According to API 570, Piping Inspection Code: In-service Inspection, Rating, Repair, and Alteration of Piping Systems, Critical Check Valves (CCV) are defined as check valves in piping systems that have been identified as vital to process safety. CCVs can be found throughout every facility’s processing-equipment units and systems. They are check valves that need to operate reliably in order to avoid the potential for hazardous events or substantial consequences, should reverse flow occur. A good rule of thumb for determining whether a check valve is "critical" in your process is, when using your site’s risk matrix, if the failure of a check valve causes a consequence of concern or if a check valve is identified as a safeguard to reduce the likelihood of a consequence of concern, hence lowering a specific risk to an acceptable level, then that check valve should be classified as critical.
Common Types of Check Valves
Swing Check Valve Piston Check Valve
Ball Check Valves Disc Check Valves
Butterfly Check Valves
How to Inspect Critical Check Valves
Critical check valves are inspected/tested using three main methods: externally, internally and, in unique situations, in-line function testing.
External Inspection
External visual inspection of check valves is part of the API 570 external visual inspection for piping circuits. The scope of this type of inspection includes a visual inspection of the piping and all the piping components, including check valves.
Internal Inspection
Internal visual inspections typically involve removing and disassembling the bonnet cover and inspecting for fouling, corrosion, and worn or missing parts. Larger valves can be removed from service and inspected. Smaller, welded valves can still be inspected by removing the bonnet during a shutdown.
According to API 570, a CCV internal visual inspection typically includes the following steps:
- Check to ensure the flapper is free to move as required, without looseness beyond tolerance due to wear.
- The flapper stop should not have wear beyond tolerance. A worn flapper stop will allow the flapper to move past the top dead central position and remain in an open position when the check valve is mounted in a vertical position.
- The flapper nut should be secured to the flapper bolt to avoid backing off in service.
Backflow leak checks of CCVs are not normally required, but might be considered for special circ*mstances. This test, also known as a bubble test, is usually performed in a valve shop by applying air to the downstream side of the check valve while the upstream side is submerged in water; then, the number of bubbles is counted and the data is compared to specific acceptance criteria. API 598, Valve Inspection and Testing, gives guidance on the criteria for performing a bubble test.
Furthermore, per API 594, Check Valves: Flanged, Lug, Wafer and Butt-welding, when a nut is used to assemble a disc or plate to the hinge arm, the nut must be positively secured to prevent separation. A single tack weld, lock washer, or lock nut is not acceptable. Also, check valves with external shafts require an inspection of the condition of the shaft retainer mechanism and seats. Check valves with internal shafts must be inspected to ensure the shaft cannot work its way out of the body.
If needed, certified valves shops can measure CCV critical dimensions and restore the valves to like-new condition. For alloy check valves, Positive Material Identification (PMI) should be performed to ensure the CCV internal components meet specifications.
In-Line Function Inspection
In-line backflow testing is not as common as external and internal inspections, but some companies do require in-line testing (function testing) to take credit for an IPL. If necessary, in-line testing requires a specific piping configuration to simulate reverse flow and measure such backflow on the upstream side of the check valve.
CCV Integrity Management
Ensuring the mechanical integrity (MI) of critical check valves is now a requirement ("shall") under API 570; but more than that, history has proven that reliable check valves prevent process safety incidents. If you do not know where to start, here are five steps to establishing your CCV MI program.
- Identify and validate the check valves that should be considered critical.
- Determine the risks associated with the failure of those identified check valves.
- Based on identified risks and process conditions, develop inspection strategies, associated frequencies, and an appropriate level of inspection to ensure the proper function of each CCV.
- Inspect and document the inspections for each CCV.
- Adjust inspection frequencies based on previous results and lessons learned.
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