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prichmon (Mechanical) (OP)
15 Aug 13 10:29
We are going to a new gas main system. The existing system is 3" black pipe at 15" WG. I am looking at changing pressure from the existing 15" WG to 7" WG, 2psi or higher. I would like your opinions for 2 reasons; 1 I am studying for my PE and wish to know the "test" method for calculating this issue and 2 real world calculations and expectations.

I chose a conservative approach for sizing in as simple a method as possible. So I chose to use each "group" as a point load at the furthest distance from the source even though all will come off the branches at specified intervals. The volume requirement falls off faster than the piping losses allowing for additional capacity or fluctuation in pressure.

I used tables from: http://www.gpta.net/classes/gas%20sizing/gaspipe%2...
I chose the following for the 2 psi sizing.
2" main @ 30 psi from Gas company
2" line teed to 2 - 2" branches;

1st run 100' 2" line BTU/hr requirement ~7 million; 2" continuing on additional 400'with a ~400K BTU/hr requirement.
2nd run 200' 2" line BTU/hr requirement ~4 million.

The machinery is assumed to be operating at 100% flow at all times.

I used the following link for calculation of initial low pressure system 7" WG:
http://www.engineeringtoolbox.com/natural-gas-pipe...

My calculation came up to ~5" line to support the required flow. Cost wise the 5" is ~2x the cost of existing 3" line.

In the calculation the pressure drop on 7" WG is listed typical at 0.5"/100'. I don't understand how higher pressure drop actually increases the flow volume? I presume the pressure drop would increase the loss and reduce overall flow?

Can the Bernoulli equation be used for this calculation to get accurate results?

Is there any reason to avoid a higher pressure system? It seems the higher pressure yields lower overall cost in material. I have tables going to 5 psi and am comfortable extrapolating to higher pressure if needed. At 5psi I can use 1.5" pipe and achieve similar flow requirements. At 35psi it appears I am safe at 1" pipe.

I wish to use PE plastic after the heating units for transmission to office heaters to reduce cost. Is there any reason to avoid PE in this application?

Thanks

Rich


BigInch (Petroleum)
15 Aug 13 17:56
It appears that your system falls within the NFPA 54 code.
Get free access to view this at www.nfpa.org
See the tables there.

Bernoulli isn't good.
If you insist on formulas, the Spitzglass equation, or other low pressure flow formulas for gases AT LOW PRESSURES would be appropriate.

A greater pressure drop implies more energy available to move the fluid thus always increases flow. At least until you reach sonic velocity. Difficult at such low pressures as you have.

Independent events are seldomly independent.

BigInch (Petroleum)
15 Aug 13 17:57
NFPA 54 also has some formulas in there. Use the one for low pressure.

Independent events are seldomly independent.

LittleInch (Petroleum)
16 Aug 13 3:41
This is a long post, but not many questions and a odd mix of sizes and flows. Pressure drop at 7" wg at 0.5"/ 100`I can only assume refers to a certain flow rate. Whilst you may know what flow equals a certain btu/hr, it is better to use volume flow rates.

Back to your questions. BI has addressed the flow calculation question very well. Is there a reason to avoid a higher pressure system? A higher pressure system will be more efficient cost wise in terms of pipe, but you will need to add a pressure regulator or check what pressure range your burners can handle so that you can see the whole package costs.

I can't see any reason not to use pe in your distribution system, but check for fire requirements and supports for pe which are different from steel/copper pipe.

My motto: Learn something new every day

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

prichmon (Mechanical) (OP)
16 Aug 13 9:27
I was using BTusince the tables were listed in BTU. In Volume I am using the conversion 1000 BTu/hr to 1cfm.

Thank you very much for the comments.
KernOily (Petroleum)
16 Aug 13 17:05
Brother BigInch, what's wrong with the Bernoulli equation for this type of work? Bernoulli is after all only an energy balance, a restatement of the First Law. The problem might arise in the use of the Moody friction factor when calculating the head loss term in Bernoulli. Fundamentals is fundamentals, I believe :) If one assumes there is no change in density (which practically there isn't for this system) then Darcy-Weisbach should work for the head loss term.

There are other issues though in play here and that is 'a reasoanble standard of care and custody' or 'what another engineer of adequate experience would do' in this situation, and in that case it's NFPA 54 because that's the way another engineer exercising a reasoanble standard of care would do it. And if push comes to shove in a litigation you need to be able to show that you exercised that infamous reasoanble standard of care. WHich means: use NFPA 54 and not Darcy-Wesibach.

BigInch (Petroleum)
17 Aug 13 2:47
At these pressures there is no change in density and no velocity, so all Bernoulli terms are zero.
Friction factors are not part of the "Original Bernoulli" equation. f is in the "Modified Bernoulli" equation. f can be from any equation that correlates to lo pressure gas flow. I recommended Spitzglass, or the NFPA equation. Why not do the calculations yourself, first using Darcy-Weisbach, then Spitzglass, then check the low pressure gas tables and report back to us on any differences you find.

Independent events are seldomly independent.

