How is the relieving pressure calculated?

P₁ = Pset·(1 + accumulation) + atmospheric. The accumulation (overpressure) is 10% for a single non-fire process device, 16% for multiple devices, and 21% for the fire case. The calculator builds P₁ from the gauge set pressure and the scenario you pick — you do not enter P₁ directly.

When is relief vapour flow critical (choked)?

For a conventional spring-loaded valve, API 520 Part I §3.6 treats the flow as critical when the backpressure ratio P₂/P₁ ≤ 0.55. Critical flow uses Eq. 3 (area independent of backpressure); above 0.55 it is sub-critical and uses Eq. 4. Balanced-bellows and pilot-operated valves follow a different rule.

How are liquid and steam relief valves sized differently?

Liquid uses API 520 Eq. 27 with the discharge, backpressure, combination and viscosity corrections (Kd/Kw/Kc/Kv); the viscosity factor Kv is found from the Reynolds number. Steam uses the Napier equation (Eq. 9) with the high-pressure correction KN and the superheat correction KSH, the latter interpolated from API 520 Table 9.

How do I pick the API 526 orifice letter?

Take the required effective area and choose the smallest standard API 526 orifice (D through T) whose effective area meets it. The calculator returns that letter and its oversize margin, plus the next size up so you can weigh turndown or future duty. If the required area exceeds the T orifice, a single valve cannot pass the load.

What backpressure and inlet limits apply?

API 520 Part II limits the non-recoverable inlet loss to 3% of set pressure (to avoid chatter) and, for a conventional valve, the built-up backpressure to about 10% of set pressure; above that use a balanced-bellows or pilot-operated valve. The open-discharge reaction force must also be carried by the outlet piping.

How is the fire case handled?

API 521 estimates the fire heat input to a wetted vessel as Q = C·F·Aws^0.82 (C = 43 200 with adequate drainage and firefighting, 70 900 without; F is the environment factor). Dividing Q by the latent heat gives the relief vapour rate, which is then sized as a vapour case with the 21% fire accumulation.

Relief valve sizing per API 520 / 521 / 526

A pressure-relief valve is sized in three moves: build the relieving pressure, compute the required effective area from the service-specific API 520 equation, then read off the API 526 orifice letter. This guide walks each step, then hands off to the live calculator.

1. Relieving pressure

The valve relieves at P₁ = Pset·(1 + accumulation) + Patm. You enter the gauge set pressure and pick the scenario — process (10%), multiple devices (16%) or fire (21%) — and the calculator builds the absolute relieving pressure internally. This mirrors how a datasheet is actually filled in.

2. Required area by service

Vapour / gas — critical (Eq. 3) when P₂/P₁ ≤ 0.55, otherwise sub-critical (Eq. 4). The coefficient C(k) and the sub-critical F₂ follow from the heat-capacity ratio.
Liquid — Eq. 27, A = 11.78·Q·√(G/(P₁−P₂))/(Kd·Kw·Kc·Kv), with the viscosity factor Kv from the Reynolds number.
Steam — the Napier equation (Eq. 9) with the high-pressure correction KN and the superheat correction KSH interpolated from API 520 Table 9.

3. API 526 orifice selection

The required effective area is rounded up to the nearest standard API 526 orifice — the lettered series D, E, F, G, H, J, K, L, M, N, P, Q, R, T. The calculator shows the selected letter with its oversize margin and the next size up, so you can leave headroom for turndown or future duty without re-sizing.

Backpressure, reaction force and fire

API 520 Part II caps the inlet non-recoverable loss at 3% of set pressure and a conventional valve’s built-up backpressure at ~10%; above that, choose a balanced-bellows or pilot-operated valve. The open-discharge reaction force is reported for the outlet piping design. For an external fire, API 521’s Q = C·F·Aws^0.82 gives the heat input, and the relief vapour rate follows from the latent heat.

Ready to size one? Open the relief valve calculator, try the free sizing tool or read the methodology & validation.

Frequently asked questions

How is the relieving pressure calculated?: P₁ = Pset·(1 + accumulation) + atmospheric. The accumulation (overpressure) is 10% for a single non-fire process device, 16% for multiple devices, and 21% for the fire case. The calculator builds P₁ from the gauge set pressure and the scenario you pick — you do not enter P₁ directly.
When is relief vapour flow critical (choked)?: For a conventional spring-loaded valve, API 520 Part I §3.6 treats the flow as critical when the backpressure ratio P₂/P₁ ≤ 0.55. Critical flow uses Eq. 3 (area independent of backpressure); above 0.55 it is sub-critical and uses Eq. 4. Balanced-bellows and pilot-operated valves follow a different rule.
How are liquid and steam relief valves sized differently?: Liquid uses API 520 Eq. 27 with the discharge, backpressure, combination and viscosity corrections (Kd/Kw/Kc/Kv); the viscosity factor Kv is found from the Reynolds number. Steam uses the Napier equation (Eq. 9) with the high-pressure correction KN and the superheat correction KSH, the latter interpolated from API 520 Table 9.
How do I pick the API 526 orifice letter?: Take the required effective area and choose the smallest standard API 526 orifice (D through T) whose effective area meets it. The calculator returns that letter and its oversize margin, plus the next size up so you can weigh turndown or future duty. If the required area exceeds the T orifice, a single valve cannot pass the load.
What backpressure and inlet limits apply?: API 520 Part II limits the non-recoverable inlet loss to 3% of set pressure (to avoid chatter) and, for a conventional valve, the built-up backpressure to about 10% of set pressure; above that use a balanced-bellows or pilot-operated valve. The open-discharge reaction force must also be carried by the outlet piping.
How is the fire case handled?: API 521 estimates the fire heat input to a wetted vessel as Q = C·F·Aws^0.82 (C = 43 200 with adequate drainage and firefighting, 70 900 without; F is the environment factor). Dividing Q by the latent heat gives the relief vapour rate, which is then sized as a vapour case with the 21% fire accumulation.