To ensure safe operations, a safety valve should always be installed after the pressure reducing valve.
It is well recognized in the plant engineering teams that reducing the steam pressure to a value sufficient to meet the process temperature requirements reduces steam consumption. Therefore, pressure reducing valves (PRV) have become a common requirement across process plants.
The boiler generates steam at a higher pressure and this high pressure steam reached the process plant where it is reduced to a pressure that is required in the process. Using steam at the lowest possible pressure for indirect heat exchange reduces the steam required and thus the fuel consumption. This is because latent heat increases with a decrease in steam pressure. However the lowest possible steam pressure depends on the required process temperatures and the design of the process equipments.
In case of an accidental failure of the PRV it is possible that the high pressure steam will reach the process equipments. This can be the cause of a major safety hazard. Hence a safety valve should be installed at the downstream of a PRV.
The safety valve must be installed after the by-bass loop of the PRV such that it is always in line with the steam that goes to the process equipments. Additionally it should be set such that the maximum allowable pressure at the process equipments is not exceeded. In such an arrangement even in the case of complete failure of the PRV, the steam pressure at the equipment end will not exceed the equipment limits thereby ensuring safe operations.