Flow Control - Jan 2012
out their entire stroke Traditionally flat diaphragms have been used in most diaphragm operated pressure reducing valves These valves can operate very effectively when operating mid stroke but can be challenged at very low flows to be stable There is a technology available called Single Rolling Diaphragm which can supply very consistent pressure throughout the entire stroke of the valve virtually from shutoff to the highest flow required This is best for pressure management systems during low flow periods A standard pressure reducing valve PRV can be a very effective tool to control pressure Once installed there are many options that can be incorporated in the initial PRVs to make this valve have different functions With a standard PRV the only limit is there is only one pressure setting downstream and this cannot be changed without manually adjusting the pressure There must also be some pressure at the inlet to the valve minimum of 10 PSI or 6 bar for the valve to function Another approach that can assist a pressure management scheme is to use a standard PRV and two mechanical pilots two pilots specific to nighttime pressure and daytime pressure and to use a simple inexpensive battery operated timer IP 68 and solenoid to change pressures based on time This approach can supply higher pressures in the daytime but cut pressures back at nighttime or off peak periods A third approach is to have a special mechanical pilot modulate pressure based on flow This approach includes the use of a proprietary pilot system and can change pressures based on flow For this technology to be most effective there must be a minimum of 45 PSI 3 bar at the inlet to the valve and only one supply point into the DMA This valve can mechanically change no batteries or power required pressure based on flow by up to 30 PSI fully adjustable This means it will lower pressure at nighttime when demand is low and pressure is high and will increase pressure in daytime when demand is high and pressure is low A fourth approach can be full SCADA control of PRVs By adding a 4 20 mA motor and incorporating pressure switches and flow information a complete system can be operated remotely through SCADA constantly adjusting pressure based on varying flow demands In addition to pressure management a well thought out program of pressurerelief valves and the associated topic of transients surge is an area that is often overlooked Anywhere pumps start and stop suddenly due to PLC failure or power loss due to lightning storms there is risk of surge damage to the pumping and piping system as well as water loss and leakage due to pipe bursts or increased stress on the piping system All pump stations regardless of having single speed motors or VFD motors should have some sort of surge protection There are numerous models of diaphragm operated control valves that can provide this protection These models include standard pressurerelief surge anticipating and rate of riseanticipating pressure relief valves best practices Battery Operated Control with Two Pressure Settings Surge anticipating pressure relief example Standard pressure reducing valve Pressure and flow control mechanical solution 20 January 2012 Flow Control
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