A check valve looks like the simplest part in the line. Flow goes one way, and it stops flow from coming back. But after 20 years of pulling chattering, leaking and stuck check valves out of instrument and process lines, I can tell you most of those failures trace back to one of three things: the wrong cracking pressure, the wrong orientation, or dirt. Here is what to look at before you order one and before you install it.
Know Your Cracking Pressure
Cracking pressure is the upstream pressure it takes to push the poppet off its seat and start flow. Check valves are offered with different spring settings, from very low cracking pressures for gravity-fed or low-pressure lines up to higher settings for systems where you want a firm, positive shutoff.
Too high, and the valve may not open reliably at your normal operating pressure, or it will rob you of pressure you need downstream. Too low, and small pressure swings can let the poppet flutter on and off the seat. That chatter wears the seat and the spring, and before long you have a check valve that doesn't check. Match the cracking pressure to the pressure the line actually runs at, not the pressure on the drawing.
Reseal Pressure Matters Too
The valve closes again when the pressure across it drops below the reseal point, which is lower than the cracking pressure. In low-pressure or gas service, a valve that never quite reseals is a common source of slow back-leakage. If tight shutoff in reverse is critical, tell your supplier about the service so you get the right seal material and spring.
Install It the Right Way Around
It sounds obvious, but a check valve installed backwards happens more than anyone admits, especially in tight panels where the body gets flipped to make the tubing line up. Every check valve has a flow arrow on the body. Confirm it points downstream before you make up the fittings. If you are working with compression tube fittings, make the connections the same way you would anywhere else: tube fully bottomed, proper turns, and a gap gauge check where the fitting maker calls for one.
Keep It Clean
A small chip of tubing or a strand of PTFE tape caught under the poppet will hold it open. Deburr and blow out the tubing before installation, keep tape and paste off the first threads, and put a filter upstream if the system carries particulate. When a check valve starts leaking back, a dirty seat is the first thing to look for.
Match the Material and Seals to the Service
Choose the body material the same way you choose your tubing. 316 stainless is the default for most instrument and process work. Then look at the seal material. Elastomer seals are rated for a temperature range and specific media, so confirm compatibility with what is going through the line, especially in hydrogen, oxygen or chemical service.
Plan for Maintenance
Springs and seals are wear items. On critical lines, put the check valve where it can be reached and isolated, and add it to your PM list. A quick inspection during a shutdown costs far less than an unplanned backflow event.
A-Best Industrial Inc. Fittings, Valves, and Tubing stocks DK-Lok check valves in a range of sizes and cracking pressures. You can browse them on our check valves page, along with the compression tube fittings to connect them.
Not sure which cracking pressure or seal fits your line? Call us at 310-833-3353 or email sales@abesttube.com. We are glad to help you get it right the first time.