Why use limit devices in temperature control applications?

07 November 2014

Ian Collins, product manager for West Control Solutions, explains why limit devices are required in temperature control applications and describes how they work. He also explains why they reduce risk.

Limit devices provide vital protection for temperature controlled process systems, ensuring product quality and the safety of both the workforce and the plant itself.

Failure to manage temperature can result in equipment damage and serious safety issues, such as fires caused by extreme temperatures that have not been identified and prevented by the system.

Without the use of a limit device every temperature control application carries these risks and the greater the size of the application and its process substances/materials, the greater the risk.

Some industries and applications are strictly regulated to reduce risk. 

The operation of ovens and kilns in the USA, for example, is regulated by the NFPA (National Fire Protection Association). NFPA 86 - the Standard for Ovens and Furnaces - was drawn up to minimise fire and explosion hazards in ovens and furnaces which are used for commercial and industrial processing of materials, and requires a limit device to be used.

Requirements made by NFPA 86 include the safe design,  installation, operation, inspection, testing and maintenance of Class A, B, C, and D ovens, dryers, and furnaces, thermal oxidisers, and any other heated enclosure used for processing of materials and related equipment. Provisions are also provided for furnace heating systems, safety equipment and application, and fire protection.

Limit devices are used in process control applications, where independent limit switching for over or under temperature is required. Typical use occurs where there is a safety or quality requirement to protect a product or appliance from damage caused by excessive temperatures.

Limit devices have both a high and a low temperature cut off, so if the pre-set temperature limit is exceeded then the system is swiftly shut down. Limit devices can typically be used with a thermocouple sensor or RTD (resistance temperature detector) and provide relay outputs for alarms. A limit controller is often used in conjunction with a loop control system, in which temperature measurements of the process material are used to dictate adjustments to the input temperature.

One reason that limit devices significantly improve safety is that if limits are exceeded, supply to the system is switched to a failsafe mode. A fixed setpoint is programmed on the limit device; so, as soon as the required temperature is reached a signal is sent to make the process safe. For example, this may be isolating power to the heaters or switch on forced cooling.

Importantly, when a process value either exceeds or falls below a pre-set limit level, it has to be manually reset before the process can continue. The fact that manual resetting is required is a key factor in reducing risk as it encourages safe conditions to be manually checked before reset of the limit device can occur.

Applications and standards 
Limit controllers are a powerful tool in many applications and standards such as NFPA 86 now specify the installation of these devices.

In Europe, the European standard, EN14597, which applies to ‘electrical or non-electrical temperature control devices which are used to control temperatures within heat generating systems by controlling the supply of energy and to limiting devices which ensure that the temperature in heat generating systems will not exceed a predefined limit.’

Limit devices should be considered an essential process tool for any temperature control application to help protect plant profitability and employee safety as well as to help raise overall performance by enabling businesses to meet new standards.

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