Water Hammer Calculation
Published on by ZEGAIT Rachid, Associate Professor at Kasdi Merbah University of Ouargla in Academic
How can I calculate the water hammer in ramified drinkable water conveyances?
How can the knocking noise in water pipes, when the water is suddenly shut off, be prevented while installing the pipes?
What can be done if the problem occurs after installation?
Taxonomy
- Water Hammer protection
- Infrastructure
- Utility Pipe Network
- Pipeline
- Pipeline Installation & Maintenance
- Pipes and Pipelines
11 Answers
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Avoid any abnormal shutdown of power failure at your pump station.
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Dear Mr Zegait,
I see that you already have a large number of useful replies, I will nevertheless add my take on the problem you describe.
I am assuming that you refer to a problem in a pipe network and not a problem in a house.
The problem you describe is influenced by a variety of factors such as flow velocity, vertical profile of the pipe, the speed with which the valve is shut and pipe material to name a few. The problem is best described by the Joukowsky equation that describes how the change in pressure (that causes the knocking) is influenced by the change in flow velocity and the celerity. The latter is a function of the pipe material properties and the properties of the liquid amongst others. Most modelling software packages are based on this equation.
Given the large number of factors it is not surprising that there are a large range of solutions and consequently there is not a "one solution fits all". I suggest that it may be worthwhile to engage an expert to assist you.
Kind regards,
Franciscus Maas
Senior Hydraulic Engineer
Opus International Consultants Ltd
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Dear Mr. Zegait,
If you need to correct water-hammer on an existing system, you can add an air tank - teed off the water line - and let the water pressurize the air in the tank during the pressure surge.
The air tank Maximum Allowable Working Pressure (MAWP) should ideally be higher than the pressure spike from the water hammer. However, you can also use an air tank of lower pressure rating and let the pressure relief valve vent the excess pressure (this is not optimal obviously).
The air tank volume can be determined by the theoretical delay in valve closure to reduce the hammer pressure wave: a) if the valve closed in x seconds instead of instantly, what volume of water would be flowing through? b) what is the acceptable volume reduction for the air before its pressure reaches the tank MAWP? This is the minimal volume for your tank.
As for everything else, try to oversize the air tank volume and MAWP.
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Water hammer/surges are the result of multiple variables! They may be created or subdued by adjusting any, or all of them. They are the pipe material, valve closing time, flow velocity, and or the length of the piping run. Attached are some references you may wish to review.
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Dear Zegait,
Some of the useful equations of calculation of water hammer in a pipe.
A water hammer commonly occurs when a valve closes suddenly at an end of a pipeline system, and a pressure wave propagates in the pipe. It is also called hydraulic shock. This pressure wave can cause major problems, from noise and vibration to pipe collapse.
Hammer calculation
From Perry you get the most common "rule of thumb":
h(wh)=a*DV/gc
where
a=sqrt(1/(den/gc*(1/K+D/b*E)))
(velocity of wave propagation)
and
h(wh)= water hammer head
V=change in fluid velocity
gc: grav. constant
den: Fluid density
K: Bulk modulus
D= pipe inside diameter
b: Pipe wall thickness
E: Young modulus for pipe material
Other equation,
P= 0.8 x W x V
Where P surge pressure psi
W Fluid density lb/cu.ft
V Velocity change ft/sec
This equation gives < 5% deviation from the standard equation
Pressure transients in pipe lines caused by a shock wave when closing or opening a valve can be calculated as
dp = 0.070 dv l / t (1)
where
dp = increase in pressure (psi)
dv = change in flow velocity (ft/s)
t = valve closing time (s)
l = upstream pipe length (ft)
1 ft (foot) = 0.3048 m
1 ft/s = 0.3048 m/s
1 psi (lb/in2) = 6894.8 Pa (N/m2)
Example - Water Hammer generated when closing a Solenoid Valve
The pressure increase (water hammer) in a 100 ft water pipe where water flow velocity is reduced from 6 ft/s to 0 ft/s when a solenoid valve close in 0.1 s can be calculated as
dp = 0.070 ((6 ft/s) - (0 ft/s)) (100 ft) / (0.1 s)
= 420 (psi)
With a closing time of 1 s (solenoid valve with damper) - the pressure increase (water hammer) can be calculated as
dp = 0.070 ((6 ft/s) - (0 ft/s)) (100 ft) / (1 s)
= 42 (psi)
Note! - it is important to open and close valves slowly and use soft starters to start and stop pumps to avoid damaging water hammers in piping systems.
http://www.engineeringtoolbox.com/water-hammer-d_966.htm
Regards
Prem Baboo
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Dear Richid，
After the pipe installation，here are 3 methods to reduce water hammer(or transient pressure):
-Follow the operation guideline strictly of pump station, especially the pump on/off
-check the air-valve along the main pipe, before or after pipe maintenance
-Assess/improve the pipe operation through installation of data logger for transient pressure
Here are some kinds of design software, such as Bently Hammer V8i, which is better to prevent water hammer in design than in operation.
BR,
Wu Hao
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BENTLEY HAMMER V8i software package
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Steve gives you good advice - just add not only how the pump starts needs to consider 2 elements when start - only after route open. Stop before route closed VSD's help with this & energy reduction. Another practical thing is to restrict the opening / closing rate of valves by restricting vent / air suply
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Dear Rachid,
you can use the application Bioroc developed by my company, either on Apple store or Goggle Play:
https://itunes.apple.com/us/app/bioroc/id902463995?mt=8
https://play.google.com/store/apps/details?id=com.sotra.calculdimensionnement&hl=en
It computes and compares the hammering for 3 materials: PVC-U, PVC-O and HDPE.
Best regards,
Hervé
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EPA.net has several softwares for the calculation including manouver time of vlaves shutting time duration. I think anti-surge tanka can be used, in a higher level ground than pipes are situated.
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There are several water network modelling packages that have a facility to calculate water hammer. https://www.epa.gov/water-research/epanet is freeware that is available. You can always use classic formulae such as provided by Webber in "Fluid Mechanics for Civil Engineers". The pipe heads, materials and other factors come into play. Recent fast recording loggers have shown that pressure transients are significant and the selection of valves and their operation can have dramatic effect as well as how pumps start up. These can all be designed out of the system and use of surge vessels on the pipeline is a common solution for major pipelines. These are directly connected tanks that are part air-filled and which act as dampers to reduce the pressure transients. They need to be maintained and as pressure vessels will require periodic inspection. Use of flexible pipes e.g. plastic (PVC, MDPE etc) will also help. Keeping pressure low will help and use of pressure reducing (management) valves on the network will ensure transients are kept to a minimum. They will also help reduce any leakage from the network so setting up a series of pressure zones in which supply water at just sufficient pressure is the modern way to design. These can be metered to monitor for leakage. At a property level again pressure reduction valves will help reduce any water hammer effects.