What Is API Recommended Practice (RP) 591? Why It Matters in Downstream Facilities
In the oil and gas industry, equipment reliability is built on a foundation of standards, specifications, codes, and recommended practices (RP) that ensure safe and consistent API RP 591: Process Valve Qualification Procedure – Sky Bear Technical Standards stands out as a critical guideline for ensuring the quality and integrity of valves used in refinery operations.
For refineries, where high pressures, extreme temperatures, and corrosive environments are the norm, RP 591 provides a structured approach to valve inspection and testing that helps prevent failures, reduce downtime, and maintain operational excellence.
What Is API RP 591?
API RP 591 is a quality‑control recommended practice published by the American Petroleum Institute | API. Unlike design‑focused codes and specifications such as API 600 or API 602, RP 591 concentrates on inspection, testing, and quality assurance for downstream facilities.
The purpose of RP 591 is to:
- Establish minimum inspection and testing requirements
- Standardize quality‑control processes across manufacturers
- Ensure valves meet refinery‑level performance expectations
- Reduce the risk of valve failure in critical service
RP 591 is widely adopted by refineries, EPC firms, and valve manufacturers as part of their internal engineering standards and procurement specifications.
What Types of Valves Does RP 591 Cover?
RP 591 applies to a wide range of refinery valves, including:
- Gate valves
- Globe valves
- Check valves
- Ball valves
- Plug valves
- Butterfly valves
These valves are essential for controlling flow, isolating equipment, and maintaining safe operating conditions.
Key Requirements of API RP 591
RP 591 outlines a comprehensive set of inspections and tests that manufacturers must perform before valves are accepted.
1. Visual and Dimensional Inspection per MSS SP-55-2025: Quality Standard for Iron and Steel Castings for Valves, Flanges, Fittings, and Other Piping Components – Visual Method for Evaluation of Surface Irregularities – Sky Bear Technical Standards
Verifies that the valve meets the required specifications, tolerances, and material requirements.
2. Material Verification
Ensures all pressure‑containing components comply with applicable standards and codes.
3. Non‑Destructive Examination (NDE)
May include:
- Magnetic particle testing
- Liquid penetrant testing
- Ultrasonic or radiographic testing
These tests detect surface and subsurface defects that could compromise valve integrity.
4. Pressure Testing to API STD 598: Valve Inspection and Testing – Sky Bear Technical Standards, API SPEC 6D: Specification for Valves – Sky Bear Technical Standards or ISO 5208:2015 Industrial valves – Pressure testing of metallic valves – Sky Bear Technical Standards
Hydrostatic and seat leakage tests confirm the valve can withstand operating pressures without leaking or deforming.
5. Functional Testing
Evaluates:
- Smooth operation
- Torque requirements
- Proper assembly
- Actuator performance (if applicable)
6. Documentation Review
Manufacturers must provide complete traceability, including:
- Material certificates
- Heat treatment records
- NDE reports
- Pressure test results
This documentation is essential for refinery QA/QC teams and aligns with broader industry standards and specifications.
Why RP 591 Is Essential in Downstream Facilities
1. Improves Safety
Refineries operate under extreme conditions. RP 591 ensures valves are tested and inspected to minimize the risk of leaks, fires, or catastrophic failures.
2. Reduces Unplanned Downtime
Valve failures are a leading cause of unplanned shutdowns. RP 591 helps ensure valves perform reliably throughout their service life.
3. Enhances Equipment Reliability
By standardizing inspection and testing, RP 591 ensures consistent quality across manufacturers and valve types.
4. Supports Compliance
Many refineries incorporate RP 591 into their internal engineering standards and procurement specifications, helping them meet regulatory and operational requirements.
5. Lowers Long‑Term Costs
Higher‑quality valves reduce maintenance needs, extend service life, and prevent costly operational disruptions.
RP 591 Is a Cornerstone Valve Quality
API RP 591 plays a vital role in ensuring that valves used in refining operations meet the highest levels of quality and reliability. By providing a structured approach to inspection and testing, RP 591 strengthens the entire ecosystem of industry standards, codes, specifications, and recommended practices that keep facilities safe and efficient.
If your facility depends on high‑performance valves and every plant does, RP 591 should be a core part of your quality‑control and procurement strategy.and environmental safety, efficiency and sustainability.
