Water Hammer, CSV versus VFD
Conserving Water Video
5HP Sub Water Hammer Video
Leaks in water lines can be anything from tiny stress fractures to blowouts that require people to be rescued by helicopter. See Figure #1; Large or small, leaks can waste up to 50% of our precious fresh water everyday. Without first addressing the cause of the problem, finding and repairing leaks can be a futile effort.
Corrosion and age of the pipe causes some leaks, but the main cause of leaks in a piping system is water hammer. Once leaks occur, they act as pressure relief for the water hammer. Repairing leaks can be a money pit if the water hammer that caused the leaks is not addressed. Repairing leaks will leave no place for the water hammer to vent, causing new leaks to reappear, as fast as the old ones are repaired.
Water hammer causes pressure spikes that can be 10 times higher than normal operating pressure. Transient pressure waves cause water hammer. Because of the incompressibility of water, transient pressure waves travel through a pipeline at speeds of 3,000 to 8,000 feet per second. When these supersonic waves hit dead ends, elbows, or tees, they create water hammer. These waves bounce off of dead ends, elbows, and tees and can ricochet back and forth many times before subsiding. A single wave can cause many water hammer events.
Transient pressure waves and the subsequent water hammer can be created on the demand or the supply side of the system. On the demand side, slow opening and slow closing valves and hydrants should be employed. Water hammer arresters, pressure relief valves, and air vents should also be installed at strategic locations. However the majority of water hammer problems are created on the supply side of the system. Most water hammer problems occur as pumps are started and stopped to allow water towers and hydro pneumatic tanks to fill and drain, or when pump control systems react too fast or too slowly.
When a pump is started, a transient pressure wave is pulsed throughout the entire plumbing system. When a pump is stopped, the water continues to travel or stretch forward, which creates a negative pressure between the pump and supply system. The water then snaps back like a rubber band, which instantly changes the negative pressure to a spike of positive pressure.
These swings from negative to positive pressures contract and expand the pipeline, and water hammer repetitively pounds away at the pipe, fittings, and thrust blocks. In the worst cases, elbows, tees, pipe, and valves can be blown off completely and major leaks spring up like geysers. At the very least, tiny stress fractures and small cracks appear in thousands of places in the pipe system. Either way, millions of gallons of water are lost through breaks and leaks, while at the same time, negative pressure waves can draw contaminants into the pipe line.
One of the first rules we learn in life, is that a body in motion wants to stay in motion, and a body at rest wants to stay at rest. Water in a pipeline takes this law of physics to the extreme. It should stand to reason that keeping the water moving continuously, verses starting and stopping the flow numerous times, should help eliminate transients and water hammer. To keep the pump running continuously, Cycle Stop Valves (CSV’s), Variable Frequency Drives (VFD’s), or other devices must vary the flow rate of a pump to match the usage. Contrary to popular belief, soft starters, VFD’s, and slow operating valves can actually cause water hammer. Delayed reactions can accentuate and even perpetuate transients and water hammer.
No matter how slow a control valve is opened, the pressure will pulse, starting a transient wave. Closing a control valve too slowly will cause the pressure to spike. No matter how slow a valve is closed, fully closing causes a negative pressure wave.
Soft starting and stopping of the pump can also cause transient waves. When a large demand is opened, waiting on a soft start, VFD, or slow opening valve can further accentuate transient pressure waves and water hammer. When a large demand is closed, the slightest delay can cause tremendous spikes in pressure.
A VFD system can be programmed to respond very fast. However, the quicker the response time programmed into the VFD, the more “hunting” or bouncing of pressure is seen and felt. Even if a VFD is programmed to respond quickly, it takes a fraction of a second for the transducer to see the pressure change, send a signal to the controller, and for the controller to change the speed of the motor/pump. By this time the transient wave, moving at thousands of feet per second, has already bounced off and is causing more destruction several thousand feet down the pipe line.
Programming a delay into a VFD is much like swinging at a baseball, after the ball has already hit the catcher’s mitt. The effort is too little and too late. It is a programmed amount of time, instead of a direct reaction to the pressure. The programmed reaction time will always be off. Even the slightest bit out of sync is like landing on a trampoline a split second after someone else, which causes a catastrophic collision and shoots the person (pressure) extremely high.
The non-closing feature of a CSV was designed to eliminate clogged bypasses that are common to other valves. However, it had another unintended but extremely helpful benefit. The non-closing feature eliminates water hammer.
The design of the CSV never allows the valve to completely close against the seat. Never completely closing, or having to pop open from a closed position, eliminates “hunting” or pulsing of the pressure.
Because CSV’s never completely close, extra large pilot controls are used to increase the valves speed exponentially. The CSV has no electronics or needle valves to determine the speed. The CSV is controlled by pressure, and therefore reacts instantly to changes in pressure, instead of working at a preprogrammed speed. The CSV reacts so fast it cancels out transient pressure waves, much the same way noise canceling technology cancels out noise. When an increase in system pressure is sensed, a CSV instantly decreases the flow rate of the pump. Likewise when a CSV senses a decrease in system pressure, flow from the pump is instantly increased. The CSV instantly meets a negative pressure wave with positive pressure, and positive pressure waves are instantly met with a negative wave. This transient wave canceling technology has proven to eliminate water hammer and line breaks in numerous water systems, large and small.
Transient pressure waves cause water hammer. Water hammer causes pipe line breaks. Pipe line breaks are time consuming, expensive to repair, waste millions of gallons of our precious fresh water everyday, and allow contamination to enter our clean water supplies. It is a waste of time and money, to repair any leaks before the root cause of the problem has been identified and corrected.
Knowing the truth about water hammer can eliminate the expense of line breaks. The consequences of wasting our water to leaks in the system, and the energy it takes to produce it, are intolerable.
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". Nowhere 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 pipeline, 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 pipeline. 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 pipeline 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 pipeline 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 toll 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.
Constant Pressure Concept Stops Line Breaks
Constant pressure is not so much a thing as it is a different idea or a concept in pumping systems. An idea, which consist of always keeping the water flowing. Changing the amount of flow produced to match the flow being used, yet never stopping the flow entirely. Constant pressure can eliminate many problems in water supply systems, increase dependability, and reduce energy cost.
For the system pressure to remain at a constant, supply and booster pumps must be able to vary their output to instantly and exactly match the demand. Any water supply system that is large enough to have more than 5 GPM going to distribution at all times is considered to have a continuous demand. Any system with the equivalent of 50 homes or more should qualify, with 350 GPD per home that is an average of 12 gallons per minute. Smaller systems can utilize a small pressure relief that returns 5 GPM to the supply tank when there is zero flow to the system. Peak demands could be as high as each system requires while the minimum system requirement is set at 5 GPM.
Even the smallest systems (such as one house) can utilize constant pressure but, must deal with zero flow conditions. In these smaller systems constant pressure can only exist when some flow is being used. If flow is at zero, pressure should be allowed to increase slightly as a pressure tank fills. When pressure has increased to a preset amount above the required constant pressure, a pressure switch will turn off the pump. Now the volume of water stored in the pressure tank must be expressed into the system before the pressure lowers to a point that restarts the pump. Any continuous demand is held at a constant pressure while zero flow conditions can utilize a pressure band width.
In the past pressure tanks and elevated tanks were allowed to fill completely at full pump capacity, then valves are closed and or pumps are stopped. When the level in these tanks is lowered, pumps are started and or valves are opened. This process is repeated over and over causing pumps to cycle frequently, pressure to vary widely, and creating devastating water hammer and pressure surges each time the flow starts or stops. Never completely stopping or having to restart the flow in supply lines eliminates these problems.
