Check valves are automatically opened or closed by the directional flow movement of the process media.
The closure member comes in several designs. The most common types in the energy industries are swing disc, ball, piston, and dual plate. The swing design is normally supplied on sizes 50mm and above whilst the piston and ball is provided on 50mm and below. Despite this industry practice the swing disc option is available below 50mm and the piston 50mm and above. Dual plate models are available from 40mm and above but are generally only used on offshore applications due to their weight and space saving design.
Check valves are available for unusual process conditions. For instance where the process may experience a sudden flow reversal an external dashpot or balance weight fitted to a swing check will slow down the closure preventing shock loading by the disc slamming into the seat face. The external balanced weight can also be adjusted to assist closure where there is a low pressure differential across the seat. An axial flow design is often specified where there is a pulsating or low flow condition where a normal swing check may cause 'chatter'. The axial valve offers the least resistance of any check valve and therefore the least prone to slam and pressure surge.
In the open position, a swing check has very little obstruction to flow, resulting in minimal pressure drop across the valve. The piston and ball type check valves have a tortuous flow path and therefore a high pressure drop. In the dual plate valve, closure members remain in the flow path when fully open, causing increased pressure drop.
Typical check valve applications are for clean process fluids or utilities. These valves are unsuitable where the process has particles in suspension as leakage can occur as a result of particles becoming trapped between the closure member and the seat during the closing operation.
The main components of steel ball, piston check and swing check valves are generally manufactured from forgings from 6mm to 50mm size. Swing check valves are also manufactured from castings in sizes 50mm to 1800mm. Dual plate valves are available in forgings or castings in sizes 40mm and above. On 50mm and below the connections offered on swing, ball and piston checks are screwed and socket weld. Flanged and butt weld connections are normally available on all sizes but generally specified as standard on sizes 50mm and above. Hub end connections are also offered in sizes 50mm and above although not widely used. The dual plate check valve is generally a wafer design suitable for inserting between flanges although flanged, butt weld and hub ends are also available in the market. Check valve ratings vary from ASME class 150 to class 4500lb or API 2000 to API 10,000.
Non Return Valves