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Water Hammer Waste Water One of the first rules of nature we learn in school is, "a body in motion wants to stay in motion and a body at rest wants to remain at rest". No where is this more true than when dealing with hydraulics. We can see with our own eyes that it takes tremendous energy from wind or earthquakes to get a calm lake or ocean churned into massive waves. When a huge wave is moving it does not want to stop and the incredible energy can devastate almost anything in it's path. A giant wave from a tsunami can reach speeds of 800 MPH in an open body of water. Enclosed in a pipe line, water can form a pressure wave or shock wave by accelerating anywhere from 3,000 to 8,000 feet per second. The speed of a shock wave in water is almost unimaginable considering that some of the fastest rifle bullets only travel at about 3,000 feet per second. A rifle bullet is small and weighs very little, but traveling at 3,000 feet per second it can cause major damage to whatever it impacts. Even a small pipe line such as a 1" pipe that is only 100' long can hold 30 pounds of water. This 30 pounds of water traveling at several thousand feet per second can cause damage to anything it impacts. The larger the diameter and the longer the pipeline the more volume or weight of water there is to cause damage. Water in a pipeline impacts the dead ends of a pipe as well as everything in between. The impact that happens when a shock wave in a waterline hits a dead end such as closed valve is called water hammer. Water hammer can create pressure in a pipe system which can be many times the normal operating pressure of the system. Pressure from water hammer can even be many times higher than the pump in the system can build. Water Hammer can be felt and heard as a (ka-thunk) when you open or close a kitchen faucet rapidly. Water hammer can also sound like a shotgun or small explosion when a check valve is slammed closed. Water hammer happens every time a valve is opened or closed and every time a pump starts or stops. Remember, "a body in motion wants to stay in motion and a body at rest wants to stay at rest". Water hammer is our punishment for forgetting or trying to ignore this simple law of nature. Water hammer incessantly "hammers" away at valve seats in faucets, showers, toilets, sprinkler valves, and all appliances. This causes dripping faucets, running toilets, and leaking sprinkler valves. Water hammer also pounds on tees, elbows, and other fittings, as well as the pipe itself in all pipe systems. This continuous pounding causes a seep at this fitting, a drip at that fitting, and even the strongest of pipe to sometimes split along its length. Usually when a pipe splits the wasted water comes to the surface and the system must be shutdown and repaired. However, a seep here and a drip there may not ever show at the surface and can waste thousands or millions of gallons of water over time. Add the gallons wasted from dripping faucets, running toilets, and small leaks in the pipe system to the water that runs down the street from the occasional broken pipe line. It all adds up to millions or billions of gallons of our precious fresh water wasted because of water hammer. Some studies estimate as much as 14% of our fresh water supplies are wasted to leaks in the system. Ironically the thicker the pipe wall and stronger the pipe and fittings the faster the shock wave. Thin walled plastic pipe will only bounce a shock wave back at 3,000 feet per second while heavy wall steel pipe will bounce a shock wave back at 8,000 feet per second. Water does not actually travel down a pipe line at these speeds. Five to seven feet per second is very fast for water to flow in a pipeline. A pressure wave or shock wave in water happens when one water molecule pushes on another water molecule and the second water molecule pushes on a third and so on. If you have a pipeline 1,000' long full of water, injecting a thimble full of water in one end of the pipe, will cause another thimble of water to almost instantaneously come out of the other end of the pipe. The non compressable nature of water is what transmits a shock wave through pipelines at such unimaginable speeds. Stronger or thicker walled pipe and fittings are better able to withstand the repeated impacts of water hammer but, as the strength of the pipe and fittings increases, the velocity of the shock wave increases causing more damage. Many people have used a locomotive pushing a train up a hill as an example of water hammer. As the locomotive begins to push the train cars up a hill, tremendous pressure is put on the first car, then the second car, then the third car, and so on before the car at the beginning of the train ever feels any movement. The same thing happens when a pump is started or a valve is opened. Pressure at the discharge of the pump or valve will spike before water at the other end of the line even begins to move. Once the train is up and moving, even as slow as 10 MPH, a sudden stopping of the locomotive will cause the train cars to break away and continue up the hill for a few feet. Gravity will cause the train cars to come to a stop then roll back down hill until they destructively slam into the stationary locomotive. Similar to a pump stopping as the water continues down the pipe. A vapor pocket is created after the pump which snaps the water back like releasing a rubber band. As the water snaps back in the pipeline, check valves are slammed shut creating a shock wave that can damage the pipe and fittings. Different from the locomotive example, water in a closed pipe line tends to bounce or ricochet back and forth many times after the initial crash. Friction loss is really the only thing that will slow down a shock wave in a pipeline. Small diameter pipe has considerable friction loss and may only bounce a pressure wave back and forth once or twice. Larger diameter pipe has less friction loss and may bounce a shock wave back and forth in a pipe system multiple times and for several minutes. In larger systems this bouncing wave can be seen as a pressure gauge swings from it's extreme minimum position to the extreme maximum pressure. As the wave begins to subside the pressure gauge will show less difference between the max and minimum pressures as it continues to bounce. Over a period of time the gauge will finally steady out at the static pressure. Even small pipe systems experience this bouncing shock wave, although it may happen so fast and such fewer numbers of times that it is hard to see with the naked eye. Small pipe systems or large, this bouncing pressure wave can have multiple damaging impacts from a single pump start or stop. As a pressure wave bounces back and forth in a pipe system every spike in pressure is followed by a dip in pressure as the wave moves away. During this time a negative pressure or vacuum can be created inside the pipe and fittings. This vacuum tries to collapse the pipe and fittings. As the bouncing wave continues the pipe system is subjected to repeated extremes in pressure. The pipe system goes from trying to collapse, to trying to explode, over and over until the shock wave subsides. Not only is this extremely hard on pipe and fittings but, in those fractions of a second when pressure in the pipe becomes lower than atmospheric, contamination can be drawn into a pressurized clean water pipeline from the same cracks and leaks created by water hammer. Water hammer can pound away incessantly until time and pressure take their tole and even the strongest of pipe or fitting is destroyed by nothing other than water. This reminds me of a movie in which a man tunneled through massive rock walls to break out of prison using only a 6" toy hammer. The same as water hammer, with enough time and pressure even a toy hammer can destroy boulders. No matter when or how water hammer causes the damage, leaks in the systems add up to a large amount of water being wasted as well as possible water contamination. Water hammer can also be caused by the opening and closing of valves. Even the valve on the kitchen sink will cause water hammer if closed abruptly. Remember the locomotive pushing the train analogy that was used earlier. Water coming out of a faucet is like the train coming out of a tunnel at 50 miles an hour. Closing the faucet rapidly is like dropping a giant boulder at the exit of the tunnel. The first train car to hit the boulder gets smashed from the other cars piling up behind it. Several cars are smashed into one before the remainder of the train is stopped. Water smashes into a closed faucet and a tremendous pressure is built inside the pipe system until the body of water comes to rest. If a small kitchen faucet can cause water hammer then larger valves such as sprinkler control valves can cause even more damage. The larger the valve and pipe size the bigger the impact on the closed valve. This type water hammer is happening far away from the pumps and controls. There is nothing that the pump controls can do to help stop this type damage. Slow closing valves are the best way to eliminate water hammer. Sprinkler valves can be made to close more slowly and manual valves close slower when you have to spin a wheel 5 turns instead of a 1/4 turn ball valve. When valves cannot be made to close slowly a surge tank or riser can be used. Just prior to the closing valve a surge tank can be installed or a riser pipe can be stubbed up a couple of feet with a cap on top. This riser pipe or surge tank contains air. When the valve is closed the water is diverted into the riser or surge tank instead of hitting a wall. The air in the riser or surge tank acts like a spring to cushion the stopping of the water flow. Usually a few riser pipes in the right locations can eliminate water hammer that can happen on the user side of a water system. The only way to completely eliminate water hammer is to remember our most important rule about a body in motion. Once our body of water is in motion, we should do our best to keep it in motion. Problems from water hammer only occur when we try to start water moving or stop water that is already moving. So as the doctor would say "don't do that". How can we keep water in motion when we are not always using as much water as the pump produces or when there is no water being used at all? The answer is "Constant Pressure Systems". In part two, see how "Constant Pressure" can conserve water by eliminating water hammer. |
| Cycle Stop Valves® is a registered trademark. All right reserved unless prior authorization is obtained. Cycle Stop Valves are patented: Patent number 5,988,984 and other patents pending. |
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