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How to prevent waterhammers. (Test & Measurement).

Waterhammer is an impact load that is the most misunderstood force known to pressure transducers today. 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 lifetime. Waterhammers occur in almost all pressure systems and usually cannot be stopped without extensive time, energy; and studies.

A common example of a waterhammer occurs in most homes every day. Simply turning off a shower quickly sends a loud thud through the house. The key phrase in the examples above was turning on or off the water "quickly" versus turning it off slowly. 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 ensure 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. The back of the train continues forward even though the front cannot 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 medium that they will be used on--such as liquids, gases, or dense liquids like motor oils--and by 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.

In most cases, the pressure transducer is connected to a meter of a recorder that updates at 2 to 3 times/sec; therefore a snubber will not affect it at all. Most sensors will exhibit a higher than normal output at zero pressure (a zero shift). This occurs because the diaphragm cannot return to zero. In severe cases, no output occurs, or the output does not change with an increase in pressure.

How a Snubber Works

The snubber would screw on to the front end of the transducer and then thread into the piping system, The snubber is located between the piping under pressure and the pressure transducer. The following equation determines the maximum pressure change that occurs during a fluid hammer. The equation assumes that the piping is inelastic.

[Delta]P = rc[Delta]v / g

Where

c for liquids = [(Eg/r).sup.1/2]

c for gases = [(KgRT).sup.1/2]

Where

P is the change in pressure resulting from the fluid hammer (pounds/[ft.sup.2])

r is the fluid density (pound mass/[ft.sup.3])

c is the speed of sound in the fluid (ft/sec)

v is the change in velocity of the fluid (ft/sec)

g is the gravitational constant (32.2 ft/[sec.sup.2])

E is the bulk modulus of the fluid media (listed in PSI but must be converted to PSF)

k is the ratio of specific heats (k = 1.4 for air)

R is the specific gas constant (ft pounds/pound mass/ degree Rankine)

T is the absolute temperature in Rankine

Waterhammer or Surge Pressures: Created by stopping and/or starting a liquid flow suddenly.

WEB RESOURCES FOR WATERHAMMERS:

www.omega.com www.kirsner.org www.pipelinesimulation.co.uk
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Publication:R & D
Article Type:Brief Article
Geographic Code:1USA
Date:Jun 1, 2002
Words:766
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