E+C August 2018

TEMPERATURE MEASUREMENT + INSTRUMENTATION

The problem with a number of process control systems in the power industry is that they do not have a built-in request system for the digital HART information. In that case, HART signals can be cat- egorised using DIP switches, then be transmitted as simple on-off discrete signals to the automation system. Thermowells provide protection In severe power plant environments, a thermowell acts as a barrier between the process and sensing element and provides protection from corrosive processes and abrasives. Thermowells also act as shields in applications where high pressure and/or flowing media are present. The use of a thermow- ell allows for the removal of the sensing element without interrupting measurement, as the sensing element is inserted into the thermowell from out- side the pipe or vessel.

to damage such as cable/sensor breakages or sensor short circuits, the natural result of which is failure of the measurement point. Overstepping the allowable sensor circuit resistance is also seen as a break in the line. This can occur in both RTDs as well as thermocouples. Cable breakