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# General gas flow equation concerns

## General gas flow equation concerns

(OP)
I 'm trying to apply the general flow equation for a simpe one straight line gas system and I would like to understand one thing.

In order to calculate the flow I need to calculate the friction factor f first which requires calculation of the Re number which is depended on the flow I am looking for. I can assume a flow and iterate the calculation but the Re is also depended on the viscocity.

Viscocity however does not remain constant as the gas flows in the pipe. What value is required here if my pressure drop is, say, 50 to 30 bar? A value related to 50 bar, 30 or other?

In addition, can the general flow equation be used for accurate results on any pressure range? In other words, can I use it for pressures around 100 bar and for pressures for residential equipment with just few mbar above atmospheric?

Thank you for any help.

### RE: General gas flow equation concerns

I think its common to ignore pressure effect on viscosity - but to divide your pipeline into smaller segments with a dP of say 5-10 bar each and then re-evaluate all parameters for each segment.

### RE: General gas flow equation concerns

In order for that equation to be valid, density must be "constant". If density is constant then velocity is constant. We know that if pressure were constant down the pipe, there would be zero flow. Consequently you set an arbitrary limit on pressure drop (usually around 5-10% depending on the flow equation you are using), then calculate an average pressure over the range and use that to calculate viscosity and friction factor. At the end, recalculate Reynolds number and verify that it is within 10% of the guess you used in the first place. Iterate until the starting Reynolds Number is within 10% of the ending Reynolds number.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

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