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Control & Monitoring
Temperature control may be provided to a system in order to:-
- Ensure temperature safety or
- Provide process temperature accuracy
Designing for temperature safety
The most important aspect in the design of a heat tracing system is in ensuring safe temperatures. This is achieved preferably by a stabilised design, or when that is not possible, by temperature control means. The use of temperature controls for ensuring safe temperatures are not required if inherently temperature-safe heaters or a stabilised design are possible.
A stabilised design ensures that, under the most onerous conditions, limiting temperatures are never exceeded. The limiting temperature may be, for example, the maximum withstand temperature of the heater construction, a maximum process temperature, or the Temperature Classification if the trace heating is in a hazardous location.
The most onerous conditions occur when :-
- The controlling thermostat (if any) fails
- The ambient temperature is at its highest value (assumed 40°C unless otherwise specified)
- The supply voltage is at the highest level (usually 10% above design voltage – resulting in 21% more power output for a constant power heating cable)
- The heater resistance tolerance is at its lowest value as per the manufacturers data (usually 10% - resulting in a 10% increase in output)
- The installed power exceeds the design heat loss (usually the case when straight tracing is applied)
Under such ‘runaway’ conditions, a pipe being freeze protected at 5°C may typically result in a temperature of, say, 100°C, and a heater surface temperature substantially higher, for example 150°C.
In this case, should the limiting temperature be higher than 150°C, then we have a safe stabilised design. If not, stabilization may be achieved by, for example, reducing the heater power (W/m) and using more passes.
But the easiest way to achieve a safe stabilised design will be when inherently temperature-safe self-regulating heaters are available. In the above case, the maximum pipe temperature may typically be 50°C with a heater sheath temperature of 60°C.
Where inherently temperature-safe heaters or a stabilised design are not possible, then temperature control to ensure safety cannot be avoided. In this case, it is necessary that:-
- In safe areas, a controller provided for process control may also act as over temperature controller
- In Zone 2 areas, two controllers, process temperature plus over-temperature are required, and
- In Zone 1 areas, two controllers, process temperature plus over-temperature are required, where the over-temperature device is a manually re-settable lock-out type, unless a monitored alarm is provided.
It is important that the sensor of the over-temperature controller is fitted to the pipe or workpiece to limit the pipe to a temperature level at which the heater will not exceed the maximum limiting temperature. A sensor fitted to the heater is more dangerous should the sensor become detached or is not in the hottest location.
Types of system control
There are basically 3 types of control systems we can use
- No control. The heating is energised all of the time
- Ambient Control. The heating is controlled by the air temperature
- Surface Control. The heating is controlled by the equipment temperature