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Why do my high pressure transmitters keep failing
25th March 2007 Not all pressure transmitters use the same sensing technology and some are more suited to measuring high pressures than others. It is high pressure spikes that are the most common cause of pressure transmitter failure and they will punch or rip a hole in a diaphragm if the right precautions are not implemented. High pressure spikes are typically generated in hydraulic systems where fluids are flowing under high pressure. It is the sudden change in momentum of the fluid in the system or the release of stored pressure from valves opening and closing that are the main cause of sudden over-pressure. The high pressure surges generate a tremendous amount of energy over a very short time period which can be very difficult to detect and if they are allowed to reach a diaphragm that is not adequately protected they can cause irreparable damage. Another cause of high range pressure transmitter failure is cavitations, which are the sudden collapse of a void generated by trapped air or fluid displacement which send out shockwaves. These resulting shockwaves are a risk to any thin material such as a diaphragm that happens to be within range.
There are three methods to protect a high pressure transmitter from high dynamic pressures and that is either i) use a pressure sensor which has a robust sensing technology, ii) install overpressure protection between a vulnerable pressure transmitter and the source of surge pressures or iii) locate the pressure transmitter in a position which mitigates the risk of pressure spikes. i) A high range pressure transmitter where the sensing diaphragm is in direct contact with the fluid media have relatively thick diaphragms which offer more protection against shockwaves caused by cavitations or pressure surges. However they are still vulnerable to low frequency over-pressure which is not so easily dissipated or absorbed unless physical mechanical stops are located behind the diaphragm to prevent it from being overstressed.
ii) The most vulnerable pressure sensor technology to hydraulic pressure spikes are those that have a oil filling between a very thin isolation membrane and the sensing diaphragm. Pressure transmitters that use this type of technology can be protected by fitting a snubber to the pressure port which will dissipate any shockwaves before they reach the thin isolation membrane. Snubbers are either sintered filters or small bore restrictions. The main disadvantages of using a snubber is that they slow down pressure measurement response and can become blocked by particles over time. iii) Finding a location in the system which offers the lowest chance of over-pressure spikes can be as easy as positioning the pressure transmitter as far away as possible from bends or restrictions in the pipe work which are potential pressure spike hotspots. But to truly understand the pressure spike behaviour of a hydraulic system it should be tested by fitting high dynamic response strain gauge output pressure transducers at different locations and examining the various operation modes with an oscilloscope or high sample rate data acquisition card. Another method is to purposely fit a lower range pressure sensor and try it in different locations and examine the zero shift of the output signal after testing at each location. The location that generates the smallest zero shift is likely to be the best position for the pressure transmitter. For safety reasons it is important that it is not possible for the burst pressure rating of the test pressure sensor to be exceeded by the hydraulic system when using this method. Pressure Measurement Resource Links Explain an IP rating for ingress protection Explanations for pressure measurement terms Create a pressure sensor specification
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