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Ensuring the proper design of inlet piping to pressure relief valves (PRVs) is crucial in maintaining system safety and efficiency. According to a detailed discussion on Control.com, it is not uncommon to locate these valves away from the equipment they protect for easier access, proximity to the effluent disposal system, or maintenance reasons. However, this can result in a notable length of inlet piping, increasing the risk of significant, non-recoverable pressure losses due to friction during valve operation. Depending on the inlet piping's size, geometry, and inner surface condition, this pressure loss can be large (reaching 10%, 20%, or even 30% of the set pressure) or relatively small (less than 5%). API recommended practice 520 part II and the non-mandatory appendix M of ASME Section VIII guide engineers to ensure the inlet pressure loss does not exceed 3% of the valve's set pressure. The practice advises calculating these losses using the device's rated capacity.

Understanding PRV Operation

When a direct-acting PRV is closed, the pressure throughout the protected system remains uniform. The inlet piping configuration will not alter the PRV's set pressure. However, the frictional losses caused by the inlet pipe become significant once the valve opens, creating a pressure differential between the system (Ps) and the valve inlet (Pv). A direct-acting PRV will reseat when the Pv matches the valve's blowdown pressure. Therefore, it is crucial that the non-recoverable inlet line losses are less than the blowdown to ensure stable lift and a single reclosure cycle. Excessive inlet losses can lead to valve chatter or rapid cycling.

Example of Valve Chatter

To illustrate this possibility, consider the following example:

• Set Pressure: 100 psig
• Reseat Pressure: 93 psig (7% blowdown)
• Non-recoverable inlet loss at rated flow: 3 psig (3% of the set pressure)

When the valve opens, it will reseat when Pv equals 93 psig, with Ps approximately at 93 psig + 3 psig (96 psig). The valve then closes, and the system pressure stabilizes at 96 psig, keeping the valve shut.

Increased Inlet Loss Scenario

In a second scenario where the inlet loss increases to 10 psig, the valve reseats at 93 psig, causing the system pressure to rise to approximately 103 psig (93 + 10 psig). Immediately upon reseating, the valve reopens because the system pressure exceeds its set pressure. This situation can trigger rapid cycling or chatter, reducing capacity and potentially damaging the valve internals and the supporting piping. Even the highest quality materials and construction tolerances may not prevent galling in these parts.

Additional Information

For more information on pressure relief valves and their applications, visit LEFLOW. Their website contains other products and resources that may be of interest.

Useful Resources

Consider reviewing this article on Wellhead Maintenance & Integrity Management - Doyenoilfield for further insights on maintenance practices.

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