FEATURE SENSORS & SENSING SYSTEMS
Considering ways to prevent pressure sensor failure
A
s part of hydraulic measurement and control systems, pressure
sensors and transducers can be found in a variety of applications. However a range of principles are required to protect the devices against hydraulic over- pressure and fatigue, as well as the environment in which they are being used. The sensors can be
“Manufacturers are able to use thicker diaphragms for the internal
body of the units which offers higher proof pressure for the same measurement range with minimal losses in performance”
specified to work under steady state or pulsation conditions; and a typical hydraulic system, whether stationary or mobile, can operate from 0-500 PSI (35 bar) to 10,000 PSI (700 bar). As most pressure sensors are specified with a ‘proof pressure’ or ‘over pressure’ specification, units can perform within specification at two times the rated pressure. In some applications, pressure sensors
can be dropped in as part of an hydraulic system and operate accurately with no modification, but sometimes pressure events can far exceed the normal operating pressure limits of the sensor. An example would be a trash compact
which includes a concrete block beneath the lip of the moving body. Here, pressure pulsations can be sent to the sensor that far exceed the normal operating pressure – at five times over the rated pressure, a standard sensor can fail. Another example is a forklift truck. If
the operator picks up a full load and lowers it to the ground, the forks can bottom out on the ground and send a pressure spike through the hydraulic lines. When this spike reaches the sensor it can exceed rated pressures, again causing sensor failure. Fast-acting valves can also create pressure transients. If a pressure sensor is located close to a valve opening and closing, the pressure event will create
30 JUNE 2014 | DESIGN SOLUTIONS
surges downstream from the valve. Pressure surges can cause a pressure sensor to fail by deforming the diaphragm or breaking the sensing elements above it. Ceramic diaphragms can crack, oil filled units can shift and wire bonds can lift off of strain gauges. Another issue can be created
if water becomes trapped within a hydraulic system that is not pressurised – if this happens to freeze it will create a simulated pressure of 1,500 PSI and will fail upon long-term exposure to these pressures.
RESISTING HIGH PRESSURE It is possible to use a restrictor plug or snubber on the inside or outside of the process connection in order to protect
diaphragms for the internal body of units which offers higher proof pressure for the same measurement
range with minimal losses in performance. Rather than having to lose resolution by
increasing the measurement range of the device,
Pressure sensors and transducers are used in many applications. Here, Greg Montrose and Karmjit Sidhu from American Sensor Technologies, look at how to reduce pressure transient effects on pressure transmitters used in hydraulic systems
the manufacturer can calibrate the same pressure range with a higher proof pressure. This solution is ideal for the ice problem as with snubbers it is possible to trap water within the transducer. Furthermore, the sensors have a cavity in
which the pressure is transmitted to the diaphragm (with the exception of flush diaphragms). A snubber can be installed inside this with a smaller inside diameter to dampen a pressure spike as it travels down the pipe. Instead of the wave of pressure shocking the diaphragm, it hits the face of the snubber and slowly feeds the liquid through the hole.
CONSIDERATIONS Many technologies are used for pressure sensors in hydraulic applications, which vary dependent upon the nature of the application and operating pressures. For example, a pressure sensor with a ceramic diaphragm having capacitive or thick film is normally specified up to 400Bar with 1.5 times of rated pressure for steady state applications. Under dynamic and pulsation applications, this pressure sensor will fail due to fatigue since ceramics do not work well in tension. Similarly, thin film-based technologies
operating under high strain levels will fail due to zero shifting. As the pressure range increases, the proof pressure drops off rapidly due to the strain level approaching the yield point of the diaphragm material.
Pressure sensor with a snubber to reduce transmitter height
Right:
In some pressure ranges a metal-to-metal seal must be used
pressure sensors against pressure spikes or rapid pressure changes. The snubber serves to dampen spikes and surges on the pressure sensor. Internal snubbers reduce the overall height of the transmitter while external snubbers make cleaning easier in hydraulic applications, which are prone to dirt. Another possibility is increasing the
proof pressure of the sensor. With the advancements in signal processing, manufacturers are able to use thicker
/ DESIGNSOLUTIONS
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