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GeneralAny change in water ow velocity creates a transientphenomenon 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 causin

pipe to collapse due to the external load of atmosphepressure and soil, damage joints, create introduction

contamination through the joints and cavitation damage

some types of pipes.

One of the most common causes of pressure surges is

sudden halt in pumping. This assay will review this even

Sequence of surges created by sudden pumping cessationFollowing the sudden stop of the pump(s), the pressure the discharge of the station drops below the static head

the pump location (“downsurge”).

This low-pressure propagates as a wave at a sonic spee

into the downstream network, reducing the head of th

entire system.

At local high-points along the pipeline, the hydrau

gradient may be lower than the pipe-elevation. In thes

sections a system water-column separation may occas the water mass is separated by gas “pockets”, whicare either lled by vapor or by air (introduced by an air

vacuum valve).

At this stage, since the hydraulic gradient is reversed,

returning ow is generated by the head of the receivinreservoir or the highest point in the network.

The return ow rells the gas pockets and is eventua

Surge- Anticipating valvesA Comparison to Air Vessels as surge-preventing means in pumping systems

Case Study

Assaf Heiman*

* Director of Application Engineering at “DOROT CONTROL

VALVES” 

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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 change of velocityand 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 th

energy and the system rests at static pressure conditions

Surge-anticipating valveoperationThe surge-anticipating valve is located on a branch of th

main pipe, downstream the check valve(s) in the pum

station. It opens instantly when the initial downsurg

Surge-anticipating valve Air vessel

Negative pressureprevention

Does not prevent negative pressures, requiresdesign of proper air valves and / or one-waylling tanks

Prevents negative pressurewhen air valves areositioned for the pipe lling stage.

Accessories Does not require any external accessory, excepta drain pipe

Requires a compressor, a backup compressor,a sophisticated control system to maintain the

proper water volume in the tank

Check valve slam Prevents mechanical slam of check valve Increases mechanical slam of the pump`s check

valve

System Cost Lower cost for larger systems- valve size is

reduced, mostly, as the pipe is longer. (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

Maintenance Low maintenance- requires only periodical tests High maintenance-compressor, control system

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

System Comparisons

Surge-Anticipating electrically operated valves

protecting an waterworks system 

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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 combined with

air/vacuum valves, designed to prevent these conditions

when sized and positioned properly.

Air Vessel (Surge Tank) OperationThe 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 itsvolume 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, insteadof the ow from the pump (which has now stopped). If sizedlarge enough, it maintains the pressure in the pipeline to

the value that prevents the creation of negative values

anywhere in network.

The returning ow re-compresses the air, which dissipate

the energy of the returning water mass, much like a sprin

SummaryThough air-vessels are the most common method of surg

protection used nowadays in design of pumping system

it is the author’s opinion that using Surge-Anticipatin

Valves should be seriously considered due to its low cosease of maintenance (cost of ownership) and reliability.

Combination of Air Release Valves with Surge-Anticipatin

valves, if properly sized and adjusted, will supply an equ

level of protection for large systems, at a much lower coand smaller footprint.

www.dorot.com

Surge-Anticipating valves protecting

an untreated sewage-pipeline