Calculating Natural Gas Pressure Drop in a 3" Riser 2

[sameeji]

Hello,
I need help in calculating pressure drop in a 3" cast iron schedule 40 Natural Gas Riser from cellar to the 19th floor of a high rise building. In the boiler room located on the 19th floor, I will have a Natural Gas Booster Pump that will be sucking gas through this 3" riser.There will be a Caterpillar Natural Gas generator installed on the rooftop which will operate at pressure of 1.5-5 PSI.
Data Available:

-Natural Gas Booster Pump can pump maximum pressure at 2.132 PSI with variable volume from 3000 ICFH to 15000 ICFH.

-Natural Gas Caterpillar Generator required minimum inlet pressure of 1.5 PSI but can operate up to 5 PSI.

-3" riser is approximately 190' tall and will have approximately ten-90 Deg elbows which will add roughly 77' of pipe length to the 190', equaling to 267' in total pipe length.

I need to find out what will be the pressure drop if there is any in this 267' of 3" cast iron pipe. One thing to remember is that the Booster Pump is located on the 19th floor boiler room and not in the cellar floor. I want to make sure that the booster pump can handle all the friction and pressure losses due to the length of this 3" riser.

Thank you very much for helping me with this project!!

 

[TBP]

For compressible fluids, you can neglect elevation change. Look at the fuel utilization code for the jurisdiction involved - all will be revealed.

I doubt that you're using cast iron pipe. Much more likely that it's steel.

 

[vzeos]

For a natural gas riser, the elevation change cannot be ignored. For low pressure gas (which is what I guess you have) the gas pressure will tend to increase with elevation. The change in pressure due to elevation must be considered in conjunction with the frictional pressure loss. Frequently the net pressure loss is less than that in a horizontal pipe and sometimes there is a net pressure gain. See the attached article on Elevation Corrections.

I agree that a cast iron riser is much too dangerous for interior building piping. It must be steel.

 

[MortenA]

Its called a compressor (or fan) not a pump when its a gas your moving i think.

Anyway for a pressure range up to 5 psig and as little elevation difference as 190' i think is safe to assume that elevation does not matter. But if you end up using some fance software and not "hand" calculations then i would included it anyway since most packages handles this "for free".

Best regards

Morten

 

[NMcC]

a quick cautionary note in reply to Vzeos statement:
For low pressure gas (which is what I guess you have) the gas pressure will tend to increase with elevation.

This applies to guage pressure only (i.e. the absolute gas pressure in the riser minus atmospheric pressure). And this is what the attachment to vzeos' post states.

Because the gas at low pressure (i.e. approx 2 bar or less) is less dense than air (approx MW 16 v.s 28.97) atmospheric pressure decreases faster with altitude than the internal gas pressure.

The static head loss inside the riser still applies, i.e. subtract rho*g*h from the available pressure at the bottom of the riser.

The absolute gas pressure in the top of the riser will be the pressure available at the bottom minus the static loss minus the frictional loss.

Regards
NMcC

 

[vzeos]

a quick cautionary note in reply to NMcC statement:
Because the gas at low pressure (i.e. approx 2 bar or less)...
In the natural gas distribution industry (at least in the US), low pressure is a specific pressure classification that is usually codified as 4”wc to 12”wc. In other words, low pressure is the pressure delivered to a residence without using a service regulator. Although the op does not specify the suction pressure of his gas booster I suspect it is low pressure because he is using a booster. MMcC’s interpretation of gage pressure is correct, however, a pressure of 2 bar (29.00) psig would be considered high pressure in the natural gas distribution industry and reference should be made to the high pressure section of the article I posted.

 

[sameeji]

Thank you very much for all of your support. I am a new memeber here and I am amazed at all the help I am receiving.

My Natural Gas serive to the high rise building is a low pressure service as VZEOS described. The utility company told me its about 4" WC which is quite low for my generator. My booster pump, pumps out at 2.132 PSI and my generator operates on 1.5-5 PSI. I know this riser will be somewhat pressurized but I want to know what will be the pressure loss due to elevation of 200 feet and would it affect my booster pumps working capacity, as I have only 0.5 psi over the required pressure for the generator. The riser is 3" diameter. Also, all the elbow's and fittings will add to the frictional losses. I assume there will be about 10 elbows. I have been researching this issue for past 2 weeks and did not get a solid answer interms of calculating the pressure drop.

 

[katmar]

sameeji, you have not given the details of your gas but if I guess the molecular weight to be 16 then I estimate the pressure drop in the 190 ft riser to be less than 1 psi at a flow of 15000 acfm. Under these conditions you can use the simple relations derived for liquids (i.e. incompressible fluids) and you do not need to get involved with the complexities of compressible flow.

The pressure drop calculator recommended by quark in thread124-231787 will probably give you all the accuracy you need.

One thing to note is that with a pressure drop of around 1 psi the gas pressure at the base of the building will be below atmospheric. If the gas riser is supplying the equipment on the 19th floor only then that is not a problem, but if you have take offs lower down you will suck air into the gas system rather supply gas to any appliances. In this case it would be better to locate your booster compressor as close to the utility supply point as possible.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[JijoP]

Hello Check these links

http://www.pipeflowcalculations.com/naturalgas/index.htm

http://www.pipeflowcalculations.com/

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

 

[vzeos]

Katmar,
you raise some important issues in booster installation design. Negative suction pressure is always a concern and is never acceptable no matter where you locate the booster. NFPA 54 paragraph 2.11 requires a low pressure shut off device that will prevent the booster from drawing a vacuum on the utility’s meter and gas mains. The concern is that a vacuum could cause the meter to collapse and cause pilot flames to be extinguished in the community. The shut off device is typically set to activate between 2 in wc and 3 in wc.

Your calculation is probably good except that sameeji did not supply the load required to properly size the riser. The booster maximum capacity is not the gas load. The gas load is the fuel consumption rate of the Caterpillar gas engine. I looked up the fuel rate of a Caterpillar 450 kW gen set for which the fuel consumption is 4472 scfh, just to do a sample calculation. Using this consumption as the gas load with an equivalent length of 267 ft, a pipe diameter of 3.068 in and an inlet pressure (P1) of 4 in wc, I get an outlet pressure (P2) of 2.255 in wc for a horizontal pipe. If you consider the elevation correction for 190 ft, P2 becomes 3.395 in wc, which would be OK for the given set of parameters.

 

[Capsaicin]

I'm probably just missing something here, but doesn't your booster run on electricity? How can it boost the pressure for your generator that is supplying the electricity?

 

[Bribyk]

You can try the calculation method below as well but it doesn't seem to account for elevation. FYI, this is the same formula as in the Canadian standard (CSA B149 1-05).

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

 

[Apakrat]

Will your local NG supplier install an industrial pressure regulator set to the 29th floor NG psi need, thus eliminating the need for installing an expensive and troublesome a booster pump?


At 74th year working on IR-One PhD from UHK - - -

 

[TBP]

Bribyk - It's almost as if the CSA standard indicates that you can neglect elevation change, isn't it? Whoda thunk it?

You guys are arguing over how many angels can dance on the head of a pin. JUST LOOK AT THE FUEL UTILIZATION CODE FOR THE JURISDICTION, AND/OR CALL THE LOCAL INSPECTION AUTHORITY.

It's just that easy. Really.