BigInch (Petroleum)
17 Aug 13 3:17
It takes many stories in a high rise to have a significant effect.

Independent events are seldomly independent.

katmar (Chemical)
18 Aug 13 5:13
Your pressures are so mixed up that it is impossible to give specific advice. From your statement "I am looking at changing pressure from the existing 15" WG to 7" WG, 2psi or higher" it is impossible to know whether you want to increase or decrease your system pressure. 2 psi is about 55" WG.

The Bernoulli equation assumes frictionless pipe. Bernoulli is really only useful for static head and velocity head. In this instance your friction head loss will be far higher than the velocity head and with gases static head is usually negligible (except in highrise buildings as mentioned by BigInch). The equation you want to use is Darcy-Weisbach. To be rigorous you should use the version integrated under the assumption of isothermal flow for gases, but if you use the downstream gas density you can use the incompressible version (much simpler) with small errors and conservative answers (i.e. slightly higher pressure drops).

But because of the entrenched practices and regulations in the gas industry you need to check the tables as well. Use D-W to know that you are correct, and use the NFPA tables and formulas to know that you are within the regulations. There is little point in using antique empirical equations like Spitzglass or Weymouth unless they are a specific requirement of the authorities.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

BigInch (Petroleum)
18 Aug 13 5:27
I couldn't make sense out of his pressures either.

NFPA 54 contains the equation that NFPA recommends. I'd eliminate potential controversy (if its an NFPA covered installation) and simply use that one.

Independent events are seldomly independent.

prichmon (Mechanical) (OP)
19 Aug 13 7:20
Local gas company at our present location supplies 15" WG
The new Gas company supplies 7" or 2psi. As per Katmar 2psi is ~55" WG.

The pressure is left to my discretion and I chose to compare pressures as supplied by the new gas company. So I first assessed for 7" WG assuming and compared pricing of piping to current. Next I assumed 2psi and referenced the tables.

Thank you very much for the information on Bernoulli and Darcy-Weisbach. I now have a better understanding of how and where to apply the equations.

Katmar: How would you make a estimation of the downstream gas density? I have presumed gas density to be constant.

PEDARRIN2 (Mechanical)
19 Aug 13 11:23
So the gas company is providing 30 psig and you are looking to change the downstream system from 15" w.c. to a lower or higher pressure.

If you are desinging (redesigning) a natural gas pipe system where you are taking the gas company pressure down to the building pressure, you will need to consult the applicable gas company requirements for sizing piping and/or the appropriate fuel gas code (whether it is the NFGC or the IFGC). They have tables and/or equations to use.

You will also have to look at changing the regulator or at least the spring.

Using the Spitzglass equation for low pressure (<1.5 psig) or the higher pressure equation (for >1.5 psig) are generally useful.

To answer the density question, natural gas is compressible, so the density will change with change in pressure. Generally, if the pressure drop is less than 10%, you can use either the density of the upstream gas or the downstream gas. If the pressure drop is 10%-40%, the general rule of thumb is to take the average of the two densities. If the pressure drop exceeds 40%, you have to use empirical equations which are quite fun to use.

Going from 15" w.c. to 7" w.c. would likely require larger pipe. I would suggest using 2 or 5 psig inside and installing additional regulators at the equipment if there are not too many of them and if the regulator vent pipes can be run easily.
katmar (Chemical)
19 Aug 13 16:16
Assuming that the gas density is constant at the upstream (supply) pressure is where you have to start. Once you have calculated the pressure drop down the line based on this assumption you have your first estimate of the pressure at the end of the line. Knowing the pressure and temperature at the end of the line you can calculate the downstream density from the ideal gas equation (which is adequate at these low pressures). From here you need to do a few iterations, calculating the pressure drop based on the newly calculated downstream pressure (or from the average of the upstream and downstream pressures if you want to be less conservative). When the calculated downstream pressure estimates stop changing then you have the answer.

When it is all written out like this it makes it seem that is better to just use the isothermal compressible form and be done with it - which is how I would do it.

Katmar Software - AioFlo Pipe Hydraulics
http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

BigInch (Petroleum)
20 Aug 13 6:07
Excel can't hardly tell the difference in gas density between 0 and 1 psig? A difference of 7% at 15 psia. NG weighs about 0.04 lbs/ft3, 7% is 0.003 lbs/ft3 0.003 lbs/ft3 /144 in2 = 0.00002 psi Not even accounts give a &$*% about that. I'd suggest you assume density is constant unless you're piping from LA to Denver.

Independent events are seldomly independent.

prichmon (Mechanical) (OP)
27 Aug 13 8:55
tHANK YOU FOR THE COMMENTS GENTLEMEN.

sorry for the caps.

Does NFPA 54 cover allowable materials for flexible connections? I have recently found out we use something similar to red rubber hose and wish to back up a change to more substantial and safer flexible connections.

Does natural gas erode or react with rubber?

Thank you.

Rich
BigInch (Petroleum)
27 Aug 13 16:18
You must use materials that are certified by the manufacturer for use with natural gas.

Independent events are seldomly independent.

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