A steam pipe will expand during a cold start condition, therefore in our layout design, provisions must be made to accommodate the expansion without any distortion to the pipework and subsequent damage occurring. It is an irresistible force, which must be controlled, and you can do this by applying the use of either expansion joints or fabricated expansion loops.
Whenever the pressure in a steam distribution system is to be reduced and the steam on lower pressure is to be carried on in another distribution system, you should use Pressure Reducing Stations (PRS). But the common use is to reduce the pressure right before the line equipment, because high pressure steam occupies less volume than that of low pressure steam and thus carrying high pressure steam requires small size pipes. This reduces the initial investment in pipes and related accessories. For that the act of reducing pressure is discussed in the steam utilization part.
Process equipment could be damaged if there's a malfunction in the PRV. In order to protect the used equipment from excess pressure, Safety Valves should be used. The safety valve should therefore be set to discharge at a pressure safely below the safe working pressure of the downstream equipment being served by the reduced pressure steam.
The capacity of the safety valve is important, it must pass at least the full capacity of the reducing valve, should the PRV fail in an open position. Anything less than that is unsafe and unacceptable.
Lastly, the safety valve must discharge to a safe place, and, if there is a rising pipe taking the safety valve discharge to a higher level than the valve then the base of the discharge must be fitted with a drain point. This is to prevent a build up of water on top of the valve clack due to "simmering" or leakage from the valve. The open end of the discharge pipe must be arranged so that it does not allow rainwater to enter it.