketch/schematic incl properties, conditions, linelength etc , maybe we can help you better.

Thx

It is indeed an intelligent pigging run (mag flux) temp/press of gas is 25 C and 60 barg
basically a straight run of 150 mm pipe 7400 metres long.pig goes in one end and is driven by gas pressure
and other end is open to flare

can I put this problem a different way ??

Imagine I have a pipe an inch diameter and 100 feet long
I put a table tennis ball (negligible mass) just slightly under an inch diameter in the pipe and apply 10 psi pressure

How can I calculate the time it will take to reach the other end ?

[ashetty]

Hello Viking,

No liquids expected during pigging? OK...

Can you elaborate on the purpose of this proposed pig run. I had assumed it was a cleaning run with the intent of removing liquid hold up and debris.

Is it a pipeline inspection with an intelligent pig?? In any case, if you can send us a brief sketch/schematic incl properties, conditions, linelength etc , maybe we can help you better.

Thx

It is indeed an intelligent pigging run (mag flux) temp/press of gas is 25 C and 60 barg
basically a straight run of 150 mm pipe 7400 metres long.pig goes in one end and is driven by gas pressure
and other end is open to flare

can I put this problem a different way ??

Imagine I have a pipe an inch diameter and 100 feet long
I put a table tennis ball (negligible mass) just slightly under an inch diameter in the pipe and apply 10 psi pressure

How can I calculate the time it will take to reach the other end ?

Pls see attached file.

Request someone to check the basis and calculation.

VikingUK please give me your feedback.

Attached Files

•  PIG VELOCITY.xls   184.5KB   82 downloads

[VikingUK]

Hello Viking,

No liquids expected during pigging? OK...

Can you elaborate on the purpose of this proposed pig run. I had assumed it was a cleaning run with the intent of removing liquid hold up and debris.

Is it a pipeline inspection with an intelligent pig?? In any case, if you can send us a brief sketch/schematic incl properties, conditions, linelength etc , maybe we can help you better.

Thx

It is indeed an intelligent pigging run (mag flux) temp/press of gas is 25 C and 60 barg
basically a straight run of 150 mm pipe 7400 metres long.pig goes in one end and is driven by gas pressure
and other end is open to flare

can I put this problem a different way ??

Imagine I have a pipe an inch diameter and 100 feet long
I put a table tennis ball (negligible mass) just slightly under an inch diameter in the pipe and apply 10 psi pressure

How can I calculate the time it will take to reach the other end ?

Pls see attached file.

Request someone to check the basis and calculation.

VikingUK please give me your feedback.

7.44 seconds for a mag flux pig to travel 7400 metres ???? I think there is something seriously wrong with that spreadsheet...!!!!

[ashetty]

Hello Viking,

No liquids expected during pigging? OK...

Can you elaborate on the purpose of this proposed pig run. I had assumed it was a cleaning run with the intent of removing liquid hold up and debris.

Is it a pipeline inspection with an intelligent pig?? In any case, if you can send us a brief sketch/schematic incl properties, conditions, linelength etc , maybe we can help you better.

Thx

It is indeed an intelligent pigging run (mag flux) temp/press of gas is 25 C and 60 barg
basically a straight run of 150 mm pipe 7400 metres long.pig goes in one end and is driven by gas pressure
and other end is open to flare

can I put this problem a different way ??

Imagine I have a pipe an inch diameter and 100 feet long
I put a table tennis ball (negligible mass) just slightly under an inch diameter in the pipe and apply 10 psi pressure

How can I calculate the time it will take to reach the other end ?

Pls see attached file.

Request someone to check the basis and calculation.

VikingUK please give me your feedback.

7.44 seconds for a mag flux pig to travel 7400 metres ???? I think there is something seriously wrong with that spreadsheet...!!!!

Yeah thats what i thought initially, but i verified the calculation using Campbell solved example (In the theory section).Come to think of it, if you had 60 barg pressure at inlet....you completly open the inlet valve, and no pressure at the downsteam of the 6" line you actually would have some serious velocity.

That is why you would have to crack open the inlet valve to the pig reciever pressurize it to maybe 0.5 barg and then the pig would probabely take 10-15 minutes @ an acceptable velocity of 10 m/s to travel the 7.4 km distance.

If you suddenly open the well at 60 barg after loading the pig,i think the pig wont be a pig anymore..it would be "Superpig", travelling faster than a bullet, shattering the sound barrier.

PLEASE CONFIRM YOUR DOWNSTREAM PRESSURE..as you know, flow and velocity depend almost entirely on the pressure differential.

Wait for someone to give 2nd opinion.

[VikingUK]

Hello Viking,

No liquids expected during pigging? OK...

Can you elaborate on the purpose of this proposed pig run. I had assumed it was a cleaning run with the intent of removing liquid hold up and debris.

Is it a pipeline inspection with an intelligent pig?? In any case, if you can send us a brief sketch/schematic incl properties, conditions, linelength etc , maybe we can help you better.

Thx

It is indeed an intelligent pigging run (mag flux) temp/press of gas is 25 C and 60 barg
basically a straight run of 150 mm pipe 7400 metres long.pig goes in one end and is driven by gas pressure
and other end is open to flare

can I put this problem a different way ??

Imagine I have a pipe an inch diameter and 100 feet long
I put a table tennis ball (negligible mass) just slightly under an inch diameter in the pipe and apply 10 psi pressure

How can I calculate the time it will take to reach the other end ?

Pls see attached file.

Request someone to check the basis and calculation.

VikingUK please give me your feedback.

7.44 seconds for a mag flux pig to travel 7400 metres ???? I think there is something seriously wrong with that spreadsheet...!!!!

Yeah thats what i thought initially, but i verified the calculation using Campbell solved example (In the theory section).Come to think of it, if you had 60 barg pressure at inlet....you completly open the inlet valve, and no pressure at the downsteam of the 6" line you actually would have some serious velocity.

That is why you would have to crack open the inlet valve to the pig reciever pressurize it to maybe 0.5 barg and then the pig would probabely take 10-15 minutes @ an acceptable velocity of 10 m/s to travel the 7.4 km distance.

If you suddenly open the well at 60 barg after loading the pig,i think the pig wont be a pig anymore..it would be "Superpig", travelling faster than a bullet, shattering the sound barrier.

PLEASE CONFIRM YOUR DOWNSTREAM PRESSURE..as you know, flow and velocity depend almost entirely on the pressure differential.

Wait for someone to give 2nd opinion.

I think even though the downstream is open to flare there would be some back prressure as the gas would have to pass through a knock out drum and then reduce to the flare pipe (50mm diameter)plus a pressure regulation valve.
I think the overall dp may be quite low (10bar) so assume a downstream pressure of 40 barg.
The flowrate I am seeing at these pressures is about 180 m3/hour which calculates to a velocity of about 1.5-2.5 m/sec in a 150 mm dia pipe