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# Safety Relief Valve Critical Flow

## Safety Relief Valve Critical Flow

(OP)
I often see .critical flow' and 'sub-critical flow' terms when reading on safety relief valve sizing. What do these terms actually mean and how are they related to the sizing formulas in API-520.

### RE: Safety Relief Valve Critical Flow

Critical flow usually refers to flow that has reached Mach 1 and is limited, or choked.

Good Luck,
Latexman

### RE: Safety Relief Valve Critical Flow

https://en.wikipedia.org/wiki/Choked_flow

From WIKIPEDIA:

Choked flow is a fluid dynamic condition associated with the venturi effect. When a flowing fluid at a given pressure and temperature passes through a constriction (such as the throat of a convergent-divergent nozzle or a valve in a pipe) into a lower pressure environment the fluid velocity increases. At initially subsonic upstream conditions, the conservation of mass principle requires the fluid velocity to increase as it flows through the smaller cross-sectional area of the constriction. At the same time, the venturi effect causes the static pressure, and therefore the density, to decrease at the constriction. Choked flow is a limiting condition where the mass flow will not increase with a further decrease in the downstream pressure environment for a fixed upstream pressure and temperature.

MJCronin
Sr. Process Engineer

### RE: Safety Relief Valve Critical Flow

Thanks for the info.

Can I take it that API-520 is only for critical flow?

Is pressure relative to critical flow? ie., at what pressure does floe become critical

How would you size a valve that has been determined as sub-critical flow?

Does critical flow apply to all fluids?

Thanks again.

*** Per ISO-4126, the generic term
'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

### RE: Safety Relief Valve Critical Flow

The equation P[sub]c= P[sub]1(2/(k + 1)[sup]k[/(k - 1) of API-520 gives the critical pressure at the valve nozzle exit. For this pressure, the flow is critical and it is the maximum flow that the valve can discharge for P[sub]1.
If the back pressure at the valve exit is greater than the critical pressure, the pressure at the nozzle exit is approximately the same and greater than the critical pressure, therefore the flow is not critical and less than before.
To size the valve,as the flow to be discharged is a known data, select a value of A (nozzle area) and apply some of the equations of API-520 that relates the upsream pressure, the flow rate and the nozzle area until obtain a flow equal or greater than the required flow.
Critical flow applies to commpressible fluids as steam and gases and also to saturated fluids as for example saturated water.

### RE: Safety Relief Valve Critical Flow

(OP)
Thanks all - Great info.

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