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Actuator pressure switch combines control and alarm

Home Engineering Electronic - Instrumentation & Control Actuator pressure switch combines control and alarm

HYDRAULIC valve actuators require pressure switches for safe operation. In control cabinets, usually, two of these measuring instruments are installed.

One switch is used to control the pump for the hydraulic pressure, while the second triggers an alarm if the pressure falls below a minimum level. However, there is also the possibility of combining both functions in one switch.

A manufacturer of ball valves for gas pipelines has realised such a solution with the model BA mechanical pressure switch by WIKA. As a result, they save the purchase costs for a second instrument and reduce the installation effort in instrumentation. Since then, they have also been able to make the control cabinets more compact. To fulfil its task, WIKA has configured the pressure switch for the valve actuators with three switch points. The instrument switches off the pump for the hydraulic pressure at a value of 190 bar and switches it on again when the pressure drops to 150 bar. If the pressure drops to 90 bar, for example, due to leakage or a pump fault, the pressure switch sends out an alarm. The operator of the pipeline can thus intervene in time and prevent a valve failure.

Pressure switches for valve actuators operate with two different switch contacts. The type B pressure switch can be configured with three switching points for a dual function.

The configuration of the model BA pressure switches for the valve actuators is based on the integration of two different switch contacts. The contact for controlling the pump has an adjustable dead band (hysteresis) depending on the task, whereas the contact for the alarm has a fixed dead band. This pressure switch version is generally designed for applications with a setting range of ≥ 16 bar. In addition, there must be a difference of at least 10 percent of the respective setting range between the switch/reset point of the adjustable dead band and the switch point of the fixed dead band. This is necessary to exclude mechanical effects of one contact on the other. The accuracy of the pressure switch remains unaffected by this.

The model BA pressure switch can switch high powers up to AC 250 V, 20 A, depending on the version, very accurately. The repeatability of the set-point is ≤ 0.5 % of span. The robust and versatile instrument is designed for pressures up to 1,000 bar and the ignition protection type Ex d (explosionproof enclosure). It works perfectly, even at temperatures down to -60 °C. For safety applications, model BA is also available in a SIL 2 or SIL 3 version.

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