Safety & Pressure Relief Valves: Types, Sizing & Standards (API 520/526)

Quick Reference

Safety valves open automatically at set pressure to prevent overpressure (explosions, equipment damage). PRVs open proportionally via controller. Types: spring-loaded (most common), pilot-operated (large relief capacity), dead-weight (test/calibration). Standard: API 526. Installed on: pressure vessels, boilers, storage tanks, piping. Last-resort protection when other safeguards fail.

What Is a Safety Valve

A safety valve opens automatically and at full lift the moment system pressure exceeds the set pressure (in bars or psi). The disc lifts, fluid discharges, and pressure drops back to normal, all mechanically, with no actuators or external controls involved.

Once pressure falls below the reseat value, the valve snaps shut on its own.

Safety valves are installed on pressure vessels, boilers, gas storage tanks, and piping systems. The governing standards are API 526 and ASME BPVC Section VIII.

Safety Valve vs. Relief Valve vs. Safety Relief Valve

The terms get used interchangeably in the field, but the distinction matters for specification, sizing, and code compliance. ASME and API define three separate device types:

A safety valve (SV) opens fully and suddenly at set pressure, with no controller or operator input. This is the snap-action behavior you see on steam boilers and gas systems. The pop action results from the expanding compressible fluid pushing against a larger disc area once the initial seal breaks.

By contrast, a pressure relief valve (PRV) opens gradually, proportional to the overpressure. A pressure controller and actuating mechanism modulate the disc lift based on the overpressure signal. PRVs typically protect liquid-filled pressure vessels where proportional relief prevents water hammer and system upsets.

pressure relief valve

The safety relief valve (SRV) combines both behaviors: snap action on gas/steam, proportional on liquids. It is the most common device in refinery and petrochemical service.

Feature	Safety Valve (SV)	Relief Valve (PRV)	Safety Relief Valve (SRV)
Opening action	Pop (full lift at set pressure)	Proportional (gradual)	Pop on gas, proportional on liquid
Media	Compressible (gas, steam, vapor)	Incompressible (liquid)	Gas or liquid
Closing action	Snap shut (blowdown 7-10%)	Gradual close	Depends on service media
P&ID tag	PSV	PSV (industry convention)	PSV
ASME code symbol	V (safety)	UV (safety relief)	UV (safety relief)
Standards	API 526, ASME Sec. VIII	API 526, ASME Sec. VIII	API 526, ASME Sec. VIII
Typical applications	Boilers, steam drums, gas systems	Liquid pipelines, pump discharge	General process vessels, refinery equipment

Comparison of safety, relief, and safety relief valves

Per API 521, the pressure in a vessel can exceed the design limit from multiple causes: blocked discharge, chemical reaction, tube rupture, fire case (external fire exposure), thermal expansion, or cooling system failure. Each event creates a different required relief rate: small mass flow for thermal expansion, enormous mass flow for a runaway chemical reaction. The engineering team must identify the worst-case governing scenario before sizing the relief device.

Safety vs. Relief: A safety valve opens fully and suddenly at set pressure (no controller needed), while a pressure relief valve (PRV) opens proportionally using a pressure controller and actuator. A safety relief valve (SRV) combines both functions and is the standard device in most process plant applications. The engineering team must determine the worst-case overpressure scenario per API 521 to size the correct relief device.

Key Terminology

These terms appear on every PSV datasheet and in API 520/521/526.

Term	Definition	Typical Value
Set pressure	Inlet pressure at which the valve begins to open under service conditions	At or below MAWP of the protected equipment
Overpressure	Pressure increase above set pressure during full valve discharge	10% (single valve), 16% (multiple valves), 21% (fire case)
Accumulation	Pressure increase above MAWP allowed during a relief event	Same percentages as overpressure (these terms are related but not identical)
Blowdown	Difference between set pressure and reseat pressure	7-10% for gas/steam, up to 20% for liquids
Reseat pressure	Inlet pressure at which the valve closes after relieving	Set pressure minus blowdown
MAWP	Maximum Allowable Working Pressure of the protected equipment	Established per ASME Section VIII calculations
CDTP	Cold Differential Test Pressure, the bench test pressure adjusted for backpressure and temperature	CDTP = Set Pressure - Backpressure Correction + Temperature Correction
Relieving pressure	Set pressure + overpressure allowance + atmospheric pressure	Used in API 520 sizing calculations
Superimposed backpressure	Pressure at the valve outlet before it opens	From other valves discharging into a shared header
Built-up backpressure	Pressure generated by flow through discharge piping after the valve opens	Calculated from discharge pipe friction losses
Simmer	Audible leakage before the valve fully opens	Occurs at approximately 90-95% of set pressure on spring-loaded valves
Chatter	Rapid opening and closing of the valve disc	Caused by excessive inlet pressure drop (> 3% of set pressure) or oversized valve

