What is Water Hammer?

The water hammer effect is an impact load that is commonly misunderstood but has a huge impact on pressure transducers. Water hammers or pulse loads are created when water flows are stopped or started suddenly and can be devastating to a pressure sensor. Since the sensitivity is to the impulse, which occurs very quickly, the effects can destroy the sensor. Most pressure systems involve water hammer effects, which can be very persistent and difficult to remove.

Snubber

The snubber has a porous metal disc available in three standard grades of porosity. Orifice type devices that prevent water hammer can get clogged easily, but this is less so for porous metal discs. They can alleviate the impacts of water hammer.

snubber]

A familiar water hammer effect is the loud crashing that can be heard throughout a house when a shower is switched off suddenly; dishwashers and washing machines make these same sounds. The pulses of noise are caused because small solenoid valves are opened and closed quickly inside the device. Extending the duration, e.g. by turning the water off slowly, can prevent water hammer.

Common industrial hardware like relief valves, solenoid valves, valves in general, centrifugal pumps, positive displacement pumps, and regulators are prone heavy hammer effects. Luckily, pressure snubbers can be fitted to these devices at low cost and will ensure that water hammer effect doesn’t destroy your sensor; they should come as standard fitted with pressure sensors.

The physical cause behind the hammer effect is because an entire train of water is stopped at once; the back end of the water train in the pipe slams into layers further forwards, resulting in shock waves that travel through the pipe. The rear end the train continues forward even as the front end is being rapidly halted; the result is a shock wave of incompressible water which travels back down the pipe. Unprotected transducers can be badly damaged by this reversing shock-wave of water.

The design of the average pressure sensor is an important aspect exacerbating the damage due to water hammer. The average pressure sensor uses a rigid diaphragm to sense changes in pressure; when the pressure changes, the deflection of the diaphragm is converted into an electrical signal. In normal use, the rigid diaphragm only deflects across distance a thousand times smaller than an inch; but the large wakes of water after a water hammer event can cause the diaphragm to be bend beyond its normal elastic limit, resulting in permanent damage.

Luckily, snubbers can avoid the damaging impacts of water hammer. You should choose the appropriate snubbers for the use medium such as liquids, gases or dense liquids like motor oils. The physical fittings that mount the snubber are also important. They operate by throttling the amount of fluid that can pass through them per unit time, preventing a rapid surge from damaging the diaphragm.

Although incompressible liquids possess the largest hammer effect, gases can still hold shock waves that can damage sensors; the snubber should be chosen for the medium of use. One can imagine the snubber as a porous sponge over the drain of a sink, ensuring the water passes through more slowly.

Snubber Frequently Asked Questions

Will a snubber affect the response time of my pressure transducer?

Given that transducers are generally connected to sensors that update more slowly than the water hammer effect, updating 2-3 times a second, the snubber should not effect the pressure sensor at all.

What are the symptoms that my sensor has been damaged by a fluid hammer?

If sensors are showing a non-zero reading at zero pressure (zero shift) it may be a sign of fluid hammer damage; the diaphragm is now permanently deformed and cannot return to zero after it’s been damaged. In severe cases the diaphragm can be disconnected from the sensor entirely, rendering it completely useless; the output won’t change or there will be no signal.

If my sensor has a large zero offset caused by this hammer effect can it be repaired?

The nature of plasticity is such that most sensors cannot be repaired; once the diaphragm has been bent beyond its elastic limit, it will be permanently deformed. Worse still, since the diaphragm is the main component of the sensor and other components are built around it, they tend to be the key element in the sensor and they cannot easily be replaced. In cases where diaphragm damage is only slight – say, less than 10% of a zero shift – the pressure sensor will probably still be linear and will give readings with an offset; but, of course, without a snubber fitted water hammer can continue to damage the sensor.

Will a snubber stop an overpressure?

Overpressure is a separate problem that cannot be solved by a snubber. They are designed to stop spikes in pressure that last for a few milliseconds due to shock waves in the water hammer effect; a permanent overpressure will damage the snubber itself. If this is a persistent problem, you may need a more resilient sensor.

How is a snubber installed in a pressure system?

Installation is simple; the snubber simply screws onto the front end of the pressure transducer. It’s then threaded into the piping system; when correctly installed, the snubber will be between the piping under pressure and the transducer that measures that pressure.

Properties of water at atmospheric pressure

Temp. Density Density Kinematic Viscosity Viscosity Surface Tension Vapor Pressure Bulk Modulus
°F lbm/ft3 slug/ft3 lbf-sec/ft2 ft2/sec lbf/ft Head ft lbf/in2
32 62.42 1.940 3.746 EE-5 1.931 EE-5 0.518 EE-2 0.20 293 EE3
40 62.43 1.940 3.229 EE-5 1.664 EE-5 0.514 EE-2 0.28 294 EE3
50 62.41 1.940 2.735 EE-5 1.410 EE-5 0.509 EE-2 0.41 305 EE3
60 62.37 1.938 2.359 EE-5 1.217 EE-5 0.504 EE-2 0.59 311 EE3
70 62.30 1.936 2.050 EE-5 1.059 EE-5 0.500 EE-2 0.84 320 EE3
80 62.22 1.934 1.799 EE-5 0.930 EE-5 0.492 EE-2 1.17 322 EE3
90 62.11 1.931 1.595 EE-5 0.826 EE-5 0.486 EE-2 1.61 323 EE3
100 62.00 1.927 1.424 EE-5 0.739 EE-5 0.480 EE-2 2.19 327 EE3
110 61.86 1.923 1.284 EE-5 0.667 EE-5 0.473 EE-2 2.95 331 EE3
120 61.71 1.918 1.168 EE-5 0.609 EE-5 0.465 EE-2 3.91 333 EE3
130 61.55 1.913 1.069 EE-5 0.558 EE-5 0.460 EE-2 5.13 334 EE3
140 61.38 1.908 0.981 EE-5 0.514 EE-5 0.454 EE-2 6.67 330 EE3
150 61.20 1.902 0.905 EE-5 0.476 EE-5 0.447 EE-2 8.58 328 EE3
160 61.00 1.896 0.838 EE-5 0.442 EE-5 0.441 EE-2 10.95 326 EE3
170 60.80 1.890 0.780 EE-5 0.413 EE-5 0.433 EE-2 13.83 322 EE3
180 60.58 1.883 0.726 EE-5 0.385 EE-5 0.426 EE-2 17.33 313 EE3
190 60.36 1.876 0.678 EE-5 0.362 EE-5 0.419 EE-2 21.55 313 EE3
200 60.12 1.868 0.637 EE-5 0.341 EE-5 0.412 EE-2 26.59 308 EE3
212 59.83 1.860 0.593 EE-5 0.319 EE-5 0.404 EE-2 33.90 300 EE3

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.

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