Check valves are pressure actuated with the operating force coming from the fluid. Some non-zero force is always needed to make a seal by preloading the valve shut and/or also from fluid pressure in the reverse direction. All check valves have their flow curves specified with at least three pairs of factors; first, an opening pressure defined as that is needed for a specific minimum flow, second is a “full flow” at some higher pressure appropriate to the end application and third, a maximum reverse leakage at some pressure or over an indicated pressure range.
Preloaded valves are called “normally closed” and will always require some degree of non-zero, positive pressure in the forward direction to make them open. Some examples include the umbrella check valve, spring loaded valves and coaxial valves.
Non-preloaded valves are called “normally open” and will begin to flow with the first application of pressure, but may also leak in the reverse direction until sufficient reverse pressure is applied to make a seal. Some examples include duckbill check valves, floating disc or ball valves.
Factors to consider when choosing a check valve:
- Chemical and/or biological compatibility
- Fixed or changing orientation
- Needed speed of operation
- Maximum reverse pressure
- Available space