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

Registered Users have access to much more information in this section.