Case Study Surge- Anticipating valves A Comparison to Air Vessels as surgepreventing means in pumping systems Assaf Heiman*
General Any change in water ow velocity creates a transient phenomenon which manifests itself as pressure oscillation in the network, more commonly called “surges” or “water hammer”. These surges are an array of alternating pressure waves, higher and lower than normal pressure conditions. High pressure surges may rupture the pipe and potentially cause damage to various accessories (e.g. check valves, water meters, air valves etc.).
Low pressure conditions may reach negative values causing pipe to collapse due to the external load of atmospheric pressure and soil, damage joints, create introduction of contamination through the joints and cavitation damage to some types of pipes. One of the most common causes of pressure surges is a sudden halt in pumping. This assay will review this event.
Sequence of surges created by a sudden pumping cessation Following the sudden stop of the pump(s), the pressure at the discharge of the station drops below the static head at the pump location (“downsurge”). This low-pressure propagates as a wave at a sonic speed into the downstream network, reducing the head of the entire system. At local high-points along the pipeline, the hydraulic gradient may be lower than the pipe-elevation. In these sections a system water-column separation may occur as the water mass is separated by gas “pockets”, which are either lled by vapor or by air (introduced by an air / vacuum valve).
* Director of Application Application Engineering at “DOROT CONTROL VALVES”
At this stage, since the hydraulic gradient is reversed, a returning ow is generated by the head of the receiving reservoir or the highest point in the network. The return ow rells the gas pockets and is eventually 36
Surge-Anticipating electrically operated valves protecting an waterworks system
stopped by the now closed Check valve(s) of the pumping-system. Both events - the re-joining of water columns and the abrupt ow stop at the check-valves generate a ch ange of velocity and consequently, a high pressure wave (“Upsurge”). The high pressure is reected as waves, initiating a water ow in the original, normal-direction which consequently causing a low pressure wave, etc. . The above sequence may continue to oscillate several times
until, eventually, the friction of the network dissipates the energy and the system rests at static pressure conditions.
Surge-anticipating valve operation The surge-anticipating valve is located on a branch of the main pipe, downstream the check valve(s) in the pump station. It opens instantly when the initial downsurge
System Comparisons Surge-anticipating valve
A i r v es s e l
Negative pressure Does not prevent negative pressures, requires prevention design of proper air valves and / or one-way lling tanks
Prevents negative pressurewhen air valves are ositioned for the pipe lling stage.
Acce Access ssor orie ies s
Does Does not not req requi uire re any any ext exter erna nall acc acces esso sory ry,, exc excep eptt a drain pipe
Requires a compressor, a backup compressor, co mpressor, a sophisticated control system to maintain the proper water volume in the tank
Chec Check k valv valve e slam slam
Prev Preven ents ts me mech chan anic ical al slam slam of chec check k valv valve e
Incr Increa ease ses s me mech chan anic ical al slam slam of the the pump pump`s `s chec check k valve
Syst System em Cost Cost
Lowe Lowerr cost cost for larg larger er syst system emss- valv valve e size size is reduced, mostly, as the pipe is longer. lo nger. (Long mains have higher friction, minimizing the return ow).
High cost for larger system. The tank size increases with capacity of the pipe, so longer pipes require larger vessel. Cheaper for small system protection
Main Ma inte tena nanc nce e
Low Low main mainte tena nanc ncee- req requi uire res s only only per perio iodi dica call test tests s
High High mai maint nten enan ance ce-c -com ompr pres esso sorr, cont contro roll syst system em
Test
Enables periodical test of operation, without stopping pumping
Does not allow testing without complete stopping the pumping
Space
Requires minimal space
Requires large amount of space
Relief function
Included in the standard valve
Not included, cannot prevent abnormal high pressure in the network through pumping
Water loss
Releases water from the network
Does not release water
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Surge-Anticipating valves protecting an untreated sewage-pipeline
materializes, and is kept in a fully open position until the returning ow reaches its location. The return-ow gets discharged out of the system instead of slamming to a halt at the closed check-valve, thus preventing the sudden velocity change that generates the high pressure wave. As the pressure rebuilds, the valve slowly closes so as to gradually reduce the returning ow in the system. The valve does not prevent the low pressure waves and the negative values. Therefore, it should be combin ed with air/vacuum valves, designed to prevent these conditions when sized and positioned properly.
Air Vessel (Surge Tank) Operation
The returning ow re-compresses the air, which dissipates the energy of the returning water mass, much like a spring.
Summary Though air-vessels are the most common method of surgeprotection used nowadays in design of pumping system, it is the author’s opinion that using Surge-Anticipating Valves should be seriously considered due to its low cost, ease of maintenance (cost of ownership) and reliability. Combination of Air Release Valves with Surge-Anticipating valves, if properly sized and adjusted, will supply an equal level of protection for large systems, at a much lower cost and smaller footprint.
The air vessel is a tank that is connected to the main pipe downstream the check valve(s). The tank is partially-lled with water and the rest of its volume is lled with air that is compressed to the pump’s normal operation pressure value. When the down surge materializes, the air expands and the water in the tank is discharges into the mains, instead of the ow from the pump (which has now stopped). If sized large enough, it maintains the pressure in the pipeline to the value that prevents the creation of negative values anywhere in network.
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