Summary:
- A comprehensive engineering guide to pressure relief valve (PRV) sizing and selection — covering the overpressure scenarios that drive PRV design, the sizing equations for liquid, gas, and steam service, and the critical importance of correct set pressure selection.
- Learn the difference between pressure relief valves, safety valves, safety relief valves, and pilot-operated relief valves, and understand when each type is the appropriate specification choice for different process applications.
- Understand the regulatory requirements for PRV design, installation, testing, and documentation under ASME Section VIII, API 520/521, and European PED requirements that govern pressure relief systems in industrial plants.
Introduction: The Last Line of Defence
Of all the valves in an industrial plant, the pressure relief valve is arguably the most important — not because it works every day, but because the day it’s needed is the day everything depends on it. A PRV is the last line of defence against overpressure events that could otherwise result in catastrophic failure of pressure vessels, heat exchangers, reactors, and pipelines. The consequences of a vessel failing due to overpressure — the term “explosion” is not an exaggeration — are measured in lives, not just equipment.
Despite their critical importance, pressure relief valves are among the most frequently misunderstood and improperly specified components in process plants. Incorrect set pressure selection, wrong sizing, inappropriate type for the service, inadequate installation, and poor maintenance are all common problems that can prevent a PRV from performing its life-saving function when it’s actually needed.
Types of Pressure Relief Devices
The family of pressure relief devices includes several distinct types, each with different operating principles and applications. Understanding the distinctions is fundamental to proper specification.
A safety valve is the term traditionally used for a direct spring-loaded pressure relief valve used in gas or steam service. Safety valves open rapidly (pop open) at the set pressure and provide a large area for relief flow. They are characterised by a “blowdown” — the difference between the set pressure (at which the valve opens) and the reseat pressure (at which it closes). API Standard 526 and ASME PTC 25 govern safety valve design and performance. Safety valves for steam service must meet specific requirements including steam-tightness at operating pressure (typically up to 90% of set pressure without leakage).
A pressure relief valve opens proportionally to the overpressure above the set point and is used primarily in liquid service. Unlike a safety valve, a PRV doesn’t pop fully open at set pressure — it opens progressively as pressure rises above set pressure, achieving full lift at typically 10-25% above set pressure (the “accumulation”). This proportional opening characteristic is appropriate for incompressible liquid service where the vessel pressure rise rate is more gradual than in gas service.
A safety relief valve is designed to perform as either a safety valve (in vapour service) or a pressure relief valve (in liquid service) and is the most versatile and widely used type in process plant applications. Most process plant PRVs are technically safety relief valves, though the terms are often used interchangeably in practice.
A pilot-operated relief valve (PORV) uses a small pilot valve to sense system pressure and control the opening of a much larger main valve. The main valve remains fully closed (seat-loaded) up to set pressure, providing superior seat tightness compared to direct-spring valves. At set pressure, the pilot opens, relieving the loading pressure from the main valve, which then opens fully to provide the relief capacity. Pilot-operated relief valves are used where very tight shutoff below set pressure is required, where the operating pressure is close to the set pressure (high backpressure ratio), and in large capacity applications where a direct-spring valve would require an impractically large spring.
A rupture disc (or bursting disc) is not a valve at all — it’s a one-time-use pressure relief device that bursts at a predetermined differential pressure. Unlike a PRV, a rupture disc cannot reclose after relieving. It is used in applications requiring leak-free service up to set pressure (PRVs always have some minor leakage potential), in applications involving slurries or materials that would foul a PRV, as a backup protection behind a PRV, and in applications where the PRV would be damaged by the relieved fluid.
Overpressure Scenarios: What Are You Protecting Against?
Every PRV installation must be justified by a specific overpressure scenario — a credible sequence of events that could cause the system pressure to rise above the maximum allowable working pressure (MAWP) without the PRV. API 521 provides comprehensive guidance on identifying and evaluating overpressure scenarios for process plant applications.
Common overpressure scenarios include: blocked outlet (a downstream valve is inadvertently closed while flow continues into a vessel); external fire (heat input from a fire causes liquid to vaporise, generating vapour pressure); cooling water failure (a condenser or cooler loses cooling water, causing vapour pressure to rise); heat exchanger tube failure (high-pressure fluid from one side of a heat exchanger breaks into the lower-pressure side); control valve failure (a control valve fails in the open position, allowing high-pressure flow into a lower-pressure system); thermal expansion (liquid trapped in a piping segment by closed isolation valves expands on heating, generating very high pressure in the confined volume); and runaway reaction (an exothermic chemical reaction accelerates beyond control, generating heat and gas faster than can be managed by normal process control).
Each scenario generates a different required relief capacity, and the PRV must be sized for the worst-case scenario (the one requiring the largest relief flow). It is critically important that all credible scenarios are identified — missing an overpressure scenario in the design analysis leaves the vessel unprotected against that specific event.
PRV Sizing: The Engineering Calculation
PRV sizing is the calculation of the required orifice area to pass the design relief flow at the accumulation pressure. The sizing equations are standardised in API 520 Part I (for sizing) and ASME Section VIII (for code-compliant design) and depend on the fluid type and flow regime.
For liquid service, the sizing equation is derived from the basic orifice flow equation: A = Q / (38 × Kd × Kw × Kc × Kv × √(ΔP/G)), where A is the required orifice area in square inches, Q is the flow in US gallons per minute, Kd is the discharge coefficient (typically 0.65 for liquid service), Kw is the back pressure correction factor, Kc is the combination correction factor (for rupture disc/PRV combinations), Kv is the viscosity correction factor, ΔP is the differential pressure across the valve, and G is the specific gravity relative to water. Getting each of these correction factors right is important — errors in input data propagate directly to errors in required orifice area.
For vapour and gas service, the sizing equation is more complex because compressible flow behaviour must be accounted for. The critical flow (choked flow) condition, where the flow velocity at the orifice reaches sonic velocity and further increasing the pressure differential doesn’t increase the flow, must be checked. API 520 provides different equations for critical and sub-critical flow conditions.
For steam service, dedicated steam sizing equations apply, using steam properties (specific volume, isentropic exponent) at the relieving conditions. Steam tables and manufacturer’s sizing software are essential tools for accurate steam PRV sizing.
Set Pressure Selection: Getting It Right
The set pressure — the pressure at which the PRV begins to open — must be set at or below the vessel’s MAWP. The relationship between operating pressure, set pressure, and MAWP must be carefully managed to ensure that the PRV provides protection without opening spuriously during normal plant operation.
The typical guidance is that the set pressure should be at least 10% above the normal operating pressure to provide adequate margin against spurious opening. A PRV that opens frequently due to operating pressure being too close to set pressure will experience rapid seat wear and may “simmer” (partial, intermittent lifting) that also damages the seats. Industry practice suggests a minimum 10% differential between normal operating pressure and set pressure for spring-loaded valves, and this differential can be reduced to 5% for pilot-operated valves which have much better seat tightness.
Installation Requirements: Getting the Environment Right
Even a correctly sized and specified PRV will fail to perform if it’s improperly installed. The installation requirements for PRVs are specified in API 520 Part II and ASME Section VIII, and must be followed rigorously.
The inlet pipe to a PRV must be sized so that the pressure drop between the protected vessel and the PRV inlet does not exceed 3% of the set pressure at the design flow rate. Excessive inlet pressure drop reduces the effective relieving capacity of the PRV and can cause instability (chatter). A common — and serious — mistake is to install a PRV at the end of a long, small-diameter inlet pipe to route it to a convenient location.
The discharge pipe from a PRV must be sized to limit the back pressure at the PRV outlet to acceptable levels. For conventional spring-loaded PRVs, back pressure above 10% of set pressure begins to reduce the relieving capacity and may affect the set pressure. Balanced-bellows or pilot-operated PRVs can tolerate higher back pressures. The discharge piping must be routed to a safe discharge location — typically a flare or vent system for flammable or toxic fluids, or an appropriate collection vessel.
Maintenance and Testing of PRVs
Pressure relief valves require regular inspection and testing to verify that they will open at the correct set pressure and provide the required relieving capacity. The required inspection frequency depends on the service — typically every 5 years for clean, non-corrosive services and more frequently (annually or at each plant turnaround) for corrosive, fouling, or high-cycling applications.
PRV testing is carried out by removing the valve from service and bench-testing it at a certified PRV testing facility. The valve is pressurised to verify that it opens at (or within a specified tolerance of) its set pressure, closes cleanly at the reseat pressure, and doesn’t leak at normal operating pressure (typically 90% of set pressure for gas service). Any valve that fails these tests must be reconditioned or replaced before returning to service.
Conclusion: PRVs Demand Engineering Rigour
Pressure relief valves are not something you can pick from a catalogue without proper engineering analysis. They require careful identification of the overpressure scenario, rigorous sizing calculations, appropriate type and material selection, correct installation, and a documented inspection and testing programme. The regulatory requirements that govern PRV design are not bureaucratic formalities — they reflect hard-won lessons about what happens when pressure systems fail without adequate protection. Treat PRVs with the engineering rigour they deserve.