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Posts: 2,316

Location: USA


Friday, September 19th 2008, 3:55am

Water hammer.

I asked this question a year or more ago and nobody responded. Let's give it another whirl. I'm dealing with water hammer. We all know what a pain that can be. I was wondering what would happen if I installed a larger size pipe before the manifold. Let's say 6 foot of 2 inch pipe just before a 3/4 inch manifold. Do you think this might slow the flow down to stop the hammering? Or will this do absolutely nothing? My landscaping buddy wants me to try some 6" pipe he has laying around. Seems excessive to me. I have a broken manifold due to water hammer I'll be replacing next week. I think I'll give it a shot to see what happens. Do you think this will work?


Supreme Member


Friday, September 19th 2008, 9:40am

While you don't specify, I'm going to assume the situation is something similar to the following based on the wording of the question:

There is a 3/4" Sch40 PVC mainline about 50 feet long between the meter and an irrigation manifold. The irrigation system is putting out 10 to 12 gpm, and therefore water velocities in the 3/4" main line far exceed 5fps and is causing water hammer when the irrigation valves close.

Your proposed solution is to create a buffer between the high velocity water and the manifold by replacing the last 6 foot of 3/4" mainline supplying the manifold with a 2" pipe.

If that indeed is the akind to the setup, the 6 foot section of buffer pipe is going to have almost no impact on the water hammer. Water hammer is caused by the sudden stop of high energy water.

Kenetic energy (K) is a function of the mass of the water (M) and the speed of the water (S) with the formula
K = 0.5 M * S * S
In other words, if you double the speed of the water, you quadruple it's kenetic energy.

Water is a nearly incompressable fluid, so when you suddenly close a valve, the speed of the entire column of moving water goes from S to zero in almost not time. All that energy has to go somewhere, when there is relatively little energy to dissapait, the energy is released in the form of a pressure spike that goes through the system. When there is relatively lots of energy to dissapait, you get not only the pressure spike, but also the water hammer.

By replacing the last 6 feet of 3/4" pipe with a 2" pipe, the speed of the water just before the manifold will have a huge drop in speed. But because water is a virtually incompressable fluid, that slow moving water can not help to absorb the energy of all the high speed water behind it.

Now it will improve things at least a little bit, because now rather than 50' of high speed water, we only have 44' of high speed water. So the total energy to disappate when the valve is closed has been reduced by about 10% (i.e. you've effectively decreased the mass M in the kenetic energy equation). You will have also increase the surface area of pipe over which to distribute this energy. So again, that might help just a little.

So unless the water hammer is minimal and this 10% reduction in energy is just enough to tip the scales, the 2" pipe will have little effect on the water hammer.


Advanced Member

Posts: 88

Location: Seattle Area


Friday, September 19th 2008, 9:52am


I know you know this, but for others who may be reading this, I offer my $.02 worth, and maybe it wil help someone.

I was taught there are 4 causes of water hammer.

1) Valve closing too quickly - Although various designs serve to reduce the intensity, if the velocity thru a valve is too fast, the change in velocity when the valve is closing will instigate a reverse shock wave we refer to as water hammer. One reason the 5 fps rule is encouraged. At any speed, as the valve closes, the velocity increases, which is why the hammer starts after the first half of valve closure. "Dampening" this retro shockwave can be accomplished by the supply side pipe, and is related its size, length, and to some degree elasticity. So depending on the length and size of the pipe enlargement, it might work. So, 6 foot of 6 inch pipe might work, and worth a try. I think the reason it would work is not so much related to the velocity, as to the dissipation of any shock wave, similar to how a shock arrestor works. Although I have never had to do this, I bet it will work. Maybe replace the 3/4 inch manifold with 1 inch (may not make a difference, but since you are there anyway). Another solution would be in the selection of a slower closing valve.

other causes of WH

2) To rapid filling of your empty downstream pipes. You did not state if the WH occurs upon closing or opening. Your friends solution would not work if this is the cause.

3) Air trapping: I have had this problem in long laterals on slopes. I have upsizesd pipes and used check vlves togeher, and one or the other helped. Perhaps, it was the replacement of the undulating smaller lateral causing the problem that helped. In class, air relief valves were suggested, but have never had to used them. But, again, I don't think your friends solution would help if this is the cause.

4) Reverse flow with pump system (I had to go back to my notes to remember this one). I don't deal with pumps, so I have no idea if your friend's solution would help.In class they mentioned check valves as a solution.

Summary. I think if your hammer follows the closing of the valve, it is due to the shock wave initiated by the incresed speed thru the valve. Upsizing the volume of the supply from a 3/4 inch (assumed, not stated) supply line to a 6 inch, effectively adds a shock arrestor, and will most probably solve the problem. I would rule out the other causes of WH first.

Best answer Keep velocities below 5 FPS when you design systems

But, as I said when I started, I think you know all this, which is why you have waited a year to ask it again. :thumbsup:

Hope this helps, and thanks for the interesting question. Jeff


Friday, September 19th 2008, 11:59am

My suggestion is replace the main line if you have hammer when valves close or add an additional mainline. either choice will slow the water moving through the pipes and fix the problem.

Adding the 6" pipe may work though.



Posts: 2,316

Location: USA


Sunday, September 21st 2008, 4:08am


Thanks for the responses guys. I fixed the broken manifold on Friday. It was surprisingly broken just right so it was a quick repair. I'm putting off trying to eliminate the water hammer til work slows a bit. I took a week vacation 2 weeks ago and I've been swamped ever since. I'll let you know what the outcome is when I find the time to fiddle around with it. Thanks again, Fix!

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