Blowdown these days is done via a ball valve and restriction orifice (RO). Control valves were used in the past and exist on some old plants but have been superceded primarily because they leaked and spring return fail open actuated ball valves are more reliable. Sizing relief valves and blowdown are two separate things, I'm just covering the blowdown here.
RW Miller will allow you to size an RO for a given flow, as would many packages inc. HYSYS and InstCalc. The hard bit is calculating the required peak initial flow, which is the RO sizing case, to give you the required drop in pressure in 15 mins. HYSYS purports to do it but two of the three options, the control valve ones (Masonellian and Fisher) are rubbish, so do it with the RO option (see above).
Cross check the HYSYS RO size by calculating the peak initial flow using Grote, compare it with the HYSYS peak initial, within about 10% is fine.
As a further check calc. the RO size (by whatever method).
Grote Equation;
Wi=Wt/[(Te/Pi)*{(Pi-Pe)/ln(Pi/Pe)}]
Where;
Wi= Peak initial flow kg/hr
Wt= total mass to be removed kg
Te= Blowdown time hrs
Pi= initial pressure bara
Pe= final pressure bara
And for your homework, remember integration!!!!;
The Grote equation is derived from the fact that pressure decay during blowdown is sonic, hence the curve is an exponential decay, with the equation;
Flow=xe^yT
Where
T=time and x and y are unknowns.
If you have any snotty wee graduates around this is the perfect task to set them, just to annoy them!!!!
