Comprehensive Stress Analysis for Pressure Safety Valve (PSV) Piping Systems: Ensuring Safety and Compliance
Comprehensive Stress Analysis for Pressure Safety Valve (PSV) Piping Systems: Ensuring Safety and Compliance Home / Learning 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
