As we have discussed here many times, check valves tend to be one of the most relied-on designs but also one of the most forgotten devices in process safety. We see check valves that have been in service for 30+ years and have never once been inspected or tested, yet they are listed every 5 years as a safeguard in the PHA. Finally, API has included some excellent inspection protocols for check valves in the latest 2024 edition of API 570.
In the 5th Edition of API 570, the guidelines surrounding Critical Check Valves (CCVs) were significantly expanded to ensure these safeguards reliably prevent hazardous process fluid flow reversals. Historically, routine thickness measurements were not strictly required for standard valves because their cast bodies are inherently thicker than the adjoining piping. However, because CCVs are often relied upon as critical safeguards or Independent Protection Layers (IPLs) in process hazard analyses, API 570 introduced more rigorous mechanical integrity expectations. Here is exactly how the code addresses CCVs and the new references to API 621:
The API 621 Connection: Minimum Thickness and CMLs
Instead of leaving valve body thickness requirements ambiguous, the new edition explicitly refers inspectors to API Standard 621 (Reconditioning of Metallic Gate, Globe, and Check Valves).
- API 621 is now the reference standard used to establish the absolute minimum acceptable wall thickness for the check valve body. This provides a concrete mechanical limit rather than forcing inspectors to estimate based on the schedule of the adjoining pipe.
- Valve bodies have complex geometries (bowls, seats, and bridges) that experience different flow turbulence and erosion rates than straight pipe. API 570 references API 621 to guide inspectors on where to establish CMLs on the valve body to effectively capture localized thinning.
Beyond thickness and CMLs, API 570 outlines specific performance and inspection criteria for CCVs to guarantee they function during an upset:
- Owner/operators must formally define and identify which check valves are “critical.” This is determined through a risk assessment evaluating the specific consequences of a backflow event, such as overpressure, inadvertent chemical mixing, or equipment damage.
- CCVs require internal inspections, which typically involve removing the bonnet during a shutdown. Inspectors are required to verify:
- The flapper is free to move without excessive looseness due to wear.
- The flapper stop is intact. A worn stop can allow the flapper to move past top-dead-center and hang open.
- The flapper nut is securely fastened to the flapper bolt so it does not back off in service (a single tack weld or lock washer is often deemed unacceptable).
- For valves with external shafts, the shaft retainer mechanism and seats must be inspected. For internal shafts, the inspector must verify the shaft cannot mechanically work its way out of the valve body.
- For alloy CCVs, Positive Material Identification (PMI) is recommended to ensure all internal replacement components meet the correct metallurgical specifications.
- In some process safety scenarios where the CCV is credited as a strict safeguard, mechanical inspection alone is insufficient. The standard acknowledges in-line function testing (backflow testing) to physically verify the valve seats and seals properly under reverse flow conditions.
Source: API 570, 5th Edition