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By Johan Cilliers, Valve Product Manager, Amiad Water Systems

Surge anticipating valves (SAVs) offer relatively low maintenance, low cost, and small footprint compared to other surge protection alternatives, but there is a reluctance to use surge anticipating valves by many end users and pump providers, frustrated by the valves “not working”.

SAVs play an essential safety role for pumped systems where there is a risk that the system could suffer from the destructive force of water hammer caused by a sudden pump stop. Slow release (“anti-slam” or “non-slam”) air valves are also effective in assisting with the surge mitigation in conjunction with SAVs.

If the (unprotected) example system experiences a sudden pump stop, a surge pattern with extreme pressures can be generated when the kinetic energy of the returning flow is converted to potential (pressure) energy, oscillating along the pipeline.

The time it takes for this pressure surge to travel to the end of the pipe and back (TC) can be calculated if the length of the pipe (L) and the pipe celerity (c) (speed at which a pressure wave will travel along the pipe) is known.

[TC = 2 x L/c]

TC can be several seconds, even minutes, e.g. for a 5000m steel pipe with celerity of 1000m/s, the high pressure surge will return to the pump station after 10 seconds (2x 5000/1000).

TC also indicates if true water hammer will occur in a system, where full Joukovski surge pressure should be expected if the event causing the surge (e.g. pump stopping time, or valve closing time) occurs in less than TC.

Even if the pipe or a fitting does not fail instantly at some point along the pipeline during this surge event, material fatigue can lead to eventual failure. If no immediate failure occurs, the surges will eventually dampen out due to pipe friction & liquid viscosity over time.

A hydraulically operated SAV consists of an automatic control valve installed on a tee off the main line, located immediately downstream of the main pump station check valve, plumbed with two hydraulic pilot valves and a hydraulic pressure sensing tube connected to the main line.

When the system is protected by a properly selected- and located SAV, the SAV will open the moment it senses the initial low pressure evident after such a sudden pump stop, “anticipating” a high pressure surge to return to the location of the SAV, which is now fully open to direct most of the returning flow out of the main pipe to atmosphere, preventing the conversion of the kinetic energy of the retuning water mass into potential (pressure) energy. In most systems the low pressure pilot valve is the only pilot valve which actually operates, and when the pressure resumes above the low pressure set point upon arrival of the retuning water mass, the SAV will start to close at an adjustable pace to eliminate secondary surges caused by too fast a valve closure.

For some long pipe systems with a long TC, combined with relatively small SAV, which limits the valve closing time due to the relatively small volume of the SAV control chamber, some secondary surges could be generated by the “too fast” closing SAV, which is then interrupted by the secondary, HIGH pressure pilot valve which re-opens the SAV sufficiently to stop the pressure rise when those secondary surges arrive at the SAV location, and the SAV  then continues to close again once the pressure drops below the HIGH pressure pilot valve set point.

The pressure drops (T1) and increases (T2) occur in the network typically in milliseconds (spikes), and given that the time to physically open the SAV depends on the speed at which the SAV control chamber can be drained through the activated pilot valve, it is sometimes necessary to add an accelerator valve to speed up the SAV opening speed, particularly for larger valves.

All automatic control valve types require a minimum operating pressure (5m or more), and the effective time available for the SAV to open before destructive pressure energy is reached during low pressure activation, is [T1 + T2].

There are two predominant reasons why SAVs could become a nuisance rather than a safety device, both due to incorrect valve selection.

The most common reason for a SAV “not working” is when it is over-sized, often leading to entire pipelines draining through the SAV which has not closed after activation, due to excessive drained flow.

Any hydraulically operated automatic control valve requires a minimum line pressure for it to operate hydraulically, and to understand why the SAV can fail to close if over-sized, we need to consider how the system pressure changes after the pump has suddenly stopped, and the SAV has opened on the low pressure signal. While the pump is running normally, the system hydraulic grade line (HGL) representing the pressure along the pipeline looks quite familiar, but once the pump no longer provides pressure energy and the SAV has been opened, the flow is effectively reversed through the same system, and the HGL changes accordingly, resulting in insufficient pressure energy at the SAV location, leaving the SAV stuck in the open position until the 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 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 down-surge 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-, installed- and 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.

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