New ATEX pressure switches that can stand the heat
Due to the increase in demand for flame proof switches, PVL Ltd has launched a range of ATEX certified pressure switches across Europe. Manufactured in India by Orion Instruments, the switches range in pressure from 1.5mbar to 200bar, and compliment PVL’s existing range of Elettrotec ATEX pressure switches.
As well as ATEX, the new range of pressure switches is certified for IECEx (International Electrotechnical Commission Explosive) by BASEEFA (the British Approval Service for Electrical Equipment in Flammable Atmospheres).
The series includes hydraulic, PD (Pressure Difference), high pressure and low pressure versions, all of which feature easily wired terminal blocks, stainless steel wetted atmosphere; for example in sewerage treatment plants, grain handling and storage areas, on metal surface grinding applications and in aluminium processing. The switches reduce the risk of ignition in applications such as machine tools, lubrication systems and compressors used in hazardous, corrosive or explosive atmospheres.
Sussex based PVL is now the unique UK and Ireland distributor for the Orion range of switches. However, the cost effectively manufactured components are available from PVL anywhere in Europe.
PVL routinely supplies conformity certificates for ATEX and IECEx when customers buy any of its certified products.
Despite this, one problem with using solid-state switches in industrial and process settings is the fact that electromagnetic interference (EMI) can corrupt signal data. In addition, a solid state switch requires an input power source to function. EMI and radio frequency interference does not affect electromechanical switches because the circuit is a mechanical switch that is either open or closed.
The first and most important step in selecting a pressure switch is to fully establish your requirements before beginning the process. Armed with that knowledge, you must consider a number of parameters in making a final selection: what kind of pressure sensor you need, cycle speed and life, pressure range, accuracy, number of switch points and deadband. You must also ask yourself if you need an adjustable or non-adjustable switch and decide between electromechanical and solid state technology.
Cycle speed, pressure range and switch point
The frequency with which the switch is activated will have direct impact on switch life, system downtime and the maintenance schedule. Due to their design, electromechanical switches are subject to metal fatigue although solid state switches aren’t.
Cycle speed will also affect switch life and preventative maintenance programmes once the design is used in anger. A solid-state switch should be selected whenever the cycle rate exceeds 50 cycles per minute so that metal fatigue is not a problem.
Establishing the right relationship between the switch point and the operating pressure range of a switch is also important. When a solid-state pressure switch is selected, the switch point should be in the upper 25% of the operating range.
For an electromechanical switch, the switch point should be in the middle of the operating range. Thus, a system that requires a switch to activate at 140 psi should use a solid-state pressure switch with an operating range of 150 psi, or an electromechanical switch with an operating range of 300 psi. The location of the switch point versus the operating range is critical to both accuracy and life.
Accuracy, pressure points and housing
Pressure switch accuracy is defined as the ability of the switch to operate repetitively at its set-point. If the switch is used to trigger an alarm, ±2% accuracy is sufficient. If one is controlling a process where the error of various devices is cumulative, then ±0.25% accuracy may be absolutely necessary. Accuracy is referenced at the high end of the operating pressure range and decreases at lower pressure.
Once the required accuracy is established, we should decide on the number of switch points need. When sensing pressure at one point, it is normal that only one switch point is required. Nevertheless, it’s not unusual for a system to require two or even four switch points to be monitored, controlled or alarmed. In designing a system, one could select a single switch for each switch point, or a single pressure switch capable of handling as many as three separate switch points.
A related issue is deadband, or the difference between the actuation point and the reactuation point in a pressure actuation switch. For example, if the device is set to operate at 100 psi on increasing pressure, the switch will close when pressure rises to that point. As pressure drops to 95 psi the switch opens - this is the re-actuation point. In this case, the deadband of this switch is five psi, the difference between the set point of 100 psi and the re-actuation point of 95 psi.
If one considers all of the factors that have to be taken into account when designing a pressure switch into an application, the hidden complexity in one of the manufacturing and process industry’s simplest components is revealed. However, despite this hidden complexity, this kind of switch remains one of the simplest to integrate and maintain. The job of the design engineer is to make that process even easier.
