PSV Design and Calculation

One of the most common dynamic fluid forces encountered in piping is the relieving force from safety relief valves. Generally, safety valve relieving systems are classified into two categories: open discharge and close discharge. In an open discharge system, the fluid is simply released into the atmosphere. On the other hand, the closed discharge system collects the discharged fluid in a drum or header for proper recycling or disposal. Each type of discharge system treats the fluid force differently.

Here in this blog, you can find the reaction force calculation, load cases, and typical supporting details.

Discharge Reaction Force Calculation

Before we start to calculate, firstly we need to list the requirements for the calculation.

Required Information

-              Material Requisition (MR) of the Product (PSV): Looking at the pressure and temperature, the opening reaction forces on elbows can be calculated by use of this information.

-              Specific Gravity (SG) of the Fluid: Some PVSs may throw liquid, so we need to know the SG of the fluid also, we need the SG of the connected Header.  

-              Temperature Change on Header: If there are several PSVs available, then the average temperature should be considered as per the example below.

TEMPERATURE CHANGE ON HEADER

All these information are required for the below formulas (API RP 520);

Discharge Reaction Force

Vapor discharge reaction force in an open system.

Discharge Reaction Force

Two phase flow discharge reaction force in an open system.

Load Cases for PSV Stress Analysis

 You can find the load cases below. You can add more cases but these are the main cases required for the analysis.

1-       Operation Case, Both Pressure Safety Valves (PSV) are close. (Including Seismic and Wind Conditions)

2-       Discharge Case (Including Seismic and Wind Conditions)

a- Close PSV and Equipment under operation conditions (Pressure and Temperature) until the PSV.

b- Considering there are 2 PSV lines, the First PSV is close and the other is open. In this case, the equipment, the second PSV (open one), and the line after the open PSV should be at relief temperature and pressure (including the discharge line of the first PSV).

3-       It’s a similar case but now the First PSV is open and the second is close. (Including Seismic and Wind Conditions)

4-       Both PSVs are open. All the continuation lines and header are at relief conditions. (Including Seismic and Wind Conditions)

5-       Both PSVs are open but in fire conditions (peak). No seismic and wind conditions are to be included in your analysis.

PSV Supporting and Restraints

PSV lines are generally located on structural platforms. Because of this, supporting the pressure safety valves is to be done symmetrically by the use of dummy supports. The restraints are welded to the platform beams.

Please see the below picture that illustrates a typical PSV support and restraint example.

Typical PSV Support and Restraint

A well-designed and well-supported pressure safety valve system is crucial for maintaining the safety of both personnel and assets in piping systems. Engineers need to follow industry standards, conduct risk assessments, and implement appropriate mitigation measures to prevent catastrophic events. Additionally, the calculation of reaction forces is necessary to ensure the structural integrity of the system and protect it from unexpected pressures.

 Overall, the combination of pressure safety valve design, robust support mechanisms, and precise calculation methodologies is essential for a resilient and dependable piping infrastructure. In the face of evolving technology and increasingly complex industrial processes, prioritizing safety is crucial. By continuously evaluating, adapting, and innovating, we can strengthen our systems against the challenges of the operating environment and ensure the long-term well-being of both people and assets in the dynamic landscape of industrial operations.

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