REGULATING VALVE APPLICATIONS PRESSURE REDUCING STATION • Parallel for High Flow Turndown to Deaerator
PURPOSE:
For reducing steam system inlet pressure to a constant outlet pressure when steam flow rates vary widely. This will help improve system rangeability resulting in more accurate control. A parallel configuration is often ideal when supplying steam to a deaerator tank as they commonly experience a wide range of steam flow rates depending on the actual make-up water heating requirements. Consult deaerator manufacturer for accurate and reasonable sizing guidelines. Because regulators are simple, self-powered devices which do not rely on an external control signal to determine valve steam position, they may not have the flow rangeability of control valves. Therefore, if an application has large flow variations (such as a deaerator), multiple regulators should be considered to improve operation and control. Typically referred to as a 1/3 - 2/3 system, one valve may be sized for approximately 1/3 of the total maximum load demand and a larger valve for the remaining 2/3. When full load is required, both valves will be open and regulating. The small regulator is typically the primary valve and is set at a pressure 2 psi higher than the larger secondary valve. This allows the small regulator to be the only one flowing when demand is low. When flow increases and the small valve cannot keep up with the demand, the downstream pressure will begin to drop which will allow the larger secondary valve to open in order to help satisfy the demand. Although the smaller regulator is commonly selected as the primary valve, either the smaller or larger regulator may be set as the primary valve based on anticipated load demand requirements. The primary valve should always be set a minimum of 2 psi above the secondary valve.
OPERATION:
INSTALLATION GUIDELINES:
(see Figure 10)
• This example depicts a parallel 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. • Proper setting of the valves is key to proper operation. The chosen primary valve should be set at a pressure approximately 2 PSI higher than that of the secondary valve. • 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 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. • 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. • When the pressure drop exceeds the recommended operation of a single-stage PRV, a 2-Stage (Series) station should be used. Orifice plates may also be considered to limit pressure drop across a single-stage PRV and reduce noise. Consult the factory for sizing and selection assistance.
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