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

Dan

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