Seawater check valves 
Check Valves or non return valves are operated by the flow of the line liquid (seawater).
Check valves are designed with a flap (disc) which can open fully alowing unobrstucted liquid flow through the valve. Any reverse of the flow or reduction of the liquid flow will cause the flap to close thus sealing the return of the liquid.
Variants of the check valve include, Lift Check, wafer check, duo check,screw down stop valve, piston check and ball check valve.
Check valves designs can include options to reduce the effect of water hammer, improving the life span of the valve and a locking device to lock the flap (effectivly closing or opening the valves)
Check valves can have flanged ends which are drilled according to pipeline compatible flange dimensional standards,although screwed and butt weld ends are also available
The ideal material for seawater is Nickel Aluminium Bronze because of its excellent corrosion fatigue and galvanic corrosion in comparason to austenitic and duplex stainless steel