Water Hammer: CSV vs VFD

Cast iron pipe broken from water hammer

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.

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