The document describes ten different ways of positioning a pneumatically actuated valve. Topics covered include positioning on/off valves, three position control, direct positioning of diaphragm actuators, I to P converters and the functions of five different types of positioner.
Does the "T" class of hazardous area equipment (e.g. T4, T5, T6) define the maximum temperature of the surrounding environment in which the equipment can be installed? For example in the case of T4 is the maximum ambient temperature 135ºC?
We have been asked this question a number of times recently and the answer is "No".
The "T" class is interpreted in one of two ways:
The maximum surface temperature of the apparatus - In the case of T4 the maximum surface temperature of the apparatus must not exceed 135ºC degrees C.
The auto ignition temperature of a particular gas - The auto ignition temperatures of all the gasses in group T4 will be greater than 135ºC and less than 200ºC.
The contents of the apparatus will increase the surface temperature of the apparatus when power is applied; during both normal operation and under fault conditions.
As an allowance needs to be made for this temperature rise, the maximum ambient temperature (T amb) of the apparatus can operate in also needs to be defined.
This maximum ambient temperature e.g.+ 40ºC or + 80ºC, will therefore be lower than the temperature defined by the relevant T class, e.g. 135ºC (T4).
A given piece of electrical equipment will therefore be approved for a stated range of ambient temperatures in which it is safe to operate.
Normally this would be T amb -20ºC to +40ºC, unless otherwise stated on the product certification.
K Controls can offer certain products with a T amb as low as -60ºC or as high as +120ºC.
In summary you will need to know the T class and the T amb (maximum and minimum) in order to specify the correct equipment.
(For further information on the T class please click here).