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Selecting & Sizing a Pressure Vessel
By: admin October 18, 2023

Manufacturing pressure vessel nozzles is a highly complex and ‘per application’ procedure that depends on the size and purpose of the pressure vessel valve. Numerous factors need to be considered when choosing the size and pressure capacity. It is crucial that the forging company has a detailed understanding of the pressure relief requirements of the system and the specific environmental conditions at the installation site select the appropriate relief valves.

Incorrectly, many times, the size of pressure relief valves is determined merely by matching them with the size of an existing available vessel nozzle or pipeline connection. However, proper, and comprehensive sizing of pressure relief valves requires a complicated and multi-step process.

This is where the expertise, application history and the forging capabilities WOT provides is invaluable.

It is important to thoroughly analyze all custom pressure vessel connections to identify any potential situations where the pressure may exceed safe limits. A suitable foundation for the design must be determined for each vessel and its components made of forged material.

The ‘design basis’ is utilized to determine the appropriate size for the pressure relief valve. Where possible, the sizing calculations should employ current, best practice methodologies that take into account factors such as two-phase flow and the reaction to heat sources.

Selecting and sizing a pressure relief valve (PRV) in accordance with the American Society of Mechanical Engineers (ASME) standards is critical to ensure that systems are adequately protected against overpressure scenarios. A properly sized PRV ensures that excess pressure is relieved in a timely manner while avoiding unnecessary discharges under normal operating conditions.

Here are several of the general steps and considerations for properly selecting and sizing an ASME pressure relief valve:

Understand the Application

– Determine the type of fluid (gas, liquid, steam, etc.).

– Note the system’s maximum allowable working pressure (MAWP).

– Determine the source of overpressure (e.g., external fire, blocked discharge, heat input).

Determine the Required Relief Capacity

– For liquid systems: Calculate the potential expansion of the liquid or the rate of liquid inflow.

For gas/vapor systems: Consider the maximum flow from a ruptured line or tank or the flow resulting from thermal expansion.

For fire cases: Use the wetted surface area and the heat input to calculate the relief rate, especially for vessels containing flammable liquids.

Select the Proper Set Pressure

– Typically, the set pressure is a percentage of the system’s MAWP. For many applications, it’s set at or below 100% of MAWP for the first relief device and up to 105% for additional devices.

Determine the Backpressure

– Note whether the system has a constant (built-up) backpressure or variable (superimposed) backpressure. This is essential for sizing and selection as backpressure affects PRV performance.

Choose Between a Conventional, Balanced, or Pilot-Operated PRV

– Conventional PRVs are best for systems with low backpressure.

– Balanced PRVs use a bellows or other mechanism to compensate for backpressure.

– Pilot-Operated PRVs are controlled by a pilot valve and can handle a wider range of conditions.

Determine Required Inlet and Outlet Piping Sizes

– Incorrect pipe sizes can lead to excessive pressure drop and chattering (rapid opening and closing) of the PRV.

– Follow the guidelines in the ASME code for maximum allowable pressure drop.

Materials and Construction

– The materials should be compatible with the fluid and the operating environment (e.g., temperature and corrosion considerations).

– For specific applications or hazardous fluids, special PRV designs or materials may be needed.

Overpressure and Blowdown

– Overpressure is the pressure increase over the set pressure before the PRV fully opens.

– Blowdown is the pressure drop after the PRV opens before it starts to close. It prevents the PRV from chattering.

Consult ASME and API Standards

– For detailed sizing and selection procedures, refer to the ASME Boiler and Pressure Vessel Code, Section VIII, and the API 520/521 standards.

Utilize Sizing Software and Tools

– Many valve manufacturers offer sizing software to aid in the proper selection and sizing of PRVs based on various inputs and scenarios. 

Safety Considerations

– Consider installing a rupture disc upstream of the PRV to protect it from process contaminants.

– Periodically inspect and maintain the PRV to ensure proper operation.


– Maintain a datasheet for each PRV detailing its specifications, sizing calculations, and other pertinent information.

And, always consult with professionals or specialists in the field when making critical safety decisions.

Obviously, the data required to perform pressure relief valve sizing calculations is quite extensive. That is why WOT can help design your custom pressure vessel connections.

As an ASME pressure vessel connections manufacturing company, our engineers have worked alongside design engineers for hundreds of companies and from all industries – petrochemical, oil and gas, industrial machinery, and power generation, so we have the expertise to forge your pressure vessel connections, custom fittings, and forged valves.

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