I am ware about double check valve assemblies where two non-return valves are assembled in series in order to avoid risk of contamination of the upstream source due to backflow
I saw such an installation once and asked the same question. Answer: Double safety. Two different types to eliminate the risk of a manufacturing fault.
The check valves are used for avoid backflow, but its PFD (Probability of failure on demand) is high, some times is recommended to install two checks in series and PFD will decrease.
some times it installed in series for economic reasons, like two small check valves in stead of a huge big one. some time it depends on the pipe rating as well, (900# vs 300#)
renin,
Probability of failure of demand (PFD) is equivalent to the unavailability of a system at the time of a process demand.
Hazardous nature of the process determines the probability of failure tolerated in a safety application
If we use one return valve then the probability of failure to execute the required safety operation (i.e Prevent Back flow)is more. To reduce the PFD (i.e Increase safety)we use two check valves. Since the chance of both return valves failing at the same time is less the PFD is reduced.
I think that we can even reduce the PFD by selecting the return valve of dissimilar types.For example if there is a design problem in a return valve the chances of both failing at the same process condition is more.
In Safety systems like PLC its called as Safety Integrity level (SIL). You can learn more about in following links.
