Water hammer is a high-pressure surge or wave created by the kinetic energy of moving fluid when it is rapidly forced to stop or change direction, altering the liquid’s velocity. Rapid stopping or starting or changing of direction of an incompressible fluid like water or oil in a system is caused by a variety of valves or pumps.
It is a myth that a pressure spike cannot exceed the normal pressure in the system. The fact is that pressure spikes can reach up to eight times the system pressure. Even though they may last only milliseconds, the potential damage can be lasting. Pressure gauges and transmitters fitted to measure normal line pressure at the receiving end of these pressure spikes are often irreparably damaged if not suitably protected.
One of the main concerns of hydraulic and other system designers is how to eliminate or minimise water hammer. Methods and formulas for calculating water hammer pressures are freely available online.
How water hammer affects pressure gauges and transmitters
Pressure gauges and transmitters are designed to measure pressures up to the full-scale value of the instrument. For short periods, these instruments may accept an overpressure of a percentage of the rated full-scale pressure without permanent damage.
The measuring sensors of pressure gauges (bourdon tube) and transmitters (diaphragm) are normally manufactured from a flexible elastomeric type of material to ensure accuracy and repeatability.
Good quality pressure gauge bourdon tubes are designed, manufactured and tested to withstand up to 1,3 times the rated pressure in normal pressure gauges for short periods. In the case of some diaphragm-operated pressure gauges that do not use bourdon tubes (but with a limited range of 2500 kPa maximum) these can handle up to five times more, without permanent damage to the instrument.
Most pressure transmitters can withstand 1,5 to two times the rated full-scale value for short periods, without permanent damage. Pressure spikes exceeding these limits can cause the elastomeric sensors to deform permanently, making them unable to return to zero. Accuracy and repeatability may be affected, and in extreme cases rupturing of the sensor may occur.
Gauges are sometimes returned to the manufacturer for investigation, completely over-pressured, and often with a ruptured bourdon tube. When SA Gauge informs the customer of the gauge’s condition, they often get responses such as: “But how can it be possible? This is a 1000 kPa gauge and the pump can generate a maximum of 600 kPa.” Water hammer is the most likely cause of damage in such a scenario.
Protecting against water hammer
The progression of the water hammer shockwave needs to be detained and slowed before reaching the instrument sensor.
SA Gauge manufactures several such devices in order to achieve this. The most common and economical is a simple threaded snubbing screw that screws into the process connection of the gauge, restricting the pressure port to minimise flow into the gauge. In severe cases, a length of small-bore capillary tube is attached to the snubber to increase the effectiveness. Other devices like external adjustable flow restrictors, piston type snubbers, and needle valves are also available to choke the offending pressure surge to an acceptable level.
It is important to note that these devices will not protect the instruments against sustained overpressure, which will still damage the instruments. An overpressure protector will be required in these cases.
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