Constant Pressure systems require a different thought process. Ground storage and elevated tanks can use a non-closing pressure reducing valve to maintain a constant level or pressure. 40 PSI constant will maintain a level in an elevated tank of 92'. 10 PSI constant will maintain a level in a ground storage tank of 23'. Maintaining a constant level in these tanks requires that the supply pump or pumps be able to produce the exact same flow as is being used from the tank. If 7 GPM is all that is being drawn from the tank, then 7 GPM is the rate that must be filling the tank. If 19,622 GPM is being drawn from the tank, then 19,622 GPM is the rate that must be filling the tank. This requires that the pump or pumps be able to match the demand instantly, exactly, and efficiently.
Maintaining a constant downstream pressure the Cycle Stop Valve attached to the discharge of the pump knows that if pressure tries to increase, less flow is needed. If pressure tries to decrease, more flow from the pump is needed. The valve will vary the size of its opening to allow the pump to produce exactly the amount of flow being used.
Multiple pumps working together is the best way to efficiently produce a wide range of flow rates. Minimum flow can be efficiently produced by a small pump while average flow may require a larger pump. Then when peak demands or fire flow is required, even larger pumps come on line until the demand is satisfied. These pumps can be in the same pump house or scattered in different locations. The Cycle Stop Valves hold a different constant pressure with each pump on the system. The pressure held constant by the small pump is the same pressure that shuts off the medium size pump. The pressure held constant by the medium size pump is the same pressure that shuts off a larger pump and so on. A system with four pumps and a minimum required pressure of 60 PSI would have the valve on the largest pump supply exactly 60 PSI. The next largest pump would supply 65 PSI. The valve on the medium size pump would supply 70 PSI, and the valve on the smallest pump would be set at 75 PSI. This ensures that only the right size pump or pumps are running to handle the particular job. When the system is running at full capacity a minimum of 60 PSI is maintained while all four pumps are running. As the system flow decreases, pressure increases, and pumps begin to turn off as they are no longer needed. Turning on or off pumps #2 through #4 as needed does not cause water hammer. The Cycle Stop Valve on each pump chokes its pump to 5 GPM before the pump is started or stopped. The system will be held at 75 PSI constant when only the small pump #1 is needed. The small pump will run all the time keeping some flow moving. As long as even a small amount of water is moving down the pipe, the flow never stops or has to be restarted eliminating water hammer completely. This system can be supplying water to a storage tank 10 miles away and can be tapped for distribution anywhere along the 10 miles of pipeline.
At the storage tank, which is 10 miles away, more booster pumps can be drawing from this storage tank. The process can be repeated to boost to distribution and or storage tanks another 10 miles or so away and so on. The same water can be pumped from one booster system to another fourteen times if there are 140 miles of pipeline to supply. Water lines can branch off anywhere along the line and go in any direction. These branches can have boosters along the way if needed as well. As the branches get smaller, so does the size of the booster systems.
If elevated tanks are not included in the system, fire protection can be achieved with increased ground storage. Booster pumps can be fitted with emergency generators. Diesel or gas driven auxiliary pumps that exercise themselves regularly, can also start up on low pressure or power off conditions and provide unlimited flow for emergency conditions. These gas or diesel engines fitted with a governor can also be controlled with Cycle Stop Valves. These emergency pumps can be huge in comparison to the regular supply pumps. Controlled by a Cycle Stop Valve they can produce enormous flows when required or continue to supply demand at any flow rate for an indefinite time period. Utilizing ground storage and backup pumps for emergencies can increase the amount of water available tremendously. Eliminating elevated storage can cut cost considerably, reduce control problems, and help with many other related difficulties.
It should also be understood that these booster systems also pick up water directly from the line at whatever pressure is left and boost again. Storage tanks along the pipeline are only needed where storage for fire protection and the like would be helpful. One booster feeds another at exactly the flow rate needed. That booster feeds another booster the exact flow it needs and so on. This saves energy by using smaller horsepower pumps. These pumps use less horse power to boost from 40 PSI to 80 PSI than they do when drawing water from a storage tank at atmospheric pressure and boosting to 80 PSI. The incoming pressure to a booster system is not wasted into a storage tank, then having to be re-pressurized from basically zero pressure back up to the required constant pressure.
The same principle applies with highrise buildings, mines, or other systems that continually rise. Starting at the bottom with a booster pump picking up city water at about 40 PSI and boosting it to 80 PSI. Seven floors up another booster picks up the 40 PSI left from the first booster system below and boost it to 80 PSI. At the fourteenth floor a third booster picks up the 40 PSI left from the second booster and boost it to 80 PSI again. This process is repeated every seven floors or so until the top floor has 40 PSI. As the demands decrease going up the building, the booster pumps get smaller. This keeps from having to pump from bottom to top at 300 PSI and having to deal with these high pressures at each floor as well as needing a high pressure piping systems.
Water supply systems can vary greatly in flow requirements, elevation, friction loss, fire protection requirements, and so on. There can be many ways to adapt these systems to constant pressure. The concept of constant pressure must first be changed in the mind. Our thought process must make a major switch from trying to supply varying demands by constantly starting and stopping flow to, simply matching the demand continuously. Once our brain has adapted to the concept of “constant pressure”, we can increase dependability, reduce energy cost, and eliminate many of the problems associated with water supply systems of the past.
"Constant Pressure" Eliminates Water Hammer, Saving Water
Constant Pressure Systems differ from old-fashioned hydro pneumatic and water tower systems in that the pump or pumps are made to produce exactly the same flow rate that is being used. The flow from a constant speed pump can be throttled with a "constant pressure valve" to vary the flow produced to match the usage. A variable speed drive can also be used to vary the RPM of a pump and motor to produce a flow rate exactly equal to the flow being used. If the pump produces even a small amount more flow than is being used the pressure will increase. If the pump produces any less flow than is being used the pressure in the system will decrease. In order to maintain a "constant pressure" the flow produced by the pump must exactly and instantly match the amount of water being used.
With an old-fashioned hydro pneumatic or water tower system, water will always be produced at the maximum flow rate that the pump or pumps can produce. When the hydro tank or water tower is filled to a higher pressure the pumps are shut off. When the hydro tank or water tower is empty, the lower pressure will start the pump or pumps again and this process is repeated over and over. The repeated starting and stopping of the pumps is what produces transient pressure waves, which causes water hammer and subsequent damage to the entire system.
Large water systems such as a city with more than 100 homes, cattle feed yard operations, and golf courses never get to a zero flow condition. Even during times when no water is actually being used, a dripping faucet here, a small leak there, and several seeps that are not large enough to ever show at the surface, add up to a continuous small flow rate. These larger systems can benefit from a constant pressure system using multiple size pumps. A jockey pump or pressure maintenance pump is a small pump that basically never shuts off and is used to efficiently keep up with leaks and small water usage. When more water is needed than the jockey pump can supply, a larger pump comes on line to make up the difference. When even more flow is needed, additional pumps come on line to supply exactly the flow rate that is being used. As the system requires less and less flow, pumps shut down when they are not needed, until only the jockey pump remains running to supply the small flows and leaks at off peak times such as in the middle of the night. This insures that the right number and size of pumps are running to efficiently supply any flow rate required. Constant Pressure systems also insures that the body of water in the pipe system remains in motion from the supply to the demand which eliminates water hammer, eliminating leaks and saving millions of gallons of water.
With "Constant Pressure" in large water systems the pump or pumps are continuously supplying the exact same amount of water that is being used. No extra water is produced, so water towers and big hydro tanks are no longer needed and an entire city can operate from a single 80 gallon bladder tank. Emergency water for fire protection can be delivered with a diesel powered pump or a backup generator for a fraction of the cost of a water tower. Not to mention that a backup pump lets you use the full volume of the reservoir for emergencies, instead of being limited to the small amount of water stored in a water tower.
With constant pressure controls for small water systems such as a single home, the body of water will stay in motion as long as any water is being used. Without constant pressure controls the pump will cycle on and off 3 or 4 times while someone takes a shower and will cycle multiple times when a sprinkler is left running. Constant Pressure controls will vary the output of the pump to exactly match the usage. The pressure remains constant and the pump runs steady any time someone is taking a shower or even if a sprinkler is left running indefinitely. These smaller water systems will have times when absolutely no water is being used. These systems need to be able to transition from having the body of water in the pipeline at rest, to being in motion, then back to rest again as smoothly as possible. This transition is handled differently with variable speed pump systems than with constant pressure valves.
With variable speed pumps the transition between no flow, flow, and back to no flow is met by ramping up and then back down the RPM of the motor. A slight drop in pressure will cause the system to ramp the motor up to the speed needed to deliver the water required. A slight increase in pressure will cause the system to ramp down the motor. Usually after the motor stays in a ramped down condition for a few seconds it is completely turned off. This keeps the pressure fairly constant which means the house is always at say 50 PSI. This also means the pump must start for every glass of water, for the icemaker, or even just to wash a toothbrush. A dripping faucet or running toilet can cause the pump to ramp up and down continuously. Any delay in response from the electronic controller to the motor RPM will cause a dip in pressure on startup and a spike in pressure before the motor shuts down. Variable speed pumps have computerized electronic controls and are only as dependable as other electronic devices such as computers, cell phones, and televisions.
Constant Pressure Valves will throttle the flow from a full speed pump to keep the house at 50 PSI as long as any water is being used. When all the taps are closed, about 1 GPM will still be flowing through the constant pressure valve. This 1 GPM has no place left to go except in to a bladder tank. The pressure will increase to 60 PSI and a standard pressure switch will shut off the pump. Water can then be used from the bladder tank for icemakers or a few flushes before the pressure drops to 40 PSI and the pressure switch starts the pump. When a tap is opened, the initial flow is started in motion by the pressure in the tank. When the tank is empty and the pump starts, the constant pressure valve is in the 1 GPM position, which is a mechanical soft start. The 40/60 pressure bandwidth is what allows the draw down from a tank to be used so the pump does not have to start for every glass of water. The constant pressure valve is also in the 1 GPM position during the time when the pump is stopped. With a mechanical soft start and stop the transition between flow and no flow is made as smooth as possible. As long as a shower, a sprinkler, or anything else is running the constant pressure valve keeps the system at a steady 50 PSI. Constant Pressure Valves are simple, dependable, have no electronics and work with standard pumps, motors, starters, and pressureswitches.
Smaller water systems have very few connections and short runs of underground pipe. Leaks are usually not a problem until water hammer makes the faucets and toilet float valves start leaking. Constant Pressure control will
eliminate water hammer from the pump starting and stopping which can save thousands of gallons of water from being being wasted to leaking faucets and toilets.
Constant Pressure systems can only eliminate water hammer during normal operations. Abnormal operations include things like initial startup and power out conditions. For these abnormal circumstances it is important to have safety equipment installed. Equipment that will help during a power outage include surge tanks, gas vessels, surge anticipator valves, and pressure relief valves. This kind of equipment cannot stop water hammer but will absorb some of the extremes in high and low pressures when needed. Software is available to help design these safety systems and can show you where in the piping system is the best place to install the safety devices.
Constant pressure valves make pumps, motors, starters, pressure switches, bladder tanks, and piping systems last longer. Constant pressure is a fundamental system change from filling and draining hydro tanks and water towers to varying the flow to exactly match the usage. Constant pressure may be the best way to completely eliminate water hammer. Water hammer not only causes equipment failure, but is a major waste of fresh water and can cause contamination even in a pressurized line.