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In most heating processes, saturated steam is commonly used. Consequently, pressure reducing valves (PRVs) are frequently seen in steam systems. However, pressure reducing systems often encounter issues such as overpressure of PRVs, activation of safety valves, pressure fluctuations, unstable heating, and frequent PRV damage. Many of these problems stem from potential risks left during the design and selection process. So how do we correctly select and configure a pressure reducing system to achieve effective pressure reduction and stability?
After choosing a reliable manufacturer, selecting the right PRV is critical. It requires a case-by-case evaluation based on actual working conditions. Below are the factors to consider when selecting a PRV:
・For conditions with low load, minimal pressure control requirements, and stable inlet steam pressure, a simple direct-acting PRV can be chosen.
・For other conditions, especially demanding situations (e.g., downstream pressure below 1 barg), pilot-operated PRVs are the best option. Specifically, diaphragm pilot-operated PRVs are required in such cases.
It is crucial to avoid oversizing the valve, as this could lead to severe consequences:
・When the valve disc is too close to the seat, wet steam passing through may cause erosion, leading to wire-drawing damage.
・Small valve movements can cause significant fluctuations in flow, making it difficult to precisely control pressure.
・Instability in pressure increases valve wear, reducing the valve’s lifespan and compromising stability.
Therefore, oversized PRVs not only increase costs but also shorten their lifespan and reduce pressure stability!
Basic Parameters for PRV Selection
Two key parameters must be understood for PRV selection: working pressure and steam flow rate. These factors determine the layout of the pressure reducing station—whether to use a single PRV, series, parallel, or combined arrangements.
A. Pressure Range: What are the inlet and outlet pressures?
・Pilot-operated PRVs typically have a pressure reduction ratio of 10:1, while direct-acting PRVs can go up to 20:1.
・For pilot-operated PRVs, if the pressure reduction exceeds 8 bar, internal components will operate at their limit, accelerating wear and mechanical fatigue. In this case, a two-stage pressure reduction system should be considered.
Key Notes for Two-Stage Pressure Reduction Stations:
・The installation distance between the two PRVs should be at least 30 times the valve diameter or at least 0.5–1 meter.
・If the first-stage PRV fails, the second-stage PRV will experience frequent overpressure or insufficient pressure. These pressure fluctuations differ significantly from those originating in boilers (which change gradually due to distance). Consequently, the second-stage PRV will over-adjust, accelerating wear and causing damage, accompanied by vibration and noise.
Recommendation:
If the pressure reduction range is less than 8 bar, avoid two-stage pressure reduction unless absolutely necessary.
B. Steam Flow Rate
Flow parameters should be categorized into normal, maximum, and minimum flow rates:
1.For high flow rates, consider parallel pressure reducing stations.
2.If the flow rate fluctuates significantly, parallel stations are also recommended.
3.For critical applications that cannot afford downtime, use parallel stations with one PRV as a backup.
Using a single large PRV for high or fluctuating flow rates can cause issues such as:
・The PRV operates at a minimal opening, leading to concentrated wear in a small area, resulting in leakage and jamming.
・Pressure control becomes unstable and inaccurate, potentially leading to PRV failure.
・Flash steam generated during pressure reduction can erode the valve seat and disc, causing cavitation and noise.
For pilot-operated PRVs, do not select based on pipeline diameter. Typically, the PRV’s diameter is smaller than the pipeline’s (by at least one size).
For optimal performance, the main valve’s operating range should be controlled between 50–80% opening. Properly selected PRVs are more durable, stable, and cost-effective.
Risks of Improper Sizing:
・Oversized PRVs experience excessive erosion on the sealing surface due to prolonged operation at minimal opening, reducing lifespan and sealing performance.
・Undersized PRVs may lead to insufficient steam supply, pressure drop, and reduced diaphragm lifespan.
・For DN15–50 valves, diaphragm pilot-operated PRVs are recommended. For DN65 and above, with downstream pressure above 1 barg, piston-type PRVs are advised.
・If no steam-water separator is installed upstream, piston-type PRVs are preferable due to their robust design and resistance to water hammer.
Regardless of the PRV type, if water or impurities enter, the valve may fail and require manual resetting or part replacement. Severe damage may necessitate replacing the valve altogether.
