Pressure Safety Valve (PSV) Piping Support and Stress Analysis

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

Firstly, we need to list the requirements for the calculation. This information is required for reaction force calculation and stress analysis.

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

Reaction Force in a Closed Discharge System:

In a closed discharge system, the reaction force is generally lower than in an open system because the momentum exchange with the surrounding atmosphere is reduced. However, the exact calculation can become more complex due to the interaction with the downstream piping and backpressure effects.

For a closed discharge system, the reaction force can be calculated using a modified version of the steady-state reaction force equation, factoring in backpressure P2​ and assuming steady flow conditions:

Where:

• Fr​ = reaction force (N or lbf)

• W = mass flow rate (kg/s or lb/s)

• v = velocity of the discharge fluid (m/s or ft/s)

• g = gravitational constant (9.81 m/s² or 32.17 ft/s²)

• A = cross-sectional area of the pipe or valve outlet (m² or in²)

• P1​ = upstream relieving pressure (Pa or psi)

• P2​ = downstream (back) pressure in the discharge piping system (Pa or psi)

Key Considerations:

1. Backpressure P2​: In a closed system, the downstream backpressure can significantly affect the force exerted. The reaction force is reduced as P2​ increases.

2. Momentum Change: Since the fluid is confined to the piping, the change in momentum plays a smaller role compared to an open discharge system.

3. Support Requirements: Closed systems often require careful consideration of support design, as the reaction forces may be lower but still significant due to fluid momentum and pressure differences.

API 520 Guidelines for Closed Systems:

API 520 also discusses the impact of backpressure and suggests methods to account for it when calculating the reaction force. When the backpressure is significant (for instance, in long or complex downstream piping configurations), additional dynamic analysis may be required to fully understand the forces generated.

Main Load Cases for PSV Stress Analysis

When performing a PSV piping stress analysis in CAESAR II, it’s important to develop appropriate load cases to capture the different operational and relief conditions. Typically, the analysis will include both sustained loads (such as weight and pressure) and occasional loads (like reaction forces during relief). Here are the common load cases to consider for a PSV analysis:

1. Sustained Load Case (Operating Condition)

This load case considers the sustained loads present during normal operating conditions, such as:

• Weight (W): The weight of the piping, insulation, and fluid.

• Internal Pressure (P): The internal pressure under normal operation.

In CAESAR II, the sustained load case would be represented as:

L1: W + P

2. Thermal Load Case (Expansion)

This load case addresses the thermal expansion or contraction of the piping system due to operating temperatures. It’s crucial for ensuring the system can safely expand and contract without excessive stress.

• T: Thermal expansion or contraction.

In CAESAR II, the thermal load case would be:

L2: T1

(Where T1 represents the operating temperature)

3. Occasional Load Case (Relief Scenario)

During PSV relieving, the dynamic forces from the sudden release of gas or liquid (jet forces) must be included. This is considered an occasional load. The relief reaction forces, combined with sustained loads, should be analyzed as an occasional load case.

• Relief Reaction Forces (F): The jet forces generated during the PSV relief.

The typical load case would be:

L3: W + P + F

(Here, F represents the dynamic relief force applied at the outlet of the PSV.)

4. Thermal Expansion + Relief Forces

In some cases, you may need to check the combination of thermal expansion and PSV relieving forces, especially if the PSV operates while the system is at elevated temperatures.

This would be represented as:

L4: W + P + T1 + F

5. Operating Case + Relief Forces

If you need to evaluate the PSV relief scenario during normal operating conditions, this would involve the sustained and operating thermal loads combined with the jet forces from PSV operation.

For this case:

L5: Operating Case + F

(Operating case includes weight, pressure, and thermal expansion.)

6. Occasional + Wind/Seismic (if applicable)

If the system is in a region where wind or seismic loads are significant, you may need to consider these as occasional loads in combination with the PSV relief forces.

L6: W + P + Wind

L7: W + P + Seismic

(Or you can combine these with the jet forces from the PSV in a separate load case.)

Load Cases Summary

Load Case Description

• L1: W + P Sustained case (Weight + Pressure)

• L2: T1 Thermal expansion (Operating temperature)

• L3: W + P + F PSV relief (Weight + Pressure + Jet force)

• L4: W + P + T1 + F PSV relief with thermal expansion

• L5: Operating Case + F Operating conditions with PSV relief forces

• L6: W + P + Wind (Optional) Sustained load with wind (if applicable)

• L7: W + P + Seismic (Opt.) Sustained load with seismic (if applicable)

Additional Considerations:

• Reaction Force Calculation: PSV reaction forces should be calculated based on the fluid properties, pressure, and flow rate during the relief scenario. These forces are usually provided by process engineers or can be calculated using formulas for momentum thrust.

• Support Design: Proper supports, restraints, and anchors must be included in the model to handle the forces during PSV operation.

By running these load cases, you ensure that the system can withstand the dynamic loads during a PSV relief event while also meeting code requirements for sustained and occasional loading conditions.

PSV Supporting and Restraints

PSV piping lines are generally located on structural platforms. Because of this, the pressure safety valves must be supported symmetrically by 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.