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Choked Tank Depressurisation Calculation

Choked Tank Depressurisation Calculation

Choked Tank Depressurisation Calculation

Hi Guys,

I have been trying to write a model to calculate the depressurisation of hydrogen out of a tank through a vent line. I've used the fanno flow calculations as the tanks are at about 350bar so the flow will be choked. The results are in the expected ballpark but no one in my company can validate the results. I'd really appreciate it if someone with more knowledge and experience in this field would check what I've done.

Much appreciated


RE: Choked Tank Depressurisation Calculation

Hi guys some information to help understand what I've done. First I used the Colebrook equation to find the frictions factor using an assumed Reynolds number, I used the goal seek function in excel to solve this. using that and the goal seek again i solved the Fanno flow equation and found the Mach number at the inlet of the vent pipe. These functions are coded in a macro and linked to the run button. from the Mach number the flow rate through the vent pip can be found and using a user defined time step the the new tank pressure and temperature can be calculated (adiabatic assumption). then assuming the Reynolds number hasn't changed significantly the friction factor and Mach number can be assumed to stay reasonably constant for the majority of the depressurisation process. the new pressure and temperature can be solved for each time step using the trapezium rule to approximate the correct depressurisation profile. I know that the Fanno flow equations breakdown at lower pressures but I think that for the majority of the depressurisation process it will be under the Fanno flow regime.

Thanks again


RE: Choked Tank Depressurisation Calculation

Sounds like the right approach, except the governing flow restriction is either the location of smallest flow area or one which has the most "oblique shock waves" , which occurs due to sudden changes in flow direction, as with many valves. The net effect of the oblique shock waves is contained in the ISA handbook of control valves method, factor Fl. The bottom line is the actual flow rate will be lower than that based on minimum flow area.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

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