Heat-generating plants are used in industry and in heating technology. The safe operation of such plants takes a large number of safety devices. They are designed to prevent, among other activities, excess temperature, overpressure and vacuum. In your community of temperature monitoring, and a large number of other instruments, safety temperature limiters are employed.
For the operation of heating plants, it really is customary to employ a mix of several closed-loop and open-loop control units. A safety temperature limiter can be used because the ?last instance?. Connected upstream are usually safety temperature monitors, temperature controllers and thermostats. The function of a safety temperature limiter would be to switch the plant to a fail-safe state when overheating occurs, i.e. when the established limit temperature is reached. When this fault occurs, the safety temperature limiter should be unlocked by hand. That is to ensure that the plant is checked and the fault, if present, is eliminated, before the plant is switched back to its normal operation. This switching characteristics for safety temperature limiters is prescribed in standard DIN EN 14597 for usage of safety devices in heating plants.
Masterclass of standard DIN EN 14597 may be the time response utilized by closed-loop and open-loop control units make it possible for safe operation of plants. It should be guaranteed these instruments perform their control or switching functions within established time parameters, after a recognised limit temperature has been exceeded.
Closed-loop and open-loop control unit
time constant in seconds for operation in:
temperature limiter / thermostat
Safety temperature limiter,
safety temperature monitor
Table: Maximum allowed values for the time constant, relative to the particular application (source: DIN EN 14597:2012-09)
Moreover, predicated on this standard, a notified body can certify, via an EC-type examination, that these instruments fulfil the directive 2014/64/EU (Pressure Equipment Directive).
Functional principle of the WIKA safety temperature limiters
The safety temperature limiter SB15 from WIKA uses a power design and is equipped with a mechanical temperature system. Sizzled is detected by the liquid expansion principle. The closed measuring system with liquid filling includes a temperature sensor, a capillary and a Bourdon tube. The change in pressure produced because of the heat put on the temperature sensor in the closed system is transmitted through the capillary to the Bourdon tube and then displayed on the dial via the angular deflection of the pointer shaft. The shifting disk, which is arranged on the pointer shaft, actuates the switching operation at the micro switch when reaching the non-adjustable switch point or in case of measuring line breakage and locks the current status. A reset is only possible, after the scale value drops below the set switch point by approx. 3% of the scale range.
Safety temperature limiters when used as safety devices contribute considerably to safe operation of heat-generating plants. Their used in heat-generating plants is regulated by standard DIN EN 14597.
Further information on the SB15 safety temperature limiter can also be found on the WIKA website.