Flow Control - Jan 2012
best practices bar from daytime to nighttime In addition pressure in a distribution system is almost always higher at the source pump station or reservoir and drops due to pressure loss as distance from the source increases and even more so if there is a topographic increase If distribution lines are feeding customers in areas close to the source it is likely too much pressure is being supplied for what is required in order to serve the most distant or highest elevation customers To reduce leakage it is best to give customers just enough pressure to get the job done but never more as that would add to leakage or wasted water A Programmatic Approach A sound approach in reviewing water loss reduction would include a pipe system annual replacement program focused on the most challenging areas and a leakage location plan where teams locate leaks and then expedite the repair of those leaks If the system is not gravity fed and pumps are involved Variable Frequency Drives VFDs can be one of many tools that can help mitigate this problem It should be noted that while VFDs can be used as an effective tool they should always be used in conjunction with a pressure management approach When considering a comprehensive water loss and leakage program it is common practice for water utilities to hire a consulting firm that specializes in this discipline After completing a water audit a consultant will usually analyze the system and begin to establish DMAs District Metered Areas which typically consist of a meter and pressure reducing valve These DMAs are often designed to be a dedicated pressure zone for between 2000 and 3000 service connections but this number can vary subject to population and the utilitys desire to find the smallest leak The meter then measures the overall flow into that specific pressure zone while the consumer industrial and commercial meters used for billing measure flows out of the zone Assuming the meters are calibrated this will give leakage rates in that zone With this information leakage location teams can now be directed into Typical DMA zone with meter and pressure reducing valve the most challenged zones to identify areas with the greatest need DMAs are always most effective if one source can be provided into a DMA zone and multiple meters and PRVs feeding off a looped system can be avoided With this approach the pressure reducing valve can be adjusted specifically to each zone and set the pressure in that zone to supply exactly what customers require never more In a small town or city of 10000 people or less the result may be four or five DMAs or pressure zones In a large city with many millions of residents there may be 100 to 200 pressure zones Each DMA will generally provide a payback of six months to one year when evaluating the cost of the water that would otherwise be lost Another interesting characteristic of pressure management is the reduction of pipe bursts that coincide with a sound pressure management program It is a well known fact that pipe bursts are usually a fairly significant portion of annual maintenance budgets Pipe bursts often happen in off peak periods or nighttime The reason why these bursts occur is the same as previously discussed i e due to demand being lower and pressure being higher By managing pressures better with DMAs or pressure zones annual maintenance budgets can be decreased and water loss associated with these pipe bursts plus consequential damages can be reduced Strategies for Pressure Reduction When selecting a pressure reducing valve its important to have the highest quality product available and minimum maintenance It is important to specify components like 316 stainless steel seats no bronze brass as this soft yellow metal will wear stainless steel fasteners on the external portion of the valve no plated or galvanized externals as they are prone to rust and always insist on heat fusion epoxy coatings on both the external portion of the valve as well as internally on all wetted surfaces avoid rusting bare ductile iron on the inside of the valve There are emerging technologies that can assist in ensuring the pressurereducing control valves are stable through Single rolling diaphragm technology 18 January 2012 Flow Control
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