REGULATING VALVE APPLICATIONS PRESSURE REDUCING STATION • 2-Stage (Series) for High Pressure Turndown
PURPOSE:
For reducing steam system inlet pressure to a constant outlet pressure when the pressure drop exceeds the recommended operation of a single-stage pressure regulating valve (PRV). This will help reduce overall velocity, resulting in less noise and improved equipment service life. The 1st stage PRV is selected to reduce the initial steam pressure to a reasonable pressure between the initial inlet and desired outlet delivery pressure. This intermediate pressure is typically selected to ensure that each PRV is within recommended turndown guidelines. However, it is also possible there will be a use for steam at a specific intermediate pressure, which must be considered when evaluating turndown and sizing guidelines. The 2nd stage PRV, installed in series with the 1st stage, then reduces pressure to the final outlet delivery pressure. Individual valve setting and operation is the same as for single-stage applications.
OPERATION:
INSTALLATION GUIDELINES:
(see Figure 8)
• This example depicts a two-stage (series) pilot-operated steam PRV pressure reducing station using HD Regulators with Pressure Pilot. An external sensing line is required to sense downstream pressure from each regulator. The end of each sensing line is placed away from the turbulent flow at the valve outlet. This helps to improve accuracy of the set pressures. Set pressure for each PRV is adjusted by turning a screw on the pilot to increase or decrease compression on a balancing spring. • 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. • Each 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 PRVs, 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 on each stage, 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. • A safety relief valve (SRV) is appropriate where applicable codes dictate their requirement, or anywhere protection of downstream piping and equipment from over-pressurization is desired. The SRV needs to handle the complete volume of steam from the regulator and bypass loop. Consult the factory for appropriate SRV sizing guidelines. • Orifice plates help reduce pipeline noise and are available as needed. Series OP orifice plates are custom engineered to maximize noise attenuation and reduce dbA to the lowest achievable value. The number and diameter of holes will be determined based on application conditions, and the plate diameter will typically be equal to the recommended downstream pipe size. The regulator(s) must be sized in conjunction with the orifice plate. For PRV and orifice plate sizing, consult the factory or SteamWorx sizing program on www.watsonmcdaniel.com.
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www.watsonmcdaniel.com •• Pottstown PA • USA • Tel: 610-495-5131
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