The moving parts usually press against the valve seat to form a seal. A small pressure must be applied to the moving parts to open the valve. Once opened, fluid power is generated to open or increase the valve opening. Fluid flow must stop before the valve will close. The fluid dynamics created by the fluid flow prevent all valves from closing. A spring may or may not be used to control the opening and assist in closing. Some check valves rely on gravity alone to provide the closing force. Valves that rely on gravity must be installed according to valve manufacturers' instructions. If horizontal pipes are specified, then the slope of a short distance local pipe must be changed.
Swing check valves are powered by gravity. As the valve opens, the force required to control the opening of the valve increases. If the balance between disc weight and fluid dynamic forces is incorrect, the valve will not fully open. Increasing the flow rate may cause unexpected corrosion or erosion damage, so gravity-driven valves must meet operating conditions.
When the valve is fully open, the disc or piston travel must be limited by a stop. A valve that is fully open, but not restrained, is susceptible to vibration. Vibration can cause rapid wear of the ammonium chain pins or piston. Valves using springs may experience early spring failure (caused by fatigue). Vibration may be caused by eddies or disturbances. When the fluid has some viscosity, fluid damping can restrain vibration. Valves using springs may be configured with springs of varying stiffness. This can be an effective vibration damper if the full travel stop includes squeezing to prevent rebound after a quick start.
Squeeze includes a seat and plate or piston design that prevents the check valve valve from slamming shut. The addition of extra material on the seat creates two squeeze areas. Try to squeeze fluid out of these areas, slowing down the valve during the snap shut. But there is a price to be paid for this. The increased area of limited clearance is an ideal site for collecting small solid particles. Controlled shut-off squeezing force protection can cause further problems due to collected solids unless there is a sufficient squeezing gap to eject the solids. Fragile solids such as coal may be crushed with a narrow seal. The squeeze area tends to expand the effective seat width and reduce the valve's ability to crush solids. This effect must be taken into account, taking into account all relevant solid properties. Ball valves often have very narrow seats and may scavenge solids for more efficient seating.
Vibration problems may be limited to small valves. When the valve is larger, the inertia of the moving parts increases. The increased inertia may effectively damp vibrations and result in a delayed shutdown after reverse flow has begun. Damping of the valve seat becomes very important. For all valves, the flow path area must be checked and flow rates calculated for design operating conditions. Disc and piston area are as important as primary orifice area. Smaller runner areas are susceptible to corrosion and cavitation wear can occur. For specific functions, the valve body of the check valve may include auxiliary connections, such as vents and drains. Valves for thermal applications may sometimes have a bypass valve to allow the system to warm up at low flow rates.