Pressure relief valve (PRV) instability from excessive inlet pressure drop causes chatter, piping vibration, and equipment failures. Equity Engineering performs API 520 inlet pressure drop analysis and 3% rule engineering assessments to evaluate PRV installations. Our experts conduct force balance calculations and acoustic interaction evaluations to prevent valve cycling while ensuring compliance, helping facilities avoid costly piping modifications.
3% Rule: What’s the Fuss?
Recent changes to the infamous 3% rule on inlet pressure drop to pressure relief valves (PRVs) have raised many questions. The ongoing efforts by API improve the current industry codes and standards for PRV instability.
PRVs should be inspected upon installation and at frequencies sufficient to maintain reliability. By maintaining safety critical pressure relieving systems and developing practical mitigation solutions, you will meet risk management targets, compliance requirements, and business objectives.
What is the Inlet Pressure Drop Rule?
The 3% rule recommends that the inlet piping between the protected equipment and the inlet flange of the PRV be designed so that the total pressure loss in the line shall be the sum of the inlet loss, line loss, and valve loss and shall not exceed 3% of the set pressure of the valve.
Why is the Inlet Pressure Drop Rule Important?
Valve instability! If the pressure drop in the inlet piping to a PRV is too high, the valve may cycle or chatter when the pressure at the inlet to the valve goes below its blowdown pressure (reseat pressure).
Best Practice: avoid inlet lines that are too small, too long, or contain too many fittings or valves.
What to Watch Out For
Chatter
The repetitive opening and closing action causes vibration in the piping and may lead to reduced PRV capacity, high piping vibration or loads, flange leaks, and piping failures.
Best Practice: Know pressure, temperature, media, flow rate, and viscosity to help design the appropriately sized valve and determine the flow characteristics of the valve when in service.
Acoustic Interaction
Acoustic interaction occurs when fluid flowing past a PRV entrance cavity can excite a standing wave, resulting in noise and vibration.
Best Practice: Recent API 520 changes recommend conducting an acoustic interaction check, particularly for liquid lines where most of the chattering valves have been identified.
Recent Changes to API 520 6th Edition
It is now possible to allow the pressure drop to go above 3% of the set pressure if a plant has completed a supplementary engineering analysis. API 520 states an engineering analysis must include a thorough review of past inspection and operating records to determine if there is any historical evidence of chatter.
Equity’s Engineering Analysis Expertise
Determining which PRV installations require costly modifications and which ones can be mitigated by future analysis will save time and money. At Equity, our team of internationally recognized experts in PRS design, optimization, and installation will conduct an engineering analysis and provide practical recommendations.
Our approach includes:
- Force balance assessment: predict valve stability by correlating flow test data with the inlet and outlet pressure drop data and PRV opening and closing characteristics.
- Acoustic interaction check: recommended on liquid lines where majority of actual chattering valves are experienced.
- Documentation: comprehensive documentation of the engineering analysis is important to maintain compliance with OSHA or other local regulatory requirements.