Pro Tip

The distinction between accumulation and overpressure confuses many engineers. Accumulation is measured from MAWP; overpressure is measured from set pressure. When the set pressure equals MAWP (the most common case for a single valve), the two values are identical. When set pressure is lower than MAWP (supplemental valves), they differ.

How a Safety Valve Works

The operating principle is straightforward: a compression spring holds the disc on the seat. When system pressure acting on the disc area exceeds the spring force, the disc lifts and fluid discharges through the outlet. When pressure drops, the spring pushes the disc back down.

In practice, this happens in four stages:

1. Normal operation: Spring force exceeds the pressure force on the disc. The valve is shut and leak-tight.
2. Set pressure reached: Pressure force overcomes the spring. The disc lifts off the seat. On a gas/steam safety valve, this happens with a “pop” because the disc snaps to full lift almost instantly as the exposed disc area increases, creating a positive feedback loop.
3. Discharge: Fluid vents through the outlet until system pressure drops below the reseat pressure.
4. Reseat: Spring force wins again, the disc snaps shut. The valve resets automatically, ready for the next event with no manual intervention.

Parts of a Safety Valve

Part	Function	Notes
Body	Houses all internals; provides the discharge flow path	Carbon steel, stainless steel, or alloy, chosen to match process conditions
Seat	Sealing surface the disc presses against	Seat finish quality directly determines leak tightness. Metal-to-metal or soft-seated (PTFE)
Disc	Lifts off seat to allow relief flow	Machined to match the seat. Can be ball, piston, or poppet type
Spring	Holds disc closed until set pressure	Spring steel for general service; Inconel for high-temp or corrosive environments
Bonnet	Covers the valve internals; secures to body	Open bonnet for atmospheric discharge; closed bonnet for toxic/flammable service
Spindle (Stem)	Connects disc to lever/actuator; transfers motion	Must be precision-machined; any binding here causes erratic set pressure
Lever	Manual lift device for in-situ testing	Not on all designs. Allows operators to verify the valve is not seized
Adjusting Screw	Sets spring compression to define set pressure	Located at the top of the spring; locked with a jam nut once set
Blowdown Ring	Adjusts the reseat pressure (blowdown)	Critical for preventing chatter. Field adjustment of the blowdown ring is one of the most common maintenance tasks on safety valves

Types of Safety and Relief Valves

Conventional Spring-Loaded Safety Valve

The spring-loaded type accounts for the vast majority of safety valves in service. It is the workhorse of overpressure protection: simple, self-contained, and field-adjustable.

Set pressure is determined entirely by the spring compression, adjusted via the adjusting screw at the top. This makes spring-loaded valves straightforward to recalibrate on the bench (though field adjustment should only be done by qualified valve technicians with the proper test equipment).

Backpressure sensitivity is the main limitation. On a conventional spring-loaded valve, any superimposed backpressure acts on the disc in the closing direction, effectively raising the set pressure. If backpressure varies (as it does in a shared flare header), the actual opening pressure becomes unpredictable.

Best for: Atmospheric venting, dedicated discharge lines, constant backpressure systems, general utility service.

Limitations: Set pressure shifts with variable backpressure; bonnet spring exposed to process fluid in some configurations; limited to about 10% superimposed backpressure.

The image shows the spring holding the disc on the seat (closed position) and the disc lifted during discharge (open position).

Spring-loaded safety valves are the standard choice for boilers, pressure vessels, piping systems, and chemical plants. For deeper technical detail, consult Leser’s guide to spring-operated safety valves.

Balanced Bellows Safety Valve

A balanced bellows relief valve solves the backpressure problem of the conventional type. A metal bellows surrounds the disc stem, isolating the back side of the disc from discharge-side pressure. The set pressure stays constant regardless of backpressure fluctuations in a shared flare or vent header.

How the bellows works: The bellows creates a sealed chamber on the back of the disc. The effective area of the bellows equals the nozzle seat area, so backpressure pushing on the bellows exactly cancels backpressure pushing on the disc. The net force balance is independent of downstream conditions.

Parameter	Conventional	Balanced Bellows
Backpressure tolerance	Max 10% of set pressure (variable)	Up to 40-50% of set pressure
Set pressure stability	Shifts with backpressure	Stable regardless of backpressure
Bellows element	None	Metal bellows on disc stem
Bonnet vent	Not required	Required (vents bellows leakage)
Cost	Lower	20-40% higher
Maintenance	Simpler	Bellows integrity must be monitored
Failure mode	Set pressure drift if backpressure changes	If bellows ruptures, reverts to conventional behavior

Best for: Shared flare headers, variable backpressure systems, corrosive service where process fluid must be isolated from the bonnet/spring area.

Bellows Integrity

The bellows is a thin-walled metal element subject to fatigue and corrosion. If it ruptures, the valve reverts to conventional behavior and backpressure now acts on the disc. API 526 requires a bonnet vent on balanced bellows valves. If fluid discharges from this vent, the bellows has failed and must be replaced immediately. Never connect the bonnet vent to a pressurized system.

Pilot-Operated Safety Valve

A pilot-operated safety valve (POSV) uses the process pressure itself to hold the main valve shut, with a small pilot valve to trigger the opening. This is a fundamentally different approach from a spring-loaded valve.

How It Works

The main valve has a piston or diaphragm with process pressure applied on both sides. In the closed position, these forces balance and the valve stays shut, with a slight net closing force because the dome (top) area is larger than the nozzle (bottom) area.

When system pressure reaches the setpoint, the pilot valve (itself a small spring-loaded valve) opens, venting pressure from above the piston. The process pressure below now has an unbalanced upward force, and the main valve snaps open. When pressure drops, the pilot closes, pressure re-equalizes above the piston, and the main valve shuts.

Why Use a POSV Instead of Spring-Loaded

The practical advantages are significant:

• Tight shutoff near set pressure: a POSV operates at up to 98% of set pressure without any leakage. A spring-loaded valve starts to simmer at about 90-95%. This matters on systems that run close to MAWP.
• Higher capacity from a smaller valve: the full bore opens at once, giving a higher Kd (discharge coefficient) than a spring-loaded valve of the same orifice.
• Backpressure immunity: the main valve opening force comes from process pressure, not a spring, so downstream backpressure has no effect on set pressure.
• Modulating discharge: POSVs can open partially, which reduces process upset and minimizes flare load.

Complexity is the trade-off. The pilot has small internal passages that can plug in dirty service, and the additional tubing and fittings create more potential leak paths. POSVs are common on gas pipelines, large storage tanks, and high-pressure process systems where operating pressure runs close to MAWP.

Field Note

Pilot-operated valves require regular maintenance - the small orifices in the pilot can clog with process debris. In dirty service, we routinely see pilot failures causing main valves to either leak continuously or fail to open when needed. Install strainers upstream for particulate-laden fluids.

Dead-Weight Safety Valve

A dead-weight safety valve uses stacked weights instead of a spring to hold the disc on the seat. The set pressure equals the total weight divided by the disc area: pure physics, no spring to fatigue or lose calibration.

The disc is typically gunmetal (corrosion-resistant), bolted to the top of a vertical steam pipe flanged to the boiler shell. When steam pressure creates an upward force exceeding the dead weights, the disc lifts and steam discharges.

You will mostly see dead-weight valves on low-pressure boilers and as calibration references for testing spring-loaded valves. They are not practical for high-pressure or variable-pressure service (you would need absurdly heavy weights). But their simplicity and inherent accuracy make them the reference standard for bench-testing set pressure.

Full Comparison of All Types

Feature	Conventional Spring-Loaded	Balanced Bellows	Pilot-Operated	Dead-Weight
Operating mechanism	Compression spring	Spring + bellows	Process pressure + pilot	Gravity (stacked weights)
Set pressure range	Full API 526 range	Full API 526 range	Full API 526 range	Low pressure only (< 15 barg)
Backpressure tolerance	< 10% variable	Up to 40-50%	Immune	N/A
Shutoff near set pressure	90-95% of set pressure	90-95% of set pressure	Up to 98% of set pressure	N/A
Response time	Fast (snap action)	Fast (snap action)	Very fast	Slow (gradual)
Maintenance complexity	Low	Medium (bellows inspection)	High (pilot, tubing, fittings)	Very low
Dirty service suitability	Good	Good (bonnet isolated)	Poor (pilot clogs)	Poor
Relative cost	Base	1.2-1.4x	1.5-2.5x	Low (but limited use)
Typical applications	General process, utilities, boilers	Shared flare headers, corrosive service	Gas pipelines, high-pressure vessels, tanks	Calibration, low-pressure boilers

Spring-Loaded vs. Pilot-Operated: Selection Guide

Selection Criteria	Spring-Loaded (Conventional or Bellows)	Pilot-Operated (POSV)
Operating pressure > 90% of MAWP	Not recommended (simmer/leakage)	Preferred (tight shutoff to 98%)
Dirty or fouling service	Preferred (no pilot to clog)	Avoid unless strainers installed
Variable backpressure	Use balanced bellows type	Inherently immune
Large orifice requirement	Larger, heavier valve needed	Smaller valve, same capacity
Remote locations	Preferred (self-contained, no tubing)	Higher maintenance burden
Cryogenic service	Standard with appropriate materials	Requires special low-temp pilot seals
Two-phase flow	Suitable	May have erratic pilot behavior
Cost sensitivity	Lower initial cost	Higher initial cost, lower lifecycle cost in some cases

Pro Tip

Always verify set pressure at site before installing a safety valve - the nameplate value is the factory setting. Temperature and elevation differences between the manufacturer’s test bench and your actual operating conditions can affect set pressure by several percent.

API 526 Standard Orifice Designations

API 526 standardizes pressure relief valve orifice sizes using letter designations from D through T. After calculating the required effective discharge area per API 520 Part I, the engineer selects the next larger standard orifice letter.

Orifice Letter	Effective Area (in2)	Effective Area (mm2)	Typical Inlet x Outlet	Common Applications
D	0.110	71.0	1” x 2”	Small vessels, thermal relief
E	0.196	126.5	1” x 2”	Small vessels, instrument air
F	0.307	198.1	1.5” x 2.5”	Medium vessels, heat exchangers
G	0.503	324.5	1.5” x 3”	Process columns, reactors
H	0.785	506.5	2” x 3”	Drums, separators
J	1.287	830.3	3” x 4”	Large vessels, compressor discharge
K	1.838	1,186	3” x 4”	Large vessels, fire case scenarios
L	2.853	1,841	4” x 6”	Storage tanks, large columns
M	3.600	2,323	4” x 6”	High-capacity applications
N	4.340	2,800	4” x 6”	High-capacity applications
P	6.380	4,116	4” x 6” or 6” x 8”	Very large vessels, fire case
Q	11.05	7,129	6” x 8” or 6” x 10”	Major relief scenarios
R	16.00	10,323	6” x 10” or 8” x 10”	Large-scale emergency relief
T	26.00	16,774	8” x 10”	Maximum capacity applications

Note

Inlet and outlet flange ratings must match the piping class of the system. API 526 provides standard flange ratings (150, 300, 600, 900, 1500, 2500) for each orifice size. The outlet rating is often one class lower than the inlet (for example, a 300# inlet with a 150# outlet) because the outlet connects to a lower-pressure flare or vent header.

Sizing Methodology (API 520 Part I)

Sizing a safety or relief valve determines the minimum effective discharge area (orifice) needed to relieve the required flow rate at the governing overpressure scenario. The process follows API 520 Part I with input from API 521 (scenario identification) and API 526 (standard orifice selection).

Sizing Steps

1. Identify all overpressure scenarios per API 521: fire case, blocked outlet, thermal expansion, tube rupture, control valve failure, chemical reaction, power failure, cooling water failure
2. Calculate the required relieving rate (W in lb/hr for gas, Q in GPM for liquid) for each scenario
3. Determine the governing scenario, which is the one requiring the largest orifice area
4. Apply the appropriate API 520 formula (gas/vapor, liquid, steam, or two-phase)
5. Select the smallest standard API 526 orifice letter that provides an effective area equal to or greater than the calculated area
6. Verify inlet pressure drop does not exceed 3% of set pressure (API 520 Part II)
7. Verify that backpressure is within the valve type’s allowable range

API 520 Sizing Formula (Gas/Vapor)

A = W / (C x K x P1 x Kb x Kc) x sqrt(T x Z / M)

Symbol	Parameter	Unit
A	Required effective discharge area	in2
W	Required relieving rate	lb/hr
C	Gas constant coefficient (function of k = Cp/Cv)	Dimensionless
K	Effective coefficient of discharge (0.975 for API-certified valves)	Dimensionless
P1	Relieving pressure (set pressure x 1.10 + atmospheric)	psia
Kb	Backpressure correction factor (1.0 for conventional; < 1.0 for balanced bellows)	Dimensionless
Kc	Combination correction factor (1.0 without rupture disc; 0.9 with)	Dimensionless
T	Relieving temperature	Rankine (F + 460)
Z	Gas compressibility factor at relieving conditions	Dimensionless
M	Molecular weight of gas	lb/lb-mol

API 520 Sizing Formula (Liquid)

A = Q / (38 x K x Kw x Kv) x sqrt(G / (P1 - P2))

Where Q is in US GPM, G is specific gravity, P1 is relieving pressure (psia), and P2 is total backpressure (psia).

Common Mistake

Using design pressure instead of relieving pressure in the formula. Relieving pressure for a single relief valve is 110% of the set pressure (which equals the MAWP) plus atmospheric pressure. For fire case or multiple-valve installations, the accumulation allowance may be 116% or 121% per ASME Section VIII. Using the wrong pressure results in undersized orifice selection.

Installation Requirements (API 520 Part II)

Proper installation is as critical as proper sizing. Many field failures are caused by installation errors, not valve defects. API 520 Part II and API 521 provide detailed guidance.

Inlet Piping

• 3% rule: The total non-recoverable pressure loss in the inlet piping must not exceed 3% of the valve set pressure. Violation of this rule is the single most common cause of valve chatter.
• Keep it short and direct: The inlet pipe should be as short as possible, with no unnecessary fittings or restrictions. Ideally, mount the valve directly on the vessel nozzle.
• Full-bore connections: The inlet pipe bore must be at least equal to the valve inlet connection size. Never reduce the bore between the vessel and the valve.

Outlet Piping

• Size for backpressure limits: Discharge piping must be sized so that built-up backpressure does not exceed the valve type’s tolerance (10% for conventional, 40-50% for balanced bellows).
• Support the discharge pipe independently: Never transmit reaction forces from the discharge pipe through the valve body. Use pipe supports and anchors.
• Drain provisions: Install a drain hole at the lowest point of the discharge piping to prevent liquid accumulation. Trapped liquid above a safety valve can cause hydraulic lock and prevent the valve from opening.

General Installation Rules

Requirement	Details
Orientation	Vertical, with the spring housing pointing up
Isolation valves	Car-sealed open (CSO) or locked open (LO) if block valves are installed upstream or downstream per API 520 Part II
Discharge routing	To a safe location: flare header, atmospheric vent, or blowdown drum
Reaction force	Calculate per API 520 Part II; anchor the discharge pipe to absorb thrust
Accessibility	Make sure the valve is accessible for testing and removal without shutting down the entire unit
Tagging	Each valve must be tagged with PSV number matching the P&ID

Critical Safety Rule

Never install a block valve between a pressure vessel and its safety valve without a car-sealed-open (CSO) or locked-open (LO) arrangement. An inadvertently closed block valve removes all overpressure protection. ASME Section VIII UG-135 governs these requirements. Many jurisdictions treat a closed PSV block valve as a code violation subject to immediate shutdown.

Safety Valve Materials

Safety valves are available in material grades from carbon steel through exotic nickel alloys, with hundreds of possible configurations when you factor in body material, trim material, spring material, seat type, and seal elastomers. The process datasheet drives the material selection; get it wrong and you get a valve that corrodes, seizes, or cracks in service.

Body and Bonnet Materials

Material	Properties	Applications
Carbon Steel (WCB, WCC)	Good mechanical properties, cost-effective	General applications, moderate temps/pressures (-29 to 425 deg C)
Stainless Steel (CF8M/316, CF8/304)	Excellent corrosion resistance	Aggressive media, high cleanliness requirements
Alloy Steel (WC6, WC9, C5)	High strength at elevated temps	High-temperature, high-pressure applications (up to 600 deg C)
Bronze/Brass	Good corrosion resistance	Water, steam, gas at lower pressures
Nickel Alloys (Hastelloy, Monel, Inconel)	Exceptional corrosion resistance	Harsh environments, acidic/alkaline conditions
Titanium	Outstanding corrosion resistance, high strength-to-density	Seawater, chlorine, acidic environments
Low-temperature steel (LCB, LCC, LC3)	Impact-tested for cryogenic service	LNG, cryogenic air separation, cold climate

Trim (Disc and Seat) Materials

Material	Properties and Applications
Stainless Steel	Common for trim due to durability and corrosion resistance. Hardened stainless steel is often used for better wear resistance.
Stellite (cobalt-chromium alloy)	Used for seating surfaces to provide excellent wear and corrosion resistance, especially suitable for high-temperature steam applications.
Tungsten Carbide and Silicon Carbide	Utilized in abrasive media applications for their extreme hardness and resistance to wear and erosion.
PTFE and other Polymers	Used for seals and seating areas in lower temperature applications. Offers excellent chemical resistance and minimal friction.

Spring Materials

Material	Properties and Applications
Spring Steel (ASTM A228, A231)	Commonly used for springs in safety valves, chosen for its high strength and fatigue resistance. Standard for -29 to 260 deg C.
Inconel X-750	A nickel-chromium alloy used for springs that require corrosion resistance and the ability to maintain strength at high temperatures (up to 540 deg C).
Hastelloy and other Nickel Alloys	Selected for springs in highly corrosive environments where conventional spring materials would fail.

Material Selection by Service Condition

Service Condition	Body Material	Trim Material	Spring Material	Special Requirements
General hydrocarbon	WCB carbon steel	316 SS / Stellite	Carbon spring steel	Standard API 526
Sour service (wet H2S)	WCB or WCC (NACE)	NACE-compliant SS	NACE-compliant	NACE MR0175 / ISO 15156 mandatory
High temperature (> 425 deg C)	WC6, WC9, C5 Cr-Mo	Stellite	Inconel X-750	Creep considerations
Cryogenic (< -29 deg C)	LCB, LCC, LC3	316 SS	Low-temp spring steel	Impact testing per ASME
Concentrated acids	Hastelloy C-276	Hastelloy	Hastelloy C	Material compatibility critical
Chloride environments	CF8M (316 SS) or Duplex	316 SS / Stellite	Inconel X-750	Avoid 304 SS (SCC risk)
Steam service	WCB or CF8M	Stellite (standard)	Carbon spring steel	ASME Section I requirements
Oxygen service	Monel or bronze	Monel	Monel or SS	Special cleaning, no hydrocarbons

Material Selection Guidelines

Match body and bonnet material to the piping material class (per the project’s piping material specification). Trim material is driven by the process fluid: Stellite-faced seats are standard for steam; stainless steel or Inconel for most hydrocarbon service; Hastelloy for acid gas. Spring material must resist the bonnet-side environment. If the bonnet is vented to atmosphere, standard spring steel works; if exposed to process fluid (conventional valve in corrosive service), upgrade to Inconel X-750.

Cost escalation from carbon steel to exotic alloys is steep; a Hastelloy C-276 body can cost 8-10x the carbon steel equivalent. Specify only what the process demands.

Common Mistake

Specifying carbon steel safety valves for wet H2S service without NACE compliance. I’ve seen new valves fail within months due to sulfide stress cracking. Always check the process fluid composition - even trace amounts of H2S at certain temperatures require NACE MR0175/ISO 15156 compliant materials.

Testing and Inspection Requirements

Safety valves must be tested periodically to verify they will open at the correct set pressure and reseat properly. A valve that fails to open on demand provides zero protection, and operators may not know it has failed until a catastrophic overpressure event occurs.

Bench Pop Test (Set Pressure Verification)

The bench pop test is the primary method for verifying set pressure accuracy:

1. Mount the valve on a calibrated test bench
2. Apply air/nitrogen (for gas valves) or hydraulic pressure (for liquid valves) gradually
3. Record the pressure at which the valve pops open
4. Verify the pop pressure is within tolerance: +/- 2 psi for set pressures up to 70 psi, or +/- 3% above 70 psi (per ASME PTC 25)
5. Adjust the spring if necessary and repeat until three consecutive pops are within tolerance
6. Set the blowdown ring to achieve the specified blowdown percentage
7. Seal all adjustments with lead seals or security wire

Seat Tightness Testing (API 527)

API 527 establishes acceptable seat leakage rates for pressure relief valves. The test applies pressure at 90% of set pressure and measures any leakage from the outlet:

Set Pressure Range	Maximum Allowable Leakage (API 527)
Up to 1,000 psig	40 SCFH air (gas valves)
Above 1,000 psig	0.60 SCFH air per inch of orifice diameter
Soft-seated valves	Zero leakage at 90% of set pressure

Inspection Intervals

Standard/Program	Recommended Interval	Notes
API 510	Not exceeding 5 years (or half remaining life)	External inspection; internal at turnaround
API 576	Based on equipment history and RBI	Specific to pressure-relieving devices
ASME Section VIII	Per jurisdictional requirements	Many jurisdictions require 3-5 year intervals
Risk-Based Inspection (RBI)	1-10 years depending on risk ranking	Allows extended intervals for clean, non-corrosive service
Severe service	Annual or every turnaround	Corrosive, fouling, high-cycle, or sour service

Documentation

Every PSV test must be documented with: valve tag number, set pressure (required and actual), blowdown (required and actual), seat leakage test results, spring and trim condition, date of test, and certification of the test facility. These records are part of the plant’s mechanical integrity program and are subject to regulatory audit.

Selection Guide by Application

Application	Recommended Type	Key Considerations
Steam boiler	Spring-loaded SV (conventional)	ASME Section I; overpressure 3%; Stellite trim
Pressure vessel (general)	Spring-loaded SRV	ASME Section VIII; API 526 orifice per API 520 sizing
Shared flare header	Balanced bellows SRV	Variable backpressure compensation
Gas pipeline	Pilot-operated SV	Tight shutoff near MAWP; large capacity
Large storage tank	Pilot-operated or weight-loaded	API 2000 for tank breathing; low set pressures
Thermal relief (liquid-filled pipe)	Small spring-loaded PRV	API 521; small orifice (D or E); liquid service sizing
Fire case on vessel	Spring-loaded SRV (sized for fire)	21% accumulation; API 521 fire case calculations
Corrosive/toxic service	Bellows type with exotic trim	Closed bonnet; discharge to closed system
Pressure vessel with rupture disc	SRV downstream of rupture disc	Kc = 0.9 combination factor in API 520 sizing
Cryogenic service	Spring-loaded with low-temp materials	LCB/LC3 body; impact-tested trim and bolting
Compressor discharge	Spring-loaded or pilot-operated SV	API 521 blocked outlet scenario

Advantages and Disadvantages of Safety Valves

Advantages	Disadvantages
Last line of defense against catastrophic overpressure (vessel rupture, explosion, uncontrolled release)	Fail if not maintained; a seized valve is worse than no valve because operators assume protection exists
Simple, self-actuated, no external power needed	Spring relaxation over time shifts set pressure; requires periodic bench testing per API 510 intervals
Relatively low cost compared to rupture discs + replacement costs	Corrosion and fouling of seat/disc cause leakage or failure to open
Applicable to gas, steam, and liquid service across all pressure classes	Proper sizing requires rigorous API 520 calculations; undersized valves give a false sense of security
Reseatable, so no replacement needed after actuation (unlike rupture discs)	Chatter in oversized valves or with excessive inlet pressure drop causes seat damage
Code-mandated on all ASME pressure vessels	Variable backpressure affects conventional type; requires bellows or pilot-operated alternative

Safety Valve Specifications and Standards

The design, manufacture, and testing of safety valves are governed by ASME and API codes. In oil and gas and petrochemical work, you will deal with these standards constantly:

Standard	Scope	Key Content
API 520 Part I	Sizing and selection of pressure-relieving devices	Orifice area calculations for gas, liquid, steam, two-phase; correction factors
API 520 Part II	Installation of pressure-relieving devices	Inlet piping, outlet piping, reaction forces, 3% inlet pressure drop rule
API 521	Pressure-relieving and depressuring systems	Relief scenarios (fire, blocked outlet, tube rupture, etc.); flare system design
API 526	Flanged steel pressure relief valves	Standardized orifice designations (D through T), dimensions, materials, pressure-temperature ratings
API 527	Seat tightness of pressure relief valves	Acceptable leakage rates and test methods
API 2000	Venting atmospheric and low-pressure storage tanks	Tank breathing valves, emergency venting
API 510	Pressure vessel inspection code	Inspection intervals; includes requirements for PSV testing
API 576	Inspection of pressure-relieving devices	Specific guidance for PSV inspection, testing, and repair programs
ASME BPVC Section VIII	Pressure vessel code	Div. 1 (general) and Div. 2 (alternative) overpressure protection requirements (UG-125 to UG-137)
ASME Section I	Power boiler code	Safety valve requirements for steam boilers (set pressure, overpressure, blowdown)
ASME PTC 25	Pressure relief devices: performance test codes	Set pressure tolerance, capacity testing methodology
NACE MR0175 / ISO 15156	Materials for sour (H2S) service	Mandatory material requirements for valves in wet H2S environments

When specifying a PSV, the data sheet must capture: set pressure, MAWP, required orifice area per API 520, orifice letter per API 526, inlet/outlet flange ratings, body and trim materials, backpressure conditions (constant vs. variable), and the governing relief scenario from API 521.

How to Select a Safety Valve

Selection starts with the process data, not the valve catalog. Work through these factors in order:

Selection Factor	What to Determine
Service fluid	Gas, steam, liquid, or two-phase? This determines snap-action (SV) vs. proportional (PRV) vs. combination (SRV)
Required capacity	Run API 520 Part 1 calculations for the governing relief scenario from API 521
Set pressure	Must not exceed the MAWP. For multiple valves, the first valve sets at MAWP; additional valves can be set up to 105% MAWP
Backpressure	Constant backpressure → conventional valve. Variable backpressure → balanced bellows or pilot-operated
Operating pressure proximity to MAWP	If > 90% of MAWP, consider pilot-operated for tight shutoff
Bonnet type	Open bonnet for steam/air/non-toxic gas venting to atmosphere. Closed bonnet when atmospheric discharge is not acceptable
Nozzle construction	Semi-nozzle for clean, non-corrosive service at moderate pressures. Full nozzle for corrosive media or high pressures (standard in process plants)
Materials	Match body and trim to the process fluid and temperature. See the Materials section above
Overpressure allowance	Steam boilers: 3-5%. Fire case: up to 21%. Liquids: 10-25%. Blowdown up to 20% for liquid service
Discharge system	Atmospheric vent (open bonnet) or closed system (flare header, blowdown drum)

Always install safety valves in a vertical position with the discharge pipe routed to a safe location (flare header, atmospheric vent, or blowdown drum). The inlet piping pressure drop must not exceed 3% of set pressure, a commonly violated rule that causes chatter and premature wear.

Frequently Asked Questions

What is the difference between a safety valve and a pressure relief valve?

A safety valve (SV) opens fully and rapidly (pop action) at set pressure and is designed for compressible fluids like gas, steam, and vapor. A pressure relief valve (PRV) opens gradually in proportion to the overpressure and is designed for incompressible fluids like liquids. A safety relief valve (SRV) combines both behaviors (pop action on gas, proportional on liquid) and is the most common type in refinery and petrochemical service. The correct selection depends on the service fluid and the overpressure scenario per API 521.

What is the difference between API 520 and API 526?

API 520 covers the sizing and selection (Part I) and installation (Part II) of pressure-relieving devices, including the orifice area calculation formulas for gas, liquid, steam, and two-phase service. API 526 standardizes flanged steel pressure relief valve dimensions, orifice letter designations (D through T), inlet/outlet sizes, materials, and pressure-temperature ratings. In practice, you use API 520 to calculate the required orifice area and API 526 to select the standard valve that meets or exceeds that area.

When should I use a pilot-operated relief valve instead of a spring-loaded type?

Use a pilot-operated safety valve (POSV) when: operating pressure is close to set pressure (above 90% of MAWP) and tight shutoff is needed; variable backpressure is present and a balanced bellows is not sufficient; a large orifice is required (POSVs achieve higher capacity from a smaller valve body); or modulating discharge is desired to minimize process upset. Avoid POSVs in dirty or fouling service where the small pilot passages may clog, and in applications where the added complexity of pilot tubing and fittings is a reliability concern.

What does the API 526 orifice letter designation mean?

API 526 assigns standard orifice letter designations (D through T) to pressure relief valves based on the effective discharge area. For example, orifice D has an area of 0.110 in2 with a typical 1" x 2" inlet/outlet, while orifice T has 26.00 in2 with an 8" x 10" configuration. Engineers calculate the required area per API 520 Part I, then select the smallest standard orifice letter that provides an area equal to or greater than the calculated value. This standardization allows interchangeability between manufacturers.

How often should safety relief valves be tested and inspected?

Testing intervals depend on the applicable code and plant inspection history. API 510 recommends intervals not exceeding 5 years for pressure vessels, with relief valve testing at each turnaround. API 576 provides specific guidance for PSV inspection programs. Many refineries test PSVs every 3-5 years based on risk-based inspection (RBI) programs. Valves in severe service (corrosive, fouling, high-cycle, or sour) may require annual testing. ASME Section VIII and jurisdictional regulations may mandate more frequent testing. All test results must be documented as part of the plant's mechanical integrity program.