A COMPLEX PHENOMENON WITH A SIMPLE SOLUTION
Waterhammer is an impact load that is the most misunderstood force known to pressure transducers today. A waterhammer is created by stopping and/or starting a liquid flow suddenly. The results of a waterhammer or impulse load are devastating to a pressure sensor. The impulse load occurs suddenly, in the millisecond time frame, but the effects of it last a life time. Waterhammers occur in almost all pressure systems and usually can not be stopped without extensive time, energy and studies.
A common example of a waterhammer occurs in most homes everyday. Simply turning off a shower quickly sends a loud thud through the house; this is a perfect example of a waterhammer. Dishwashers and washing machines make these same sounds, because inside them small solenoid valves are being opened and closed quickly, producing this pulse noise. The key phrase in the examples above was turning on or off the water “quickly” verses turning it off slowly. In the shower example, if you turn the water off slowly, the waterhammer will not occur. Common industrial hardware like relief valves, solenoid valves, valves in general, centrifugal pumps, positive displacement pumps, and regulators can and will cause heavy hammer effects. A simple solution to this devastating effect is to protect each sensor with a pressure snubber. Snubbers are low ticket items that will insure that this hammer effect will not render your costly sensor useless. All pressure sensors should utilize snubbers for all installations.
The hammer occurs because an entire train of water is being stopped so fast that the end of the train hits up against the front end and sends shock waves through the pipe. This is similar to a real train, instead of slowing to a stop, it hits into a mountain side. The back of the train continues forward even though the front can not go anywhere. Since the water flow is restricted inside the pipe, a shock wave of incompressible water travels back down the pipe deflecting everything in its path. An unprotected transducer in the path of this monstrous wake is without question, going to sustain heavy damage.
To understand the damage caused by the waterhammer forces, it is necessary to understand the principles behind the sensor. Most pressure sensors utilize a rigid diaphragm as the primary sensing element. The diaphragm deflects due to the pressure, and its deflection is transformed to an electrical output via various methods. The key component is the rigid diaphragm. The rigid diaphragm deflects only on the order of a thousandth of an inch. With a large wake of fluid hitting the sensor, it is no wonder the diaphragm is bent beyond its elastic limit and permanent damage is done. Remember that a snubber eliminates this effect and therefore should always be installed on every pressure system.
Snubbers are chosen by the media that they will be used on such as liquids, gases or dense liquids like motor oils, and their physical mounting fittings. Snubbers only let so much fluid pass through per unit time, eliminating the surge from hitting the diaphragm. Liquids possess a large hammer effect because they are incompressible, but gases can also possess a hammer effect large enough to render a sensor useless. A practical analogue to a snubber is a sponge in the drain of a sink. The sponge ensures that the sink empties slowly, instead of all at once. A lot of common questions are asked about hammer effects; the following are just a few.
WILL A SNUBBER AFFECT THE RESPONSE TIME OF MY PRESSURE TRANSDUCER?
In most cases, the transducer is connected to a meter of a recorder that updates at 2 to 3 times a second; therefore a snubber will not affect it at all.
WHAT ARE THE SYMPTOMS THAT MY SENSOR HAS BEEN DAMAGED BY A FLUID HAMMER?
Most sensors will exhibit a higher than normal output at zero pressure (a zero shift). This occurs because the diaphragm can not return to zero. In severe cases no output occurs or the output does not change with an increase in pressure.