2000 Hook-up Book

Pressure Reducing Stations

PRV Station Components A stop valve is usually needed so that the steam supply can be shut off when necessary, and this should be followed by a line size strainer. A fine mesh stainless steel screen in the strainer will catch the finer particles of dirt which pass freely through stan dard strainers. The strainer should be installed in the pipe on its side, rather than in the conventional way with the screen hanging below the pipe. This is to avoid the screen space acting as a collect ing pocket for condensate, since when installed horizontally the strainer can be self-draining Remember that water which collects in the conventionally piped strainer at times when the reducing valve has closed, will be carried into the valve when it begins to open. This water, when forced between the valve disc and seat of the just-opening valve, can lead rapidly to wire-drawing, and the need for expensive replacements. Pressure gauges at each side of the reducing valve allow its per formance to be monitored. At the reduced pressure side of the valve, a relief or safety valve may be required. If all the equipment con nected on the low pressure side is capable of safely withstanding the upstream pressure in the event of reducing valve failure, the relief valve may not be needed. It may be called for if it is sought to protect material in process from overly high temperatures, and it is essen tial if any downstream equipment is designed for a pressure lower than the supply pressure. Steam Safety Valve Sizing When selecting a safety valve, the pressure at which it is to open must be decided. Opening pres sure must be below the limitations of the downstream equipment yet far enough above the normal reduced pressure that minor fluc tuations do not cause opening or dribbling. Type “UV” Safety Valves for unfired pressure vessels are tested to ASME Pressure Vessel Code, Section VIII and achieve rated capacity at an accumulated pressure 10% above the set-to

open pressure. Safety valves for use on boilers carry a “V” stamp and achieve rated capacity at only 3% overpressure as required by Section I of the Code. The capacity of the safety valve must then equal or exceed

the capacity of the pressure reducing valve, if it should fail open when discharging steam from the upstream pressure to the accumulated pressure at the safe ty valve. Any bypass line leakage must also be accounted for.

SYSTEM DESIGN

Figure 37 Typical Installation of Single Reducing Valve with Noise Diffuser

Bypasses may be prohibited by local regulation or by insurance requirements

Safety Valve

Pressure Sensing Line

Separator

Diffuser

Downstream Isolating Valve is needed only with an alternative steam supply into the L.P. System

Reducing Valve

Trap Set

Figure 38 Typical Installation of Two Reducing Valves in Parallel

Figure 39 Two-Stage Pressure Reducing Valve Station with Bypass Arrangement to Operate Either Valve Independently on Emergency Basis

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