Pressure Relief Valve Sizing Guide: How to Size Relief Valves Correctly

Proper pressure relief valve sizing ensures the valve can discharge enough flow to prevent dangerous over-pressurization while avoiding the problems of oversized or undersized valves. An undersized relief valve cannot protect your system—it will simply blow open and stay open while pressure continues rising. An oversized valve may chatter, leak, or fail to open at all due to insufficient flow velocity.

Relief valve sizing is not guesswork, and it’s not simply “match the pipe size.” It’s a technical process governed by established codes and standards (API 520/521, ASME, ISO) that considers fluid properties, flow conditions, pressure requirements, and discharge scenarios. Get it wrong, and you’ve installed a valve that provides a false sense of security while leaving your equipment vulnerable to catastrophic failure.

Whether you’re sizing a relief valve for a water heater, boiler, pressure vessel, or process system, this guide walks you through the critical factors, calculations, and selection process to ensure proper protection.

Why Relief Valve Sizing Matters

The Consequences of Wrong Sizing

Undersized Relief Valve

What happens:

• Valve opens but cannot discharge enough flow
• Pressure continues rising despite open valve
• System exceeds design pressure
• Equipment fails catastrophically

Real-world example:

Boiler rated 150 PSI, relief valve set at 150 PSI
Boiler produces steam faster than relief can discharge
Pressure rises to 200+ PSI despite relief wide open
Boiler explodes

Oversized Relief Valve

What happens:

• Disc doesn’t lift fully at normal relief flow
• Valve operates in unstable region
• Chattering damages valve and creates noise
• Premature seat wear
• May not seal properly after opening

Real-world example:

Water heater needs 30 GPM relief capacity
Installer uses 100 GPM valve "to be safe"
Thermal expansion creates only 5 GPM flow
Valve lifts slightly, chatters, leaks
Constant dripping ruins valve, wastes water

The Goldilocks Principle

Relief valves must be “just right”:

• Large enough to handle required relief flow
• Small enough to operate in stable, efficient range
• Matched to actual system conditions

Key Sizing Parameters

Before sizing any relief valve, you must know these critical parameters:

1. Required Relieving Capacity

Question: How much flow must the valve discharge?

This depends on the relief scenario:

Critical point: You must identify the worst-case scenario that produces maximum required relief flow.

2. Set Pressure

Definition: The pressure at which the valve begins to open (crack pressure)

Guidelines:

• Must not exceed Maximum Allowable Working Pressure (MAWP) of vessel
• Typically set at 90-100% of MAWP
• For ASME vessels: usually set AT the MAWP
• Consider operating pressure (should be well below set pressure)

Example:

• Vessel MAWP: 150 PSIG
• Normal operating pressure: 100 PSIG
• Relief valve set pressure: 150 PSIG

3. Allowable Overpressure

Definition: How much pressure can exceed set pressure during relief

ASME Code allowances:

• Normal operation: 10% overpressure (165 PSIG for 150 PSIG set)
• Fire case: 21% overpressure (181 PSIG for 150 PSIG set)

This determines the accumulation the valve must handle.

4. Back Pressure

Definition: Pressure in the discharge system

Types:

• Superimposed: Exists before valve opens (from other sources)
• Built-up: Created by valve’s own discharge

Impact: High back pressure reduces valve capacity and may require balanced bellows design

5. Fluid Properties

For liquids:

• Specific gravity
• Viscosity
• Temperature

For gases/vapors:

• Molecular weight
• Compressibility factor (Z)
• Specific heat ratio (k)
• Temperature

Critical: Some fluids require special calculations or valve materials

Relief Valve Sizing Methods

Different fluids require different sizing approaches.

Sizing for Liquids

Liquid relief valve sizing uses this basic formula:

Q = A × K × √(ΔP / G)

Where:

• Q = Flow rate (GPM)
• A = Effective discharge area (in²)
• K = Discharge coefficient (typically 0.65)
• ΔP = Pressure differential (set pressure – back pressure, PSI)
• G = Specific gravity (water = 1.0)

Step-by-Step Liquid Sizing

Step 1: Determine Required Flow (Q)

Calculate worst-case relief flow:

Thermal expansion example:

Blocked water line, 40°F to 140°F temperature rise
100 feet of 2" pipe

Volume expansion ≈ 100 ft × coefficient × ΔT
Flow through relief = expansion rate
Required Q = 15 GPM

Step 2: Calculate Pressure Differential

Set pressure: 150 PSIG
Back pressure: 5 PSIG (atmospheric + pipe resistance)
ΔP = 150 - 5 = 145 PSI

Step 3: Determine Specific Gravity

Water at 140°F: G ≈ 0.98

Step 4: Calculate Required Area

Q = A × K × √(ΔP / G)
15 = A × 0.65 × √(145 / 0.98)
15 = A × 0.65 × 12.15
A = 1.90 in²

Step 5: Select Valve Size

Consult manufacturer’s capacity tables for valve with effective area ≥ 1.90 in² at specified set pressure.

Typical result: 1″ × 1-1/2″ relief valve

Sizing for Gases and Vapors

Gas/vapor sizing is more complex due to compressibility effects.

Basic gas formula (critical flow):

W = C × K × A × P₁ × √(M / (Z × T))

Where:

• W = Flow rate (lb/hr)
• C = Constant based on gas properties
• K = Discharge coefficient
• A = Effective area (in²)
• P₁ = Relieving pressure (PSIA)
• M = Molecular weight
• Z = Compressibility factor
• T = Temperature (°R)

Simplified Approach for Steam

For saturated steam:

W = 51.5 × A × P₁ × K

Where:

• W = Steam flow (lb/hr)
• A = Effective area (in²)
• P₁ = Set pressure + overpressure (PSIA)
• K = Correction factor (Napier factor)

Example:

Required steam relief: 10,000 lb/hr
Set pressure: 100 PSIG (114.7 PSIA)
Overpressure: 3% (118 PSIA)

W = 51.5 × A × 118 × 1.0
10,000 = 51.5 × A × 118
A = 1.65 in²

Select valve with area ≥ 1.65 in²

API 520/521 Standard Method

For process industry applications, API standards provide detailed procedures:

API 520 Part I: Sizing and selection API 521: Guide to pressure relief and depressuring systems

These cover:

• Fire scenarios (external fire on vessel)
• Thermal expansion in blocked liquids
• Gas blowdown calculations
• Two-phase flow conditions
• Reaction scenarios

Sizing Charts and Tables

Water Relief Valve Sizing Chart

For water at 100°F, 150 PSIG set pressure:

Note: Actual capacities vary by manufacturer and model

Steam Relief Valve Sizing Chart

For saturated steam, set pressure 100 PSIG:

Boiler Relief Valve Sizing

Minimum required capacity (ASME Section I):

For boilers:

• Capacity must equal or exceed maximum steam generation rate
• Based on maximum burner input or heat release
• Multiple valves may be used (largest must be ≥67% of total)

Quick estimate:

Required capacity (lb/hr) = Boiler HP × 34.5

Example:
100 HP boiler
Required capacity = 100 × 34.5 = 3,450 lb/hr
Select orifice "G" (3,056 lb/hr) + additional capacity

Special Sizing Considerations

Fire Case Sizing

When vessels are exposed to fire, dramatic increases in vapor generation occur.

API 521 fire case formula:

Q = C × F × A^0.82

Where:

• Q = Required relief rate (GPM or lb/hr)
• C = Constant depending on fluid
• F = Environment factor
• A = Wetted surface area (ft²)

Key points:

• Fire dramatically increases relief requirements (10× normal is common)
• Wetted surface area is critical parameter
• Insulation significantly reduces required capacity
• Often the governing case for sizing

Two-Phase Flow

When liquid flashes to vapor during relief (most challenging scenario):

Challenges:

• Neither pure liquid nor pure vapor equations apply
• Flow regime affects capacity
• Requires specialized calculation methods
• Often requires larger valve than pure liquid or gas

Approaches:

• Omega method (API 520)
• Homogeneous equilibrium model
• Specialized software (ProMax, Aspen, etc.)

Conservative approach: Size for liquid flow, verify adequate for vapor

Thermal Expansion in Blocked Liquid Lines

Common in process piping and water systems:

Formula:

Q = β × V × ΔT / t

Where:

• Q = Relief flow rate
• β = Volumetric expansion coefficient
• V = Trapped volume
• ΔT = Temperature rise
• t = Time for temperature rise

Water thermal expansion:

β = 0.0004 per °F (approximate)

100 gallons water trapped
Temperature rise 40°F to 140°F (100°F increase)
Time = 1 hour

Q = 0.0004 × 100 × 100 / 1
Q = 4 gallons/hour = 0.067 GPM

Small valve required (1/2" adequate)

Selection Process: Step-by-Step

Step 1: Define the System

Document:

• [ ] Equipment protected (vessel, piping, heat exchanger)
• [ ] Design pressure (MAWP)
• [ ] Normal operating pressure
• [ ] Operating temperature
• [ ] Fluid properties (specific gravity, molecular weight, etc.)

Step 2: Identify Relief Scenarios

List all credible overpressure scenarios:

• [ ] Blocked outlet with continued input
• [ ] Fire exposure
• [ ] Heat exchanger tube failure
• [ ] Thermal expansion
• [ ] Cooling water failure
• [ ] Control system failure
• [ ] Runaway reaction (if applicable)

Determine which scenario produces maximum required relief capacity.

Step 3: Establish Design Criteria

• [ ] Set pressure (at or below MAWP)
• [ ] Allowable overpressure (10% or 21% for fire)
• [ ] Back pressure (existing and built-up)
• [ ] Temperature at relief conditions
• [ ] Required certifications (ASME stamp, etc.)

Step 4: Calculate Required Capacity

Using appropriate method:

• [ ] Liquid: Calculate GPM
• [ ] Gas/vapor: Calculate lb/hr or SCFM
• [ ] Two-phase: Use appropriate correlation
• [ ] Fire case: Apply API 521 methodology

Step 5: Size the Valve

• [ ] Calculate required effective discharge area
• [ ] Add safety margin (typically 10-25%)
• [ ] Consult manufacturer capacity tables
• [ ] Select valve with adequate certified capacity

Step 6: Verify Selection

Check:

• [ ] Adequate capacity at set pressure + overpressure
• [ ] Back pressure within allowable limits
• [ ] Body rated for system pressure and temperature
• [ ] Materials compatible with fluid
• [ ] Inlet/outlet sizes appropriate
• [ ] Mounting orientation acceptable
• [ ] Certified for code requirements

Step 7: Design Discharge System

• [ ] Determine discharge destination (atmosphere, containment, flare)
• [ ] Size discharge piping (minimize back pressure)
• [ ] Ensure safe discharge location
• [ ] Consider noise and thrust reactions
• [ ] Meet environmental regulations

Common Sizing Mistakes

Sizing Software and Tools

Hand Calculations vs Software

Recommended Software

Process simulation packages:

• Aspen Plus (with pressure relief utilities)
• ProMax
• HYSYS

Specialized relief sizing:

• API 520 calculators (various vendors)
• Pressure Relief Valve Sizing (PRV) by HTRI
• Manufacturer sizing tools (Emerson, Baker Hughes, etc.)

For simple cases:

• Excel-based calculators (many free versions available)
• Online calculators (verify with standard)

Regulatory and Code Requirements

ASME Boiler and Pressure Vessel Code

Section I (Power Boilers):

• Capacity = maximum steam generation
• Must have ASME stamp and capacity certification

Section VIII (Pressure Vessels):

• Relief valve required for all pressure vessels
• Capacity must prevent pressure exceeding MAWP × 1.10 (or 1.21 for fire)
• Certification required

API Standards

API 520/521:

• Industry standard for sizing methodology
• Required for process industry applications
• Covers all major scenarios

National Board Inspection Code

• Requires periodic inspection and testing
• Capacity verification required
• Recertification after repair

Pressure Relief Valve Sizing Checklist

Use this checklist for every sizing project:

System Information:

• [ ] Equipment description and tag number
• [ ] Design pressure (MAWP)
• [ ] Operating pressure and temperature
• [ ] Fluid composition and properties
• [ ] System volume/capacity

Relief Scenarios:

• [ ] All credible scenarios identified
• [ ] Worst-case scenario determined
• [ ] Fire case evaluated
• [ ] Required relief flow calculated

Valve Selection:

• [ ] Set pressure determined
• [ ] Allowable overpressure specified
• [ ] Back pressure calculated
• [ ] Required effective area calculated
• [ ] Valve size selected from certified data
• [ ] Adequate capacity verified

Installation Design:

• [ ] Inlet piping sized (minimize pressure drop)
• [ ] Outlet piping sized (minimize back pressure)
• [ ] Discharge safely routed
• [ ] Mounting orientation verified
• [ ] Supports and thrust restraints designed

Documentation:

• [ ] Calculations documented
• [ ] Valve datasheet completed
• [ ] P&ID updated
• [ ] Nameplate information specified
• [ ] Maintenance requirements noted

Frequently Asked Questions

Can I use a larger valve “to be safe”?

No. Oversizing causes operational problems:

• Chattering and instability
• Premature wear
• Poor sealing
• Use 10-25% margin maximum

Do I need to account for temperature effects?

Yes. Fluid properties change with temperature:

• Density affects liquid calculations
• Vapor pressure affects gas/vapor calculations
• Always use properties at relief conditions

How do I handle variable operating conditions?

Size for worst case:

• Highest required relief flow
• Lowest set pressure needed
• Maximum back pressure expected
• If wide variation, consider multiple valves

What if my calculation shows I need between standard sizes?

Always round up to the next available standard orifice size. Never round down—this provides inadequate protection.

Can I use multiple smaller valves instead of one large valve?

Yes, with considerations:

• Total capacity must equal or exceed required
• Largest valve should be ≥67% of total (ASME requirement)
• All must be set at same pressure (or within code limits)
• More complex piping and maintenance

Conclusion

Pressure relief valve sizing is a critical safety calculation that requires careful consideration of system conditions, credible relief scenarios, fluid properties, and applicable codes. Undersized valves cannot protect equipment, while oversized valves create operational problems and premature failure.

Key principles for successful sizing:

• Identify the worst-case relief scenario (often fire case)
• Calculate required relief flow using appropriate method for fluid type
• Account for all pressure losses (back pressure, inlet losses)
• Size valve to provide required capacity with 10-25% margin
• Verify selection against certified capacity data
• Design discharge system to minimize back pressure
• Document all calculations and assumptions

Following established standards (API 520/521, ASME) and using appropriate calculation methods ensures your relief valve will protect equipment when needed while operating reliably during normal conditions.

When in doubt, consult a qualified pressure relief specialist—the cost of proper sizing is insignificant compared to the cost of equipment failure or safety incidents.

Need assistance with relief valve sizing? Consult with certified pressure relief engineers or contact valve manufacturers’ technical support for application-specific guidance.