In industrial piping systems, controlling pressure within safe limits is vital to prevent hazardous over-pressurization. Pressure Safety Valves (PSVs) are crucial in this regard, as they are designed to release pressure once it reaches a pre-set limit, protecting equipment, the surrounding environment, and personnel from potential harm. However, when a PSV activates, it exerts dynamic forces on the connected piping, creating stress-critical conditions. Therefore, a thorough stress analysis of PSV-connected piping systems is essential for both safety and compliance.
This article provides an in-depth look at the stress analysis of PSV systems, from the fundamental PSV types and discharge classifications to the load cases needed for accurate modeling. Understanding the critical loads and forces involved enables engineers to design safer, more reliable systems that meet industry standards.
Types of PSV Discharge Systems: Open vs. Closed Discharge
The two primary PSV discharge systems—open discharge and closed discharge—handle fluid forces differently:
Open Discharge: In this system, the PSV discharges directly into the atmosphere. This approach is often used for non-toxic or non-flammable gases, where environmental and safety risks are minimal.
Closed Discharge: Here, the fluid from the PSV is directed to a closed system, such as a discharge header or drum, where it can be collected for disposal or recycling. This system is typically employed for hazardous or high-value fluids that require containment.
Each discharge type imposes distinct forces on the piping, which must be accounted for during stress analysis to ensure stability and prevent failures under relief conditions.
Design Principles for Pressure Relief Devices
The key principle behind pressure relief devices is intrinsic safety; they must either “fail safe” or not fail at all. Solutions to issues in pressure relief piping should rely on solid design practices, as any failure is unacceptable. Prioritizing simplicity and established guidelines is vital for ensuring reliability. Here are four primary reasons to approach the engineering of pressure safety valves and discharge systems with precision:
- Flow Capacity Restrictions: The design of inlet and outlet piping can hinder flow, potentially compromising the valve’s ability to release pressure safely.
- Operational Performance: Poor design can negatively influence the operation of the PSV, altering its opening or closing pressures.
- Risk of Mechanical Failures: The thrust generated during valve discharge can result in mechanical failures within the piping system.
Safety Valve Inlet Piping :
To function effectively, safety valves should be mounted vertically, either directly on the vessel nozzle or via a short connection that allows for unobstructed flow. This principle should also apply to safety valves safeguarding piping systems.
Pressure Drop:
The pressure drop between the vessel and the safety valve inlet should be minimal to prevent starving the valve or causing it to chatter.
Preventing Piping Overstress :
It is crucial to avoid overstressing the inlet piping or mounting nozzle on the vessel. Consider the reaction force during valve operation along with forces from the discharge piping; minimizing the length of the inlet piping can help reduce strain.
Safety Valve Discharge Piping :
The allowable back-pressure on a safety valve depends on several factors, including its back-pressure rating, which may differ from the ASA rating of the outlet flange. This should be verified with the manufacturer. Conventional safety valves should not experience back-pressure exceeding 10 percent of the net setting, and it must always remain at least 5 psi lower than the opening pressure.
Drain Hole Management :
In applications where liquids may accumulate at the valve discharge, it’s essential to remove the drain hole plug. This applies in scenarios where condensate can form or precipitation may enter the discharge line. If the plug is removed, the drain must be piped safely for disposal, particularly if the fluid poses a hazard or if sudden discharges could endanger personnel.
Piping Support Design
Safety valves need to measure pressure within a 3% accuracy and fulfill specific control functions. Excessive strain on the valve body can hinder its performance. Therefore, piping supports should be designed to minimize load on the valve. In high-temperature applications, excessive loads can cause permanent deformation, and even at lower temperatures, distortion may result in leaks below the set pressure.
Discharge piping should be supported independently from the valve and carefully aligned to reduce forces during normal operation. Properly designed expansion joints or long-radius bends should be included to prevent excessive strain.
Discharge Piping Stress Analysis is mainly subjected to stresses from thermal expansion and discharge reaction forces. The rapid release of compressible fluids can create impact loads and bourdon effects at directional changes, necessitating adequate anchoring to prevent sway or vibration during discharge.
Minimizing Pressure Loss
To limit pressure loss in discharge piping, the system should be as direct as possible, employing long-radius bends and avoiding tight-fitting connections. The discharge pipe’s cross-sectional area must never be smaller than that of the valve outlet.
Main Load Cases for PSV Stress Analysis
Effective PSV stress analysis involves defining load cases that accurately capture the operational and occasional forces exerted on the system. Below are essential load cases typically evaluated for PSV piping systems:
Sustained Load Case (Operating Condition): This load case models sustained forces that occur under normal operating conditions, including:
Weight (W): Accounts for the weight of the piping, insulation, and fluid.
Internal Pressure (P): The pressure within the piping during standard operation.
Thermal Load Case (Expansion): Thermal expansion or contraction occurs as the piping temperature changes. It is essential to account for this to prevent excessive stress during temperature fluctuations.
Key Component: Thermal Expansion (T1): Thermal forces associated with the operating temperature.
Occasional Load Case (Relief Scenario): During PSV activation, sudden pressure release generates dynamic jet forces, which classify as occasional loads. These forces need to be considered along with sustained loads to simulate a PSV relief event.
Key Component: Relief Reaction Forces (F): Resulting from the rapid release of gas or liquid.
Thermal Expansion with Relief Forces: If the PSV is triggered while the system operates at high temperatures, combining thermal expansion with relief forces becomes necessary to account for this additional stress.
Operating Case with Relief Forces: This load case combines sustained and thermal loads with the dynamic relief forces from a PSV release under standard operating conditions.
Occasional Loads Including Wind/Seismic Events: In regions prone to seismic activity or strong winds, it’s critical to assess these occasional loads along with relief forces.
Additional Considerations for PSV Stress Analysis
- Reaction Force Calculation:
- Accurate calculation of PSV reaction forces is essential to determine the stress impact on the connected piping. These forces are typically based on the properties of the fluid, such as flow rate, pressure, and temperature during relief. Engineers can calculate preliminary reaction forces using standards like API RP 520, though values should be verified against vendor data when available.
- Support Design and Restraints:
- PSVs are generally installed on structural platforms, and their piping systems require sturdy support due to the significant reaction forces they experience during relief. Dummy supports are often used to ensure even distribution of forces, and restraints are welded to platform beams to maintain stability. Proper support design minimizes strain, reducing the risk of damage or misalignment under load.
- Structural Platform Placement and Support:
- PSV systems, commonly mounted on structural platforms, need to be supported symmetrically. This involves using appropriate dummy supports and welding restraints to platform beams. This careful design ensures stability and reduces the impact of reaction forces during relief scenarios.
- Stress Analysis for Complex Load Cases:
- Complex load cases, such as combined thermal expansion and jet forces, need specialized analysis to ensure that the piping can handle the additional strain. These scenarios are especially relevant in elevated temperature operations or during high-pressure releases.
Conclusion
Conducting thorough stress analysis for PSV-connected piping systems is critical for safe and efficient operation. By developing accurate load cases, calculating reaction forces, and designing robust support mechanisms, stress analysis engineers can significantly reduce the risk of failure. A well-designed PSV system protects both assets and personnel, ensuring compliance with industry codes and the integrity of the overall piping infrastructure. Through rigorous analysis and adherence to codes and standards we can enhance safety in increasingly complex industrial environments, reinforcing resilience against the demands of high-stakes applications.
MECS Engineering specializes in comprehensive stress analysis, offering expertise in modeling loads, evaluating material behavior, and ensuring compliance with industry standards. Our experienced engineers can help identify potential issues and optimize the design of your piping systems, enhancing safety and operational efficiency.
