hniaboujemaa, thanks for a very interesting reply. Two curiosity questions:
1) Do you know if your last sentence (non-cert liq eqns for vapor trim) is actually written down in a standard ?
2) What about liquid certified valves with a vapour relief case ? Are the normal vapour phase equations...
Milutin, some companies put it to a flare header. The Flare Gas Recovery system is then used to send it to an amine unit. Several risk though (like air into your flare system).
zdas, this is a very strange explanation of choked flow. Some proof is needed before this explanation can be regarded as correct.
I think what really happens is much simpler. Taking your example of 10,000 psig PSV discharging into 50 ft of 2-inch pipe:
(a) If the PSV nozzle area is very small...
Rated flow = Design flow * (Full open area / Design area).
Sequence of calculations are as follows:
- Calc required relief flow from scenarios (design flow).
- Calc design orifice area from design flow.
- Select a standard relief valve with full open area larger than design area.
- Calc rated...
Okay, let me take a stab at it:
Design = Required flow rate from your relief scenarios.
Rated = Certified flow rate based on full open valve.
Max = 1.1 times Rated flow (as ASME derate by 10%).
"Actual" I assume is whatever happens in a real relief case.
KernOily,
With Excel the easiest way is probably as follows:
- Calculate the mass flow rate using the isothermal compressible equation for a range of pipe outlet pressure (say from Pin to close to zero).
- This is a simple calc that is not iterative (except for f maybe) and depends on the pipe...
ione,
2) No, you can (and usually do) have a shock wave by simply going to sonic flow (assuming adiabatic). No need to go supersonic to get a shock wave.
Cilliers
KernOily,
1) Partly. If you increase upstream pres (at fixed downstream) then the flow will eventually choke at the pipe outlet.
If you continue to increase upstream pres, the mass flow will increase, but the flow will remain choked at the pipe outlet.
2) Partly. The flow will never go...
KernOily, download the trail version of Korf Hydraulics from www.korf.co.uk and use it to solve this problem.
Email me if you are interested in the equations used in the program.
Cilliers (please note I am involved with Korf).
