T ECH N I C AL pipe section is completely drained... If the SAV is sized correctly, it often appears to be under-sized to the uninitiated, but what actually happens is that the smaller SAV size causes a higher flow resistance (headloss), effectively generating sufficient “back-pressure” for the SAV to start closing hydraulically. The ideal SAV size generates a hydraulic resistance (H2) at returning flow (Q2) as near as possible to static lift, which theoretically results in energy release while the pressure in the pipeline increases to the steady state static conditions over a shorter pressure range, but the SAV still needs to be large enough for sufficient energy release. A SAV installation should always have an isolating valve installed upstream of the SAV for maintenance purposes, and it is unfortunate that some less experienced designers are sometimes left with no alternative but to use these isolating valves to artificially generate “back-pressure” to enable SAV operation, by semi-closing the isolating valve during commissioning stages. However, this “workaround” is based only on guesswork and trial-and error attempts with considerable risk to the system, and when it eventually comes
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to maintenance of the SAV, everyone has “forgotten” how to re-adjust the isolating valve to the same opening position, or why it was done in the first place. Under-sizing the SAV on the other hand is also a risk to the system since it could prevent sufficient pressure energy release and can result in repeated re-opening of the valve by the secondary HIGH pressure pilot valve, resulting in excessive water losses. The second most common reason of SAV “failure” occurs if the initial downsurge after the sudden pump stop is not low enough to activate the primary “anticipating” pilot valve, leaving the SAV permanently inactive (closed). The root cause of this type of “failure” is a randomly specified SAV, installed “just in case”, without performing a surge analyses to prove that it was actually required in the first place. There are some system characteristics which could prevent a hydraulically operated, low pressure activated SAV from working properly, and for such cases an electrically activated SAV should be used instead, e.g. where the static lift is <20m which results in a too short required valve opening time, and the relatively close set point of the low
pressure pilot valve could trigger false SAV activation, and where distant peaks along the pipeline experience column separation after sudden pump stop, with subsequent water hammer when these columns rejoin, which is then reflected back to the pump site. Instead of a low pressure activating hydraulic pilot valve, the electrically activated SAV type involves a battery- operated timer which is set to start closing the SAV after an adjustable (calculated) period to allow for the high pressure surges to be released before it starts closing. Surge anticipating valves do protect systems, and will continue to protect those systems, as long as they are actually required, then sized-, installedand commissioned properly, hence the only way to achieve a successful surge anticipating valve is to have an experienced surge specialist analyze the system to specify accordingly. Acknowledgements Contributions from- and peer reviewed by Giora Heimann, Specialist Consultant for Dorot Valve Manufacturers and Jamie Pickford, WA State Manager for Amiad Water Systems.
pump industry | November 2013 | Issue 5
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