Water Hammer v.2

Back in February I wrote a blog about “water hammer”. Water hammer is the most common form of the banging noise that you get when turning a faucet on or off or when the dishwasher, washing machine or sprinkler system cycles on or off. Well maybe because of the weather or just the cyclical nature of how problems begin to reappear on the forums, but water hammer has again begun to be a frequent topic of discussion. My previous blog on the subject can be found at:


http://nationalbuildersupply.blogspot.com/2009/02/whos-that-knocking-on-my-pipes.html

For today’s blog I want to discuss it further with the hope to give some even better understanding. Lets start with a basic question that I ran across on the Do it Yourself Chatroom:

http://forum.doityourself.com/plumbing-heating-cooling-c-fireplaces-air-filtering-water-conditioning-systems-124/

“Q: When the water sprinkler system changes stations (from 1 to 2 and so forth) there is a extremely loud bang in my house. This has been going on for some time. There are also some banging noises when the hot water is being used inside the house and when the toilets are flushed. I need some advice as to how I could fix this problem.

A: Sprinkler systems and wash machines use quick opening valves, you should have shock-trols at those locations. If you have other noise it may be loose pipes, I would deal with the obvious first and see if it helps the prob. First thing is to check your water pressure. Go to Ace, True Value, a big box store or wherever and get a pressure gauge that attaches to a hose bib. They're less than $10. The pressure should be about 50 to 60 PSI. If it's anything over 60 to 70 PSI, install a PRV so you can bring it into a normal range.

Things like sprinkler valves, washing machines, ice makers in the 'fridge -- these use electric solenoids to control the water. The shut off is instantaneous, and they will cause hammering. If there is any play in the pipes, the condition will be worse.

Installing a hammer arrestor is usually pretty easy, especially in the sprinkler system since that's probably PVC piping and readily accessible. All it takes for a hammer arrestor is a length of pipe about 12" long oriented vertically with a cap on the top end. This will act as a 'shock absorber' since it will be full of air, not water. For the sprinklers, simply tee it off of the manifold and have it pointing up. (Where on the manifold it is doesn't matter.)

The entire hot water system doing it means you may need to install an expansion tank at the WH.

The other places you are having the problem will be more difficult to access -- they are behind sheetrock, which means you'll have to be patching the sheetrock after the arrestor is installed.”

This answer is very concise as to some of the causes and easy corrections with the emphasis being on water pressure which is the most frequent culprit. Folks just seem to believe that the higher your water pressure the better which is far from the truth. Not only does high water pressure cause water hammer but can do permanent damage to valves in appliances and faucets and lead to their early demise. If the pipe is suddenly closed at the outlet (downstream), the mass of water before the closure is still moving forward with some velocity, building up a high pressure and shock waves that’s the water hammer. This can also cause pipelines to break if the pressure is high enough and certainly causes additional noise and potential damage to piping that is inadequately anchored in the walls when initially installed. Air traps or stand pipes (open at the top) are sometimes added as dampers to water systems (see pic above) to provide a cushion to absorb the force of moving water in order to prevent damage to the system. If your plumbing is loose or has become loose due to long term water hammer you’re in for a more difficult repair which may include opening walls to re-anchor the pipes. On the other hand, when a valve in a pipe is closed, the water downstream of the valve will attempt to continue flowing, creating a vacuum that may cause the pipe to collapse. This problem can be particularly acute if the pipe is on a downhill slope. To prevent this pressure relief valves, or air vents, are installed just downstream of the valve to allow air to enter the line and prevent this vacuum from occurring..

The following characteristics may reduce or eliminate water hammer:

• Low fluid velocities. To keep water hammer low, pipe-sizing charts for some applications recommend flow velocity at or below 5 ft/s (1.5 m/s).

• Slowly closing valves. Toilet flush valves are available in a quiet flush type that closes quietly.

• High pipeline pressure rating (expensive).

• Good pipeline control (start-up and shut-down procedures).

• Water towers (used in many drinking water systems) help maintain steady flow rates and trap large pressure fluctuations.

• Air vessels work in much the same way as water towers, but are pressurized. They typically have an air cushion above the fluid level in the vessel, which may be regulated or separated by a bladder. Sizes of air vessels may be up to hundreds of cubic meters on large pipelines. They come in many shapes, sizes and configurations. Such vessels often are called accumulators or expansion tanks.

• A device (as pictured above) similar in principle to a shock absorber called a 'Water Hammer Arrestor' can be installed between the water pipe and the machine which will absorb the shock and stop the banging.

• Air valves are often used to remediate low pressures at high points in the pipeline. Though effective, sometimes large numbers of air valves need be installed. These valves also allow air into the system, which is often unwanted.

• Shorter branch pipe lengths.

• Shorter lengths of straight pipe, i.e. add elbows, expansion loops. Water hammer is related to the speed of sound in the fluid, and elbows reduce the influences of pressure waves.

• Arranging the larger piping in loops that supply shorter smaller run-out pipe branches. With looped piping, lower velocity flows from both sides of a loop can serve a branch.

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Hopefully that’s enough information to help you better understand the water hammer and more importantly the potential for damage to your plumbing which is far worse than just the aggravation of the noise. Don’t wait for the water to come pouring from a wall or for your washing machine to self destruct before doing some about it. Consider the noise as an alarm bell and look for and repair the problem…NOW.