Gas (Methane) Pipe Sizing

[PEDARRIN2]

I have a project which is using a gas ~60% methane to run a generator. I have a gas analysis which indicates the BTU/ft3.

I am responsible for sizing the piping to the generator. I know how to size the piing for natural gas but have not sized piping for a lesser concentrated gas.

I have a general approach but want to see if anybody has actual experience with this.

 

[sshep]

It sounds like if you knew the ACFM flow you could size the minimum pipe requirement.

1.) Calculate the flow (SCFM) required using your given heating value, maximum generator power requirement, and power transfer efficiencies.
2.) Convert to actual flowing conditions (ACFM) at the temperature and pressure of the system.
3.) Size your pipe using whatever tables or program you have used in the past.

Are we missing something? -SShep

 

[Montemayor]

PEDARRIN2:

I also believe we are missing something here. As engineers we are trained to design piping for any gas - whether natural, mixed, or pure.

Are you using a simple nomograph for natural gas? I ask because you should be using the combination of the Moody chart and Darcy's relationship. Whether you use equivalent pipe lengths, K method, or the 2-K method is up to you. But you have all these analytical methods and it should be a piece of cake if you have done it before. Besides, there is a multitude of computer programs out there that will do 95% of the job for you -- if and when you input the correct and necessary basic data.

If there is more to this story, please tell us.

Art Montemayor
Spring, TX

 

[saxon]

Peddarn2, So what is the pressure and flow requirements at the generators prime mover? This info needs to be supplied by the manu'f. of the generator/driver set. After you have this info., size the line as you would for any other gas.

saxon

 

[friartuck]

I don't know what your gas regs are like in your country, but in the UK we have a limit of allowable pressure drop. The gas enters the building at 8.5" wg (20.7mB) and we are allowed a 1 milli Bar drop (about 0.3"wg) which makes our gas mains quite large. If you are not restricted by pressure drop then your mains can be a lot snmaller.

Also, if you DO decide to 'guesstimate' using the tabulated gas tables for Methane,beware that these usually apply only to low velocity low pressure and incompressible flow. When boosted gas is used, a different approach may be needed. For higher pressure systems, we are allowed to drop the pressure by 10% of the incoming pressure, i.e. if you start at 40"wg, you are allowed to drop to 40-10"=36" final pressure.

Friar Tuck

 

[PEDARRIN2]

I use the International Fuel Gas Code to size piping for Natural gas. This is mostly used for piping inside the building, not for large or long transmission lines. It is based upon flows measured in cubic feet per hour which is roughly analogous to 1000 BTU/hr which is based upon natural gas which is ~93% Methane and a specific gravity of 0.6.

My gas is off of an anaerobic digestor and the lab analysis is 63% methane and the specific gravity is 0.9 and the heating content of the gas is about 530 BTU/hr.

As I said, I can size it with natural gas because that is what the tables use. But I am looking for how to size the piping using a less concentrated and heavier gas mixture.

I have calculated an increased pipe size based upon a ratio of the heating values and also based upon a ratio of the percent of Methane. The pipe size approximately doubles.

Just looking for somebody who has done something like this to tell me that my approach is accurate.

 

[TD2K]

Heating values are normally reported in BTU/scf, I assume your BTU/hr is a typo.

What's the balance of the gas other than methane? Given your heating value, I assume it's an inert gas. Given your SG for the mixture, it back calculates to an SG of about 1.4 which is close to that of CO2 (1.52) but I'm not sure you get CO2 off an anaerobic digestor.

You know the SG of the gas mixture and the heating value. You can easily calculate the flow rate you need to deliver the BTUs that your final equipment requires. Once you have that, it's a simple calc to see what size pipe you need to deliver the flow rate of gas depending on the available pressure drop you can afford to take.

 

[PEDARRIN2]

The other gas is CO2. It is a byproduct of the anaerobic process.

The BTU/hr is not a typo. It is just a convention used when sizing natural gas piping inside buildings. It has to do with the fact that the heating value for natural gas is close to 1000 BTU/ft3

Thanks for your help.

 

[quadtracker]

Just a point to remember, the governor on the engine will take up most of the changes in your fuel. If the engine needs more power, the governor controlling the fuel supply will just open up until its satisfied. The governor will maintain the speed you set. Most of the time the governor runs maybe 40% to 60% max.