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


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.

E2G'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 E2G, 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.

Recommended Reading

Pressure Relieving Systems Expertise

The process technology team leverages its extensive experience with state-of-the-art technology to provide practical design, operation, and inspection considerations to manage risk and maximize the damage tolerance of equipment in process units, including pressure vessels, tanks, heat exchangers, and piping. 

Our experts: 

  • Have 100+ years of experience 
  • Have served as API 520 task force chairman for more than 25 years 
  • Are the primary authors WRC 528 and WRC 562 
  • Serve as lead investigators on API 579 and API 571