REGULATING VALVE APPLICATIONS PRESSURE REDUCING STATION • Using Spring-Loaded Pilot and Trip-Stop Valve
PURPOSE:
For system protection after pressure reducing valves when applicable codes allow for installation of a Trip-Stop valve in lieu of a Safety Relief Valve (SRV) to prevent over-pressurization. Consult factory for allowable use.
OPERATION:
The Trip-Stop pilot is installed on a separate HD Series main valve downstream of the pressure reducing valve. If the downstream control pressure increases above the factory-set pressure on the Trip-Stop pilot, the pilot trips to release the diaphragm pressure on the main valve, closing the valve to stop steam flow. Once the pressure downstream is below the set point, the pilot may be manually reset.
INSTALLATION GUIDELINES:
(see Figure 6)
• This example depicts a pilot-operated steam PRV with a separate trip-stop valve, whereby an external sensing line is required to sense downstream pressure. The end of the sensing line is placed away from the turbulent flow the trip-stop valve outlet. This helps to improve accuracy of the set pressure. Set pressure is adjusted by turning a screw on the PP pressure pilot to increase or decrease compression on a balancing spring. • The trip-stop main valve should be sized and selected to provide minimal pressure drop and to allow full steam flow downstream of the main PRV. A low-differential or low-pressure main valve is typically required for the trip-stop valve. Consult factory for appropriate sizing guidelines. • For optimum operation and service life, maintain recommended minimum piping straight runs before and after the PRV. Inlet pipe diameters could be 1-2 sizes larger and outlet pipe diameters 2-3 sizes larger than the end connections of an appropriately sized PRV. The purpose of increasing the pipe size downstream of the regulator is to keep the steam velocity constant on both sides of the regulator. • The pressure sensing line should slope downwards, away from the regulator, to prevent condensate from entering the pilot. • Eccentric reducers, if required, are used on valve inlets to prevent accumulation of condensate which could become entrained with high-velocity steam, possibly resulting in dangerous waterhammer. • While the separator shown upstream is appropriate for protection of the PRV, it is not always required as a properly sized drip leg with steam trap may be sufficient. It is recommended for systems where steam is known to be “wet” and the entrained moisture could affect valve performance and/or result in component damage. • Consider installing a properly sized bypass line with globe valve to provide continuous operation should regulator maintenance be required. • Consider low-cracking pressure (1/4 PSI opening pressure) check valves after steam traps when discharging into condensate return lines. Check valves eliminate the possibility of condensate backing up through the steam trap into the system.
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www.watsonmcdaniel.com •• Pottstown PA • USA • Tel: 610-495-5131
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