How do you size a pressure relief valve?

First identify the credible overpressure scenario, such as blocked outlet, external fire or control valve failure, and calculate the required relief load in mass per unit time. Then determine the relieving pressure, which is the set pressure plus the allowable accumulation. Using the API 520 equation for the fluid phase and flow regime, calculate the required effective orifice area, applying the discharge coefficient and the backpressure and other correction factors. Finally, select the next larger standard API 526 orifice.

What is the difference between API 520 and API 521?

API 521 identifies the overpressure scenarios and calculates the required relief loads, including the external fire case and thermal expansion. API 520 takes those loads and sizes the relief device, giving the equations for the required orifice area for gas, liquid, steam and two-phase flow. API 526 then defines the standard orifice letters and dimensions used to specify a real valve.

What is the governing relief case?

A vessel can be exposed to several credible overpressure scenarios, each producing a different required relief load. The governing case is the one that demands the largest relief area, because the relief device must protect against the worst credible single event. This calculator evaluates each scenario and identifies the governing case automatically.

How is the fire relief load calculated?

For an external pool fire, API 521 gives the absorbed heat as a constant times an environmental factor times the wetted surface area raised to the power 0.82. The constant is 43200 in SI units with adequate drainage and firefighting, or 70900 without. The relief load is then the absorbed heat divided by the latent heat of vaporisation of the fluid, which gives the mass rate of vapour that must be relieved.

What is critical or choked flow in relief valve sizing?

When the ratio of downstream to upstream absolute pressure falls below a critical value, the gas reaches sonic velocity at the nozzle throat and the flow chokes - it will not increase with further pressure drop. The critical pressure ratio is two divided by k plus one, all raised to the power k over k minus one, about 0.53 for air. Above the critical ratio the flow is subcritical and a different equation applies.

What is an API 526 orifice letter?

API 526 defines a set of standard effective orifice areas identified by letters from D, the smallest at 71 square millimetres, to T, the largest at about 16774 square millimetres. After the required area is calculated, the valve is specified by choosing the next larger standard orifice letter, so that the installed capacity meets or exceeds the requirement.

Can this PSV calculator be used for final design?

No. This tool is preliminary and screening grade, intended for study, checking and design support. Final relief valve sizing must use manufacturer-certified discharge coefficients and be carried out and verified by a qualified engineer to the current edition of the governing code, because relief system design is safety critical. Two-phase relief in particular should follow a full DIERS analysis.

How do you size for two-phase relief?

Two-phase relief, where a mixture of liquid and vapour is discharged, is sized using the DIERS omega method or an equivalent, which accounts for flashing as the pressure drops through the valve. It requires the fluid physical properties at relieving conditions and can give a much larger required area than a single-phase calculation. This calculator provides a screening estimate; a full DIERS analysis by a qualified engineer is required for design.

AIEnginear^TM · PSV Relief Load Calculator

Name: AIEnginear PSV Relief Load Calculator Ultimate
Author: AIEnginear

Ultimate · Professional Engineering Calculation Suite