Tracking Over-Temperature Protection Devices

Overtemperature protection and the need to provide automatic temperature protection

Description of the process

It is a common practice to include over-temperature protection in the design of furnace and oven control systems. In general provision is made for a simple single alarm output device which provides protection for the apparatus. If this single device is also required to provide protection for the process the setpoint would need to preset for each operation requiring frequent adjustment by the operator. This is not desirable on some audited processes and is not suitable for programming control systems.

There is a growing requirement to provide automatic temperature protection of the process and include apparatus protection in one secure unit. This is particularly required where the value of the process work is high or where audited performance has to be guaranteed.

The following benefits can be gained by adopting a method of dual process and apparatus over-temperature
protection.

1. The apparatus is protected by a secure “high – high” alarm where the setpoint adjustment can be hidden from operational use.
2. The process is protected by a single deviation high alarm or dual alarms dependent on the desired strategy.
3. High value processes can adopt a range of strategies to provide alternative alarm and shutdown.
4. The application continues to fulfill the need to maintain autonomy between the control device and the over-temperature protection unit.
5. Alarm setpoints can be made secure removing the need for operator adjustment.
6. Tracking of the setpoint continues throughout programming profiles.

The application requires that the process controller and the protection device are both able to accommodate communications of some type. It is possible to fulfill this requirement using analogue communications on 800 series and 900 series controllers. Alternatively Digital communications can be used between the EPC900 and other digitally communicating controllers.

A new lower cost option is available using the 2000 range of equipment where PDSIO is employed to communicate between the two units.

This application considers the use of the 2000 units to fulfill this requirement as follows:-

Process Controller

This can be any of the 2400 series controller/programmers configured to include PDSIO retransmission of setpoint.

The setpoint is retransmitted to the protection device for use in maintaining constant track of the process temperature cycle.

Over-Temperature protection device

This can be any of the 2400 or 2200 range of devices configured to include PDSIO remote setpoint input. To
provide the necessary autonomy the unit is connected to an independent thermocouple and in the simplest form the device uses two alarm outputs.

Alarm 1 is a full scale high alarm setpoint set at the maximum safe working level of the apparatus and
provides protection for the equipment.

Alarm 2 is a deviation high alarm which tracks the remote setpoint by an amount pre-determined as being safe for the process.

Both the alarm values can be made secure from the operator or promoted for use as desired.

Occasionally, it is also desirable to detect other alarm conditions on the process particularly low temperature deviation or sensor break alarm it is possible to include this in the alarm strategy.

Since the alarms on the 2000 range can be combined onto a single relay output it is possible to tie the physical output to a number of software features as desired.

For the strategy of the apparatus it is important that the high alarm maintains a single over-riding function and is configured as an absolute alarm which is not dependent on the retransmitted setpoint between the two units.

By incorporating remote setpoint and local setpoint tracking on the protection device it is possible to lock the setpoint at the last good value if for any reason the remote setpoint is interrupted.