Upload
dorotcontrolvalves
View
221
Download
0
Embed Size (px)
Citation preview
8/12/2019 Surge- Anticipating valves A Comparison to Air Vessels as surgepreventing means in pumping systems
http://slidepdf.com/reader/full/surge-anticipating-valves-a-comparison-to-air-vessels-as-surgepreventing-means 1/3
36
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”
8/12/2019 Surge- Anticipating valves A Comparison to Air Vessels as surgepreventing means in pumping systems
http://slidepdf.com/reader/full/surge-anticipating-valves-a-comparison-to-air-vessels-as-surgepreventing-means 2/3
37
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
8/12/2019 Surge- Anticipating valves A Comparison to Air Vessels as surgepreventing means in pumping systems
http://slidepdf.com/reader/full/surge-anticipating-valves-a-comparison-to-air-vessels-as-surgepreventing-means 3/3
38
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