Water Hammer
Water Hammer
(OP)
Hello,
We're experiencing water hammer in a fire water network. The pump closed valve pressure is 15 bar, but when the pump is shut down, we experience an instantaneous pressure spike up to 21 bar (measured with a paper disk type pressure recorder). The spike happens when the swing check valve slams shut.
There have been some suggestions that replacing the check valve with a quicker acting valve, such as a double disk check valve, will reduce the backflow through the valve at time of closing and reduce the pressure spike.
I'm looking for advice and suggestions of other methods that I could investigate to eliminate the problem.
Thanks in advance,
Tom
We're experiencing water hammer in a fire water network. The pump closed valve pressure is 15 bar, but when the pump is shut down, we experience an instantaneous pressure spike up to 21 bar (measured with a paper disk type pressure recorder). The spike happens when the swing check valve slams shut.
There have been some suggestions that replacing the check valve with a quicker acting valve, such as a double disk check valve, will reduce the backflow through the valve at time of closing and reduce the pressure spike.
I'm looking for advice and suggestions of other methods that I could investigate to eliminate the problem.
Thanks in advance,
Tom





RE: Water Hammer
RE: Water Hammer
There are a few weapons you can use to counter the problem.
Surge valve, which releases a small amount of liquid causing the overpressure to an outside vent or tank.
such as,
h
There are also in line fast acting pressure control valves, etc.
you can find examples of possible solutions here,
http://www.water-hammer.com/
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: Water Hammer
I disagree. It depends on the system characteristics. Noreva non slam check valves are routinely used to mitigate pressure transients.
The system must be analysed. The major challenge with fire networks is that the pump may be diesel driven. On pump trip the pump stops instantaneously. The negative wave travels and is reflected as a positive wave which may combine with other reflected waves from branches in the network.
BTW the pressure your have recorded may not be the peak. Conventional instruments are not fast enough acting to catch the peaks. You need fast acting pressure transducers and a data logger that can capture 50 points per second.
So rather than guessing what mitigation measure to use have the system analysed. The attached presentation gives you some idea of the myriad of mitigation measures available. Chek out the notes with the slides and you will find numerous references and links that may be of use. Lord kelvin had it right.
http://waterhammer.hopout.com.au/
"In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be." Lord Kelvin
"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.
http://waterhammer.hopout.com.au/
RE: Water Hammer
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: Water Hammer
Regards,
SNORGY.
RE: Water Hammer
Certainly a faster acting check will only tend to make it worse, but I don't think Stanier was suggesting that. I would also certainly advocate some transient analysis work to confirm the proposed solution, whatever it might be, so I don't see where there is any disagreements, unless Stanier only wants us to consider softer check valves.
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: Water Hammer
Couple of good resources over at Cheresources here.
When I first started out one of the service engineers told the story of how he had been on site where they had a fast acting solenoid valve in a water main. Every time it shut the free pipe thrashed around until one day the pipe burst just before the valve and they flooded the Elephant and Castle. These stories make things stick in the mind and illustrate just how serious these problems can be.
JMW
www.ViscoAnalyser.com
RE: Water Hammer
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: Water Hammer
The concept of a non slam check valve is that they do close faster. Noreva valves close in less than 0.4s. The idea is that a non slam chack valve has low mass of moving parts and a short travel to closure. Thus the valve is closed and a reversing liquid column cannot accelerate and then brought to a standstill by a closing check valve.
Counterweighted swing check valves and dual flap check valves are some of the worst offenders as there is a high mass to acclerate or a long distance to close. Tilt check valves are an improvement because the travel distance is reduced.
An excellent coverage of this is by Prof. ARD Thorley in his book Fluid Transients in Pressure Systems.Also Delft Laboratories Utah State University have published papers on the behaviour of check valves.
The Nuclear Regulatory Authority has also published papers on failures of dual flap check valves.
"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.
http://waterhammer.hopout.com.au/
RE: Water Hammer
It doesn't matter, theoretically, how fast the check valve closes or doesn't close. At the instant of velocity =0, we have a full MAX pressure in the pipe, initial + converted kinetic, Po + V^2/2/g, just downstream of the check valve. At this point any amount of liquid passed by the check valve as it takes its time closing will only tend to reduce pressure downstream of the check plate, and increase pressure upstream of the check plate. Note that full max pressure, static + kinetic, has already been reached in the pipe. Whether that pressure spike makes it through the check and into the pump, or whatever else is upstream of the check, may not matter at that point.
I don't see any way that you can get past the theory, "as dT goes to zero, dP increases to static + V^2/2/g. Or even worse, how velocity reaches infinite values as dT approaches zero. I would be interested to know how the NRC handled this theoretical issue.
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: Water Hammer
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RE: Water Hammer
The pressure immediately down stream of the check valve can be negative on pump trip. Column separation may occur and very high pressures can occur when columns rejoin. That is why Joukowsky's formula doesnt cover all events and can under estimate the magnitude of a pressure transient event.
Even if the column has reached V=0 the pressure wave may be travelling through the liquid. i.e. check valve closed. This pressure wave may be -ve or +ve. Depending upon the check valve Vr/ vs dV/dT its opening and closing speed will be determined by the pressure wave.
Refer section 3.8.2 Dynamic Performance of Check Valves ARD Thorley.
"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.
http://waterhammer.hopout.com.au/
RE: Water Hammer
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: Water Hammer
I can only agree to stanier, as he knows nozzle check valves for decades.
For info please feel fre to have a look at the attached video comparing a slow closing, long stroke swing check valve with a fast closing, short stroke, spring assisted nozzle check valve in a vertical upward pumping system.
It is quite obvious, how good they perform and how they prevent water hammer. (Put your speakers on full volume!)
RE: Water Hammer
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: Water Hammer
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: Water Hammer
I dont have a video of big size valves, but maybe the attached 3in horizontal comparison of nozzle and dual plate check valves is also impressive.
The little boomph that you can hear on the nozzle check is what you should expect from any size of a proper designed nozzle check valve.
RE: Water Hammer
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso
RE: Water Hammer
The point you make about the short time of high pressure spikes is relevant to system analysis. Many a time I have had a client install fast acting transducers and data loggers that can capture 100 points per second just to get that high peak. The use of a SCADA system is no good to measure pressure transients. The SCADA devices have slow response times and the SCADA system has been dumbed down to avoid noise. Thus the peaks are missed as the fast pressure spike passes the tapping point. The PLC latency also results in peaks being missed.
Ventomat illustrated this in their research on the behaviour of air valves when they were trying to analyse failures. Until they measured the system spikes correctly they were at a loss to know why their equipment failed. www.ventomat.com.
"Sharing knowledge is the way to immortality"
His Holiness the Dalai Lama.
http://waterhammer.hopout.com.au/
RE: Water Hammer
Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso