Heat-generating plants are employed 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 Failure of temperature monitoring, in addition to a large number of other instruments, safety temperature limiters are used.
For the operation of heating plants, it is customary to use a combination of several closed-loop and open-loop control units. A safety temperature limiter can be used as the ?last instance?. Connected upstream are usually safety temperature monitors, temperature controllers and thermostats. The event of a safety temperature limiter is 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. This is to guarantee that the plant is checked and the fault, if present, is eliminated, before the plant is switched back again 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.
Disgusted of standard DIN EN 14597 may be the time response used by closed-loop and open-loop control units to enable 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
operating mode
time constant in seconds for operation in:
water
air
oil
Temperature controller
continuous
130
120
160
Temperature controller,
temperature limiter / thermostat
two-point behaviour
45
120
60
Safety temperature limiter,
safety temperature monitor
two-point behaviour
45
120
60
Table: Maximum allowed values for the time constant, relative to the particular application (source: DIN EN 14597:2012-09)
Moreover, based 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 an electric design and is equipped with a mechanical temperature system. The temperature 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 due to the heat applied to the temperature sensor in the closed system is transmitted through the capillary to the Bourdon tube and displayed on the dial via the angular deflection of the pointer shaft. The shifting disk, that is arranged on the pointer shaft, actuates the switching operation at the micro switch when achieving the non-adjustable switch point or in the event 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.
Conclusion
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.
Note
Further information on the SB15 safety temperature limiter may also be on the WIKA website.g