Pressure drop and flow control for low pressure gas. 2

[stanfea]

Hello All

I am in the process of laying out a gas line (coke oven gas, sg=0.36) that has an inlet pressure of only 3.25 psig. The flow is 220 000 SCFH. The dilema I am facing is that the tolerance of the allowable pressure drop is only 0.75 psi, by the end of the run. I am a bit skeptical of working within such a narrow band of drop. I am using the Crane program and the technical article it is based on to do the analysis. Is such a small drop within the limits of a realistic answer? In other words in typical situations 1 or 2 psi would be meaningless, in this case it is not. All comments appreciated.

Thanks
Stan

 

[CRG]

A pressure drop of 23% is reasonable to calculate. I would do the calculations for pipe sizing using the Crane formulas as you intended with a safety factor based on the certainty of pipe routing, fittings, valve coefficients, etc.

Then for a quick check, I would look at the International Fuel Gas Code, Table 402.3(8) 2 psi gas with 10% drop, Table 402.3(9) 5 psi gas with 10% drop, and Table 402.3(24) correction multiplier for specific gravity other than 0.6. I would then use the tables too look up the required pipe size for 2 psi gas and 5 psi gas with a 10 % drop in pressure.

The calculated pipe size using the Crane formula should be somewhere in-between the 2 psi and 5 psi pipe sizes located with the corrected tables. The results of both methods should give you some comfort level with the accuracy of your calculations.

 

[jay165]

Try the Spitzglass formula, GPSA, Eq. 17-29.

 

[katmar]

Hi Stan,

The Crane program is the ideal method for this problem. Your fluid is Newtonian and the Reynolds number will be high so all the standard assumptions will be satisfied. If you put a bit of effort into getting the correct lengths and fitting details I would expect you to get an accuracy of about 20% on the pressure drop. Your pipe will be a reasonable diameter so the roughness will not play a big part.

One of my hobby-horses is telling people that a design is a decision and not a calculation. Lots of calculations need to be done, but at the end of the day it's a decision on what to do. The advantage of having a computer program to do all the calcs is that you can try so many alternatives to see the sensitivity of the problem to the different variables - and then make your decision.

As an example I did some calcs on your data (I had to guess some of it) and worked out pressure drops for a 1000 ft equivalent length. The pressure drops I got were (based on pipe ID's)

8" ID = 2.6 PSI
10" ID = 0.76 PSI
12" ID = 0.31 PSI

Obviously an 8" pipe is no good. Now the decision. Do you take a chance with the 10" pipe or go for the 12" pipe? It all depends on your circumstances. Who pays for the extra cost of the 12" pipe? Who pays if it does not work? How much safety factor have you got in your allowable pipe pressure drop? in the supply pressure? in the control valve pressure drop?

Remember that the pressure drop varies with the fifth power of the diameter, so making a change of one standard pipe size has a very significant effect on the pressure drop.

regards
Katmar