How to understand what natural gas flow we can expect through 4" piping to our equipment 1

[USAeng]

At our plant we have a 4" gas line coming to some recently placed equipment. We are trying to understand if the existing 4" pipe with our 8.5psi supply pressure will allow enough gas for the new equipment.

Critical piece of equipment needs 5psi minimum and gas flow of 13,000SCFM. It is 75' from the supply pipe to the equipment with a 3" pipe. There are 2 other pieces of equipment also - 1 requires 12"WC at 10,000SCFM with 2" pipe and the other at 12" WC at 8,000 SCFM with 2" pipe- both of these have approx. 100' piping to the equipment.

Some people here are saying this engineering toolbox chart says we will not have enough gas. I do not understand what this chart is supposed to explain. Can someone explain what it should be used for? There is a small section of pipe that goes over a bridge from the meter to our plant (about 100' that is 3"). So those people are using the 3" section as the pipe size and saying you can only get 23,400-33,860SCFM through the 3" pipe. I do not think they are using the chart correctly.

http://www.engineeringtoolbox.com/natural-gas-pipe-sizing-d_828.html

We have tried this from Google

http://www2.iccsafe.org/states/oreg...dix A_Sizing and Capacities of Gas Piping.pdf

using pages A-7 and A-9 I find expected flow to be 45,000SCFM using P1 as 8.5psi and P2 as 5psi, 75' pipe length and 3" pipe

We have also tried using this Renouard equation which gives about 50,000SCFM

Are these acceptable ways or estimating our gas usage? Also something we don't understand is while we get the high numbers for flow using the chart and equation for the 75' of pipe - if I take the same equation or chart and put in the numbers that exist at our gas Meter (10psi) with 1.8psi drop to the point on our property which is about 1700' away - numbers for flow equal close to 16,000 SCFM

Can anyone help explain how guys normally would do this? It feels like we going about this wrong. Sorry for the long post

 

[BigInch]

CFH cubic feet per hour.
Your equipment needs 13,000 SCFM = 780,000 SCFH = 18.7 MMSCFD

The Toolbox chart says that 100 ft of 4.5" OD steel pipe, with inlet pressure of 8 psig will have a downstream pressure of 7.6 psig when the flow rate is 40440 SCF/HOUR NOT MINUTE. That is 674 SCF/MINUTE.

That pressure drop is 8.0-7.6 = 0.4 psi which equals 11 inches of water.
Figure A.6(a) in Appendix A, with a pressure drop of 10 in of water across 100 feet of 2.5 in OD pipe, says it will give you 10,000 SCF/HOUR or 166 SCF/MINUTE

The same with a 4" OD pipe looks to give around 30,000 SCFH, which is 500 SCFM



you must get smarter than the software you're using.

 

[USAeng]

sorry I meant SCFH for all. Many appologies

 

[BigInch]

You get such low number (16,000 SCFH) for 1700 ft of pipe because it travels 17 times the tabled values and gas transmission at low pressures is very inefficient. Pressure drop through cumulative lengths (each 100 ft segment) is not linear and each 100 ft length towards the downstream end will have a larger pressure drop, even though the mass flow through all remains constant. As the pressure reduces, the gas' volume expands causing velocity, friction and pressure drop to increase in each successive segment.

you must get smarter than the software you're using.

 

[SEP87]

Just a note regarding:

We have tried this from Google

http://www2.iccsafe.org/states/oregon/08_residenti...

using pages A-7 and A-9 I find expected flow to be 45,000SCFM using P1 as 8.5psi and P2 as 5psi, 75' pipe length and 3" pipe

The table on A-9 requires P1 and P2 to be absolute pressures. So P1 should be 23.2psi and P2 should be 19.7psi (approximately).

Also Regarding your 1.8 psi drop through 1700' of 4" pipe.
Using Figure A.6(b) P1 = 27.7 P2 = 22.9 This gives you about 40,000 SCFH which appears ample for your requirements.

I would usually do this from the Darcy–Weisbach equation & Moody Diagram. With this sort of pressure loss I assume a constant density (a few psi here or there will still give a good estimate).

 

[LittleInch]

You get low numbers because I don't think you read the document properly. The number (P1^2 -p2^2/ length in hundreds of feet is in absolute pressure i.e. gauge plus 14.5 psi. Doing this (using absolute pressure) for the figure A6b gives you about 25,000 scfh whereas using the guage pressure is about 16,000. On that basis 10 psig to 5 psig gets you about 40,000 scfh.

Basically sounds like you're at the far end of the capacity for a 4" steel pipe and at these sorts of pressures the odd 1 psi one way or the other can make a lot of difference. These types of graphs are not wildly accurate, but intended to show that your chosen pipe size is well in excess of your flow requirement.

The other question to ask is whether you're sure these pipes are steel and not PE. PE is quite a lot smoother than steel and has different flow nonagrams and usually more flow capacity for the same ID.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[SEP87]

Got my calcs a bit muddled:

Using Figure A.6(b) P1 = 27.7 P2 = 22.9 This gives you about 40,000 SCFH which appears ample for your requirements.

Should read:

Using Figure A.6(b) P1 = 24.7 P2 = 22.9 This gives you about 25,000 SCFH!

 

[katmar]

There are 2 details that are not clear to me, but which don't make too much difference to the conclusions. The first is the section of 3" pipe. If this is part of the 1700' of 4" pipe then the equivalent length of 4" pipe should be taken as 2000'. The second detail is the pressure drop available for the 4" pipe. Is it the 1.8 psi given, or is it 1.5 psi (calculated as 10.0 - 8.5).

Using the 1700' of pipe and a pressure drop of 1.8 psi gives me a flow rate of 23,000 scfh, but this decreases to 18,900 scfh if the length is 2000' and the pressure drop 1.5 psi.

However, the pipes from the 4" main to your users seem to be more than adequate. The 75' to the new user should have a pressure drop of less than 1 psi, and the 2 low pressure users would also need less than 1 psi. Assuming the total flow rate in the 4" pipe is 31,000 scfh and the supply pressure is 10 psi you should get 6 psi at the end of the main (i.e. 4 psi pressure drop). Provided the regulators for the users are at the equipment end of their branches it should all work, but you will not get the expected 8.5 psi at the end of the main.

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

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