Pressure Relief Valve Sizing: A Practical Guide

Introduction

This guide provides a practical overview of how a pressure relief valve is sized: the standards that apply, the data the calculation requires, and the procedure used to determine the correct orifice. It is intended as general reference for those responsible for specifying relief valves. The referenced standards, including AS 1210, AS 1271, API 520 Part 1, ISO 4126-1 and AD 2000-Merkblatt A2, are the authoritative source for the sizing process, and the current edition of the applicable standard should always be confirmed before use, as standards are periodically revised.

Step 1. Establish the pressure limit to be protected

The vessel's design pressure and the accumulation permitted above it set the ceiling the relief system must work within. For equipment designed to AS 1210, Section 8 requires the protection, sets the required relieving capacity, and defines the accumulation limits: at least one relief device set at or below the design pressure, any additional devices set no higher than 110% of the design pressure, and a limit of 121% of the design pressure in a fire case. The set pressure sits at or below the design pressure. The valve's rated discharge capacity comes from the manufacturer's type-test certification (see Step 9).

Step 2. Identify the governing relief scenario

The valve is sized for the worst credible overpressure event, for example a blocked outlet, control valve failure, thermal expansion, or external fire. The scenario that produces the largest required flow governs the size, and the fire case is considered separately, as it carries its own accumulation limit. Where data is incomplete, the worst credible case is assumed, in which all causes of overpressure occur simultaneously.

Step 3. Determine the required relief flow

The required flow is the mass or volume the valve must pass under the governing scenario. This is the required capacity.

Step 4. Define the fluid and its state

The fluid state may be gas or vapour, steam (saturated or superheated), liquid, or two-phase. Each requires its own properties:

• Gas or vapour: molecular weight, ratio of specific heats (the isentropic coefficient), and compressibility factor.
• Liquid: density and viscosity.
• Two-phase: the properties of both phases, in accordance with API 520 Part 1 Annex C.

Step 5. Establish the relieving conditions

The relieving conditions are the set pressure, the available overpressure, the back pressure, and the relieving temperature. The valve is sized at the relieving temperature, not the operating temperature, because that is the condition present during discharge. The operating temperature is also relevant, as it affects valve selection.

Step 6. Select the sizing standard

The sizing standard is matched to the design code to which the equipment is built and certified:

• ASME Section VIII or I, with API RP 520 for the formulas, API 521 for the fire and thermal expansion cases, and the API two-phase method.
• ISO 4126-1.
• AD 2000-Merkblatt A2.

For a vessel designed to AS 1210, the standard itself sets the required relieving capacity and accumulation limits (Section 8.6), permits the API RP 520 method for the fire and gas case (Clause 8.6.2.4), and takes the valve's rated discharge capacity from AS 1271 or equivalent (Clause 8.6.5).

Step 7. Calculate the minimum orifice area

The minimum required orifice area is calculated for the required flow using the sizing equation for the fluid state given in the selected standard. For ASME and API work this is the relevant formula in API 520 Part 1 (gas or vapour, steam, or liquid), with two-phase flow covered in API 520 Part 1 Annex C; ISO 4126-1 and AD 2000-Merkblatt A2 contain their own equivalent equations. The calculation applies the valve's certified discharge coefficient, which accounts for the difference between actual and ideal nozzle flow.

Step 8. Select the next larger certified orifice

The selected orifice is the next certified size equal to or larger than the calculated minimum. It is never smaller than the calculated minimum.

Step 9. Verify the certified capacity

The selected valve's rated discharge capacity must meet or exceed the required flow, established from its certified coefficient of discharge and effective area. This capacity may be certified to AS 1271 or established by international type testing such as ASME or ISO 4126. The Australian standards permit both: AS 1271 (Clause 3.5.4) allows the discharge capacity to be rated on the basis of tests conducted outside Australia, supported by the test documentation, and AS 1210 (Clause 8.6.5) refers to capacity certified to AS 1271 or equivalent.

Step 10. Specify the valve build

The materials, connections and configuration are specified to suit the application:

• Materials: body material, trim material, and disc seal type (metal, soft or stellited).
• Connections: connection type and standard, DN or NPS size, PN or pressure rating, and flange facing.
• Configuration: valve type (conventional, bellows, pilot operated or pressure/vacuum), discharge type (atmospheric or piped), bonnet type (open or closed), nozzle type (full or semi), lifting device (closed cap, plain lever, packed lever or easing gear), and the design code and design standard.

Information required for sizing

The following inputs are needed to size and specify a relief valve.

Sizing inputs:

• Set pressure
• Relieving temperature and operating temperature
• Required capacity (mass or volume flow)
• Fluid designation and fluid state
• Fluid properties for that state (gas: molecular weight, ratio of specific heats, compressibility factor; liquid: density, viscosity; two-phase: per API 520 Part 1 Annex C)
• Back pressure (Constant/Variable Superimposed, and Built-Up Back Pressure)

Specification inputs:

• Body material, trim material, disc seal type
• Connection type and standard, DN or NPS size, PN or pressure rating, flange facing
• Valve type, discharge type, bonnet type, nozzle type, lifting device
• Design code and design standard

Once the valve has been sized and specified to the chosen standard, it can be procured against that specification.