Pressure Relief & Safety Valves: API 520/526 Types, Sizing & Selection

Quick Answer

A pressure relief valve (PRV/PSV) is the last line of defence against overpressure: it opens automatically at a predetermined set pressure, flows a rated capacity at a specified overpressure, and recloses when pressure returns to a safe level. Three types exist, and backpressure decides which you can use. Conventional spring-loaded valves are simplest and cheapest but tolerate only up to about 10% backpressure — above that the set pressure shifts and capacity collapses. Balanced bellows valves use a metal bellows to isolate the spring chamber from backpressure, handling roughly 10–50% (some sources up to 30–50% superimposed); they require a bonnet vent that must never be connected to a pressurised system, because discharge from that vent signals bellows failure. Pilot-operated (POSV) valves use process pressure itself to hold the main valve shut, tolerating backpressure up to about 90%, allowing operation very close to set pressure (above 90% of MAWP) with tight shutoff, and suiting large orifices. Sizing follows API 520 Part I (flow equations for gas, liquid, steam, two-phase; Part II covers installation), the relieving scenarios come from API 521 (blocked outlet, fire case, thermal expansion, gas blowby, runaway reaction), and API 526 standardises flanged steel valve dimensions and the orifice letter designations D through T. Typical allowable overpressure is 10% for a single device, 16% for multiple valves, and 21% for the fire case.

Every pressurised system in a plant has a worst case. A control valve fails closed and blocks the outlet. A heat exchanger tube ruptures and high-pressure fluid enters the low-pressure side. A fire engulfs a vessel and boils its contents. A reaction runs away. In each case, pressure climbs toward — and past — the equipment's design limit, and the only thing standing between that and a rupture is a pressure relief valve.

This is why relief valves are engineered and documented differently from every other valve in a plant. A gate valve that fails simply stops isolating. A relief valve that fails, or that was sized for the wrong scenario, allows a vessel to burst. The design work is not really valve selection at all — it is scenario analysis: identifying every credible way the system can be overpressured, calculating the relief load each one generates, and sizing for the governing case.

The second thing engineers get wrong is backpressure. A relief valve is not a standalone device; it discharges into piping, often a shared flare header. The pressure in that header pushes back on the valve, and if the valve type cannot tolerate it, the valve opens late, chatters, or delivers a fraction of its rated capacity — while the calculation on the datasheet says everything is fine.

For process and mechanical engineers, safety practitioners, and procurement teams — this guide covers pressure relief and safety valves: the three types and when each applies, the API standards framework, set pressure and blowdown terminology, sizing basics, and the mistakes that cause relief systems to fail when needed.

For related equipment, see Industrial Valves Guide and Chemical Factory Setup.

Terminology

Getting these right matters, because they are frequently confused:

Safety valve vs relief valve: conventionally, a "safety valve" pops fully open (used for compressible fluids/steam) while a "relief valve" opens proportionally (used for liquids). "Safety relief valve" covers both. In common industrial usage PRV and PSV are used interchangeably.

Typical Allowable Overpressure

• 10% — single relief device, standard code applications
• 16% — multiple relief valve installations, and pilot-operated valves in overpressure-only service
• 21% — fire case exposure
• 3% or 5% — steam boiler service (ASME Section I), much tighter
• 10% or 25% — common for liquids, with blowdown values up to 20%

The Three Valve Types

Conventional Spring-Loaded

A direct spring force holds the disc against the seat. The valve opens when inlet pressure exceeds the spring force.

• Simplest and lowest cost, reliable, widely used
• The spring bonnet vents to the discharge side, so backpressure acts on the disc and directly affects opening pressure, closing pressure, and capacity
• Limit: no more than about 10% backpressure as a percentage of set pressure
• Above that limit the set pressure shifts dangerously and capacity plummets

Use when: backpressure is low and stable, and the service is straightforward.

Balanced Bellows

A metal bellows around the stem isolates the spring chamber from backpressure, so backpressure no longer acts on the effective disc area.

• Handles superimposed backpressure up to roughly 30–50% of set pressure without shifting the opening point (commonly cited working range 10–50%)
• Also protects the spring and bonnet internals from corrosive process fluid
• Requires a backpressure correction factor (Kb for vapour, Kw for liquid) in the sizing calculation — this applies to bellows valves; conventional and pilot-operated use Kb = 1.0
• Typical Kd flow coefficient around 0.885 for bellows valves vs 0.975 for conventional direct-spring valves at 10% overpressure

The bellows is a wear item. It is a thin-walled metal element subject to fatigue and corrosion. If it ruptures, the valve reverts to conventional behaviour and backpressure acts on the disc again.

API 526 requires a bonnet vent on balanced bellows valves. Two rules follow:

1. Never connect the bonnet vent to a pressurised system — doing so defeats the balancing
2. If fluid discharges from the vent, the bellows has failed and must be replaced immediately

Use when: backpressure is variable or exceeds 10%, or the process fluid would corrode the spring chamber.

Pilot-Operated (POSV)

A fundamentally different approach: the process pressure itself holds the main valve closed, and a small pilot valve triggers opening.

The main valve has a piston or diaphragm with process pressure applied to both sides. Because the dome (top) area is larger than the nozzle (bottom) area, there is a slight net closing force and the valve stays shut. When system pressure reaches the setpoint, the pilot (a small spring-loaded valve) opens and vents pressure from above the piston — the net force reverses and the main valve opens.

• Tolerates backpressure up to about 90%
• Allows operation very close to set pressure (above 90% of MAWP) with tight shutoff — no simmer or seat leakage as set pressure is approached
• Suits large orifice requirements
• More complex; the pilot line and sensing arrangement must be protected from plugging and freezing

Use when: operating pressure sits close to set pressure and tight shutoff is needed; backpressure is high or highly variable; or a large orifice is required.

Type Selection by Backpressure

The API Standards Framework

Three standards work together, and knowing which does what saves confusion:

API 520 — Sizing, Selection and Installation

• Part I: the sizing calculations — governing flow equations for vapour/gas (critical and subcritical flow), liquid (with viscosity correction), steam, and two-phase service; also the selection logic between conventional, bellows, and pilot-operated types based on backpressure
• Part II: installation requirements — inlet and outlet piping, manifolds, discharge systems

API 521 — Pressure-Relieving and Depressuring Systems

• Defines the relieving scenarios to evaluate: blocked outlet, fire case exposure, thermal expansion, gas blowby (e.g. control valve failure), runaway reaction, and others
• Each scenario produces a different relief load; the governing case determines valve size
• Contains the fire-case heat input equations (adequate drainage, inadequate drainage, confined fire)

API 526 — Flanged Steel Pressure Relief Valves

• Standardises dimensions, inlet/outlet sizes, materials, pressure-temperature ratings
• Defines orifice letter designations D through T, each with a standardised effective area

In practice: use API 521 to identify scenarios and relief loads → API 520 Part I to calculate the required effective orifice area → API 526 to select the standard valve whose orifice meets or exceeds that area.

Note that API 526 orifices are standard sizes, not requirements — you may specify a certified non-standard area where appropriate. API 526 also does not cover pilot-operated valve dimensions, and steam boiler safety valves fall under ASME Section I rather than this framework.

Sizing Basics

The sequence:

1. Identify all credible relieving scenarios (API 521)
2. Calculate relief load for each — mass or volumetric flow to be discharged
3. Determine the governing case — usually the largest load, but check each
4. Establish relieving conditions — pressure, temperature, and fluid properties at relieving conditions, not normal operating conditions (a frequent error)
5. Apply the correct flow equation for the phase (gas critical/subcritical, liquid, steam, two-phase)
6. Apply coefficients — discharge coefficient Kd, backpressure correction Kb (vapour) or Kw (liquid), combination factor Kc where a rupture disc is installed upstream
7. Calculate required effective orifice area
8. Select the next standard API 526 orifice (letter D–T)
9. Verify against ASME Section VIII for UV-stamped applications

Two-phase flashing flow requires specialised correlations from API 521 Appendix C or vendor methods — the standard single-phase equations do not apply.

Rupture disc combinations: where a disc is installed upstream of the PRV, a combination capacity factor applies (commonly 0.9, or 0.62 where sizing is for a rupture disc alone under specific conditions) — check the applicable edition.

Construction and Materials

Nozzle type:

• Semi-nozzle — works well for non-toxic, non-corrosive media at moderate pressures
• Full nozzle — suitable for corrosive media and extremely high pressures; standard in the process industry

Materials:

• Body: ASTM A216 WCB (standard carbon steel), A351 CF8M (stainless), bronze for low-pressure service, LCB/LCC for low temperature
• Trim: 316 stainless standard; Stellite 6-faced seats for high-temperature steam or corrosive service
• Special materials for corrosive fluids or high temperatures — Hastelloy, Inconel, Monel

Options: open or closed bonnet, lifting lever (required for some steam and air services), test gag, soft seats for tight shutoff, heat/steam jacketing.

Installation Requirements (API 520 Part II)

• Inlet piping pressure drop should not exceed 3% of set pressure at rated flow — excessive inlet loss causes chatter, which destroys the valve and the seat within minutes
• Discharge piping must be sized and supported for reaction forces and thermal expansion
• No isolation valve between the equipment and the PRV unless administratively controlled (car-sealed open, or a full-capacity spare in a changeover arrangement)
• Drainage — discharge piping should self-drain; liquid accumulation causes hydraulic hammer on relief
• Bonnet vent on bellows valves must discharge to a safe location and never be plugged or connected to pressure

Common Specification Mistakes

After 15+ years supplying valves and pressure equipment to industrial projects:

Mistake 1: Conventional Valve on a High-Backpressure Service

Conventional PRV installed on a shared flare header where backpressure exceeds 10%. The set pressure shifts and capacity drops — the valve cannot protect the equipment.

Prevention: Calculate total backpressure (superimposed + built-up). Above 10%, specify balanced bellows; above ~50%, pilot-operated.

Mistake 2: Sizing at Normal Operating Conditions

Fluid properties taken at normal operating temperature and pressure instead of relieving conditions. The valve is undersized for the actual relief event.

Prevention: Always evaluate fluid properties at relieving pressure and temperature for the governing scenario.

Mistake 3: Missing the Governing Scenario

Only blocked outlet considered; the fire case (which often produces a much larger load) is never evaluated.

Prevention: Systematically evaluate every API 521 scenario — blocked outlet, fire, thermal expansion, gas blowby, runaway reaction, tube rupture, utility failure.

Mistake 4: Excessive Inlet Pressure Drop

Long, small-bore inlet piping to the PRV. Inlet loss exceeds 3% of set pressure; the valve chatters and destroys its seat.

Prevention: Keep inlet piping short, straight, and full-bore. Verify inlet pressure drop is under 3% at rated flow.

Mistake 5: Bellows Bonnet Vent Piped to a Pressurised System

Bonnet vent connected into the flare header "to avoid emissions". Backpressure now acts on the bellows and the balancing is defeated.

Prevention: Vent to atmosphere at a safe location. Never connect to a pressurised system. Monitor the vent — discharge means the bellows has failed.

Mistake 6: Ignoring Two-Phase Flow

Standard single-phase gas equation used for a flashing liquid relief. The valve is badly undersized.

Prevention: Use two-phase correlations (API 521 Appendix C or vendor methods) where flashing or two-phase flow is credible.

Mistake 7: No Relief on a Positive Displacement Pump

A PD pump installed without discharge relief. A closed valve causes pressure to climb until piping or pump fails.

Prevention: Every PD pump discharge needs a relief valve sized for full pump capacity — see Positive Displacement Pumps.

Supply from Kasko Makine

Kasko Makine supplies pressure relief and safety valves for oil & gas, petrochemical, power, and process applications:

Valve types:

• Conventional spring-loaded safety relief valves
• Balanced bellows safety relief valves
• Pilot-operated safety relief valves (POSV)
• Steam safety valves (ASME Section I)
• Thermal relief valves
• Vacuum relief and breather valves
• Rupture discs and PRV/disc combinations

Standards: API 520 / 521 / 526, ASME Section VIII (UV stamp), ASME Section I for boiler service, PED for European projects

Sizes and orifices: API 526 orifice designations D through T, flanged inlet/outlet per API 526 dimensions, plus certified non-standard areas where required

Materials:

• Body: A216 WCB, A351 CF8M, LCB/LCC low temperature, bronze
• Trim: 316 stainless, Stellite 6-faced seats, special alloys (Hastelloy, Inconel, Monel)
• Full nozzle and semi-nozzle construction
• Soft seats for tight shutoff, steam jacketing, lifting levers, test gags

Engineering support:

• Relieving scenario review (API 521)
• Orifice sizing per API 520 Part I including backpressure correction
• Valve type selection based on total backpressure
• Inlet/outlet piping review against the 3% inlet loss rule
• Material selection for corrosive and high-temperature service
• Documentation for authority and third-party approval

Certification: EN 10204 Type 3.1/3.2, set pressure test certificates, ASME UV stamp where applicable, capacity certification, NACE MR0175 for sour service, third-party inspection available

Logistics: Relief valves shipped from Istanbul to projects across Africa, the Middle East, Central Asia, and beyond. Standard valves 6-10 weeks; pilot-operated and special alloys 12-20 weeks.

Need pressure relief valves? Send us the relieving scenario and load, set pressure, superimposed and built-up backpressure, fluid and phase (gas, liquid, steam, two-phase), relieving temperature, and required code stamp to info@kaskomakine.com or WhatsApp +90 (537) 521 1399. We'll verify the orifice size, recommend the valve type for your backpressure, specify materials, and provide a quotation with certification within 48 hours.

Continue Reading: Valve & Equipment Guides

• Industrial Valves Guide — All valve types and how to choose
• Globe Valve vs Gate Valve — Isolation and throttling
• Check Valves — Backflow protection
• Chemical Factory Setup — Plant design and protected equipment
• Positive Displacement Pumps — Mandatory discharge relief

Frequently Asked Questions

Q: What is a pressure relief valve?
A: A pressure relief valve (PRV, also called a pressure safety valve or PSV) is a safety device that protects equipment and piping from overpressure. It opens automatically at a predetermined set pressure, flows a rated capacity at a specified overpressure, and recloses when system pressure returns to a safe level. It is the last line of defence against scenarios such as blocked outlet, external fire, thermal expansion, gas blowby from control valve failure, tube rupture, and runaway reaction. Three types exist: conventional spring-loaded, balanced bellows, and pilot-operated, with the choice driven primarily by how much backpressure exists in the discharge system. Sizing follows API 520, scenario definition follows API 521, and standard dimensions and orifice designations follow API 526.

Q: What is the difference between conventional, balanced bellows, and pilot-operated relief valves?
A: A conventional spring-loaded valve uses direct spring force on the disc — it is simplest and cheapest but tolerates only about 10% backpressure, above which set pressure shifts and capacity collapses. A balanced bellows valve adds a metal bellows around the stem that isolates the spring chamber from backpressure, handling roughly 10–50% backpressure while also protecting internals from corrosive fluid; it requires a bonnet vent that must never be connected to a pressurised system, and discharge from that vent indicates bellows failure. A pilot-operated valve uses process pressure itself to hold the main valve shut with a small pilot triggering opening — it tolerates backpressure up to about 90%, allows operation above 90% of MAWP with tight shutoff, and suits large orifices.

Q: What is the difference between API 520, API 521 and API 526?
A: The three standards work together. API 521 defines the relieving scenarios to evaluate — blocked outlet, fire case exposure, thermal expansion, gas blowby, runaway reaction — and provides the fire-case heat input equations; each scenario generates a different relief load and the governing case determines valve size. API 520 Part I contains the sizing calculations, with flow equations for vapour/gas (critical and subcritical), liquid (with viscosity correction), steam and two-phase service, plus the selection logic between valve types based on backpressure; Part II covers installation requirements including inlet and outlet piping. API 526 standardises flanged steel relief valve dimensions, materials, pressure-temperature ratings, and the orifice letter designations D through T. In practice you use API 521 for scenarios, API 520 to calculate required area, and API 526 to select the standard valve.

Q: How does backpressure affect relief valve selection?
A: Backpressure is the pressure in the discharge piping, made up of superimposed backpressure (present before the valve opens) and built-up backpressure (created by the relieving flow itself). It determines which valve type is acceptable. Conventional spring-loaded valves are limited to about 10% backpressure as a percentage of set pressure — beyond that, set pressure shifts dangerously and capacity plummets. Balanced bellows valves tolerate roughly 10–50% because the bellows isolates the spring chamber, though sizing must apply a backpressure correction factor (Kb for vapour, Kw for liquid). Pilot-operated valves tolerate up to about 90%. Note that built-up backpressure does not affect when a conventional valve opens (it is zero before opening) but does reduce flow capacity during relief.

Q: What are API 526 orifice designations?
A: API 526 defines standard orifice letter designations from D through T, each corresponding to a standardised effective flow area, together with standard inlet and outlet flange sizes, face-to-face dimensions, materials, and pressure-temperature ratings for flanged steel pressure relief valves. In practice, you calculate the required effective orifice area using the API 520 Part I equations for your governing scenario, then select the next larger standard API 526 orifice letter. Note that these are standard sizes rather than mandatory requirements — a certified non-standard area can be specified where standard orifices are insufficient or excessive, such as two-phase flashing flow, high-backpressure applications with large correction factors, or steam service above 500 psig.

Q: What allowable overpressure is used for relief valve sizing?
A: Standard allowable overpressure is 10% of set pressure for a single relief device in code applications. Multiple relief valve installations allow 16%, as do pilot-operated valves in overpressure-only service. Fire case exposure allows 21%, reflecting the accepted risk basis for that scenario. Steam boiler service under ASME Section I is much tighter, typically 3% or 5%. For liquids, common overpressures are 10% or 25% with blowdown values up to 20%. The relieving pressure used in sizing calculations is the set pressure plus the allowable overpressure plus atmospheric pressure, and fluid properties must be evaluated at those relieving conditions rather than at normal operating conditions — using normal operating properties is a common and serious sizing error.