Check valves have a bad rap simply because they're misunderstood, and usually for the wrong reasons. Some argue these valves do not work because they're riddled with problems, such as water hammer (slamming), vibration, reverse flow, leakage, component wear and damage – all of which are detrimental to a downstream system.
However, the real cause of these problems usually stems from poor sizing and selecting of the check valve for the application; therefore, the problem originates with the application, not the check valve itself. Here's why and what to do about it.
Most check valves are selected on line size and the desire for the largest Cv available. This completely ignores the fact that flow conditions determine the internal performance of a check valve because its disc is directly in the flow path. Disc movement will occur inside the valve if there isn’t enough flow resulting in wear, potential for failure, and a higher pressure drop. A component failure can result in the valve not performing its function.
How is pressure drop affected? Pressure drop is calculated based on the check valve being 100% open. If there isn’t enough flow to achieve full opening, the pressure drop will be greater than calculated because the flow will be restricted by the disc in the flow path – ultimately resulting in failure.
An analogy: Some people say that check valves are like doors. If you open up the door to your office in the morning, you close it at the end of the day. Now, if you're opening and closing the door consistently throughout the day, the hinge pins will eventually wear out and fail, since they’re the “weak link” in the operation of the door.
Check valves face a similar situation.
Pins, stems, springs, or other components that are constantly cycled can fail. A check valve with a high Cv in a low flow application is doomed from the start. It’s not the check valve's fault, but the fault of the wrong valve selection for the application. The valve would have worked well for proper flow conditions.
For proper check valve selection, consider the following:
- Material compatibility with the medium
- Valve rating (ANSI)
- Line Size
- Application data
- Fluid characteristics, including temperature & pressure
- Seat type
- Horizontal or vertical
- End connection
- Envelope dimensions
- Leakage requirements
For more information about check valves, including how they work, check out an earlier post – The Quick And Dirty Guide to Check Valves.
Always review the application and service conditions with the manufacturer, or contact the Crane Engineering Valve team before making your next check valve selection!