Harper International Article_6252: Pressure Reducing Control Valve

The Downfeed Demon: Maximizing Pressure

The Problem

The Solution:  Pressure Reducing Control Valves

A pressure reducing control valve station, feeding down, (reducing pressure to a lower elevation zone), creates a problematic hydraulic condition. Simply put, the valve is last to open, and last to close, while outlet pressure varies with down-flowing water.

The valve does not open until after demand has occurred, especially during high demand. The condition is exaggerated when high demand occurs at the bottom half of the zone. Such high demand create a precipitous drop in outlet pressure at the master station, tending to “pull” the valve open wider. The valve is “over-compensating.” Hard valve closure – even slamming and high pressure surges may happen during periods of intermittent high-demand, since the valve has become caught open when flow is diminishing.

The Solution

In many facilities, retrofits did not incorporate an upfeeding system, which dictates a continuum of the downfeed-style master pressure reducing stations. Careful selection of control valves, and a clear understanding of the total system demand are key to avoiding the problem. Since the pressure-reducing valve (at the highest elevation) is the last component to open, numerous smaller, faster acting valves, in conjunction with a larger pilot operated valve work effectively and avoid slamming and high-pressure surges.

Commercial water-works applications experience this same condition. This (down-feed condition) presents itself when the pressure-reducing station is located on a hilltop to protect the lower elevations from higher pressures. Due to the total system demand, it may not be practical to apply numerous smaller, fast-acting pressure reducing valves. However, careful valve selection, with tuned pilot systems can mitigate this condition.

Harper International’s goal is to reduce customers’ total cost of ownership, and to guarantee uninterrupted water supply. We specify and configure top-of-the-line valves so that water demands are consistently and cost-effectively met. Installing the proper valves may increase the initial investment, but ultimately saves money in maintenance and replacement.