What is the difference between API 520 and API 521?

API 521 (Pressure-relieving and Depressuring Systems) covers identifying and quantifying overpressure scenarios — determining which relief case governs and what relief load it produces. API 520 (Sizing, Selection, and Installation of Pressure-relieving Devices) covers how to size and select the actual relief device — orifice area, valve type, and installation details — once the governing relief load is known. In practice the two are used together: 521 tells you how much to relieve, 520 tells you what valve does it.

What is the difference between set pressure, overpressure, and accumulation?

Set pressure is the inlet pressure at which the valve begins to open. Overpressure is the increase in vessel pressure above set pressure while the valve is relieving, expressed as a percentage of set pressure (commonly 10% for a single valve on fire/other cases, up to 21% for fire-only cases per code). Accumulation is the resulting maximum vessel pressure during relief, expressed as a percentage above the vessel’s maximum allowable working pressure (MAWP) — code limits accumulation to 10-21% of MAWP depending on the scenario.

Which relief scenario usually governs the required relief rate?

It depends on the vessel and process, but external fire exposure is very often the governing (largest) case for vessels containing volatile liquids, because the API 521 fire heat-input equation scales with wetted surface area and can produce very large vapor-generation rates. Blocked outlet, control valve failure, and thermal expansion of trapped liquid must all still be checked — the largest calculated relief load across every credible scenario sets the required capacity.

Why does backpressure matter for relief valve selection?

Backpressure is the pressure on the outlet (discharge) side of the valve, from the header or downstream piping. Excessive backpressure on a conventional spring-loaded valve can reduce its lift and capacity, or cause chatter. Built-up backpressure above about 10% of set pressure typically requires a balanced-bellows valve (which shields the disc from backpressure effects) or a pilot-operated relief valve, both of which maintain rated capacity and stable operation at much higher backpressures.

How is a relief valve orifice area calculated?

API 520 gives standardized sizing equations for gas/vapor, liquid, and two-phase/steam relief, each a function of the required mass flow (from the governing scenario), the relieving pressure, fluid properties (molecular weight, temperature, compressibility, or specific gravity/viscosity for liquids), and empirically derived coefficients of discharge. The calculated area is then rounded up to the next-larger standard API orifice letter designation (D through T), never down.

Engineering·13 min read·July 12, 2026

🚨 Pressure Relief Valve (PSV) Sizing

A working-engineer guide to pressure relief valve sizing per API 520/521 — relief scenarios (fire, blocked outlet, thermal expansion), orifice sizing calculations, and set pressure vs. backpressure.

📖Chemical & Process Engineering Handbook (Full Access)

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Why Overpressure Protection Exists

Every pressure vessel, pipeline, and piece of process equipment has a maximum allowable working pressure (MAWP) beyond which it is not guaranteed to hold. A pressure relief valve (PRV or PSV) is the last independent line of defense: a purely mechanical device that opens automatically when pressure exceeds a preset limit, vents the excess fluid to a safe location, and recloses once pressure returns to normal. Sizing this device correctly — neither too small (it won't protect the vessel) nor needlessly oversized (it chatters, wastes flare capacity, and costs more) — is one of the most consequence-heavy calculations a process engineer performs. Get it wrong in the unsafe direction and a vessel can rupture catastrophically; that possibility is why relief system design is governed tightly by industry codes.

Topics covered

pressure relief valvePSV sizingAPI 520API 521relief valve sizingrelief scenariosfire caseblocked outletthermal expansion relieforifice sizingset pressurebackpressureaccumulationoverpressure protectionrupture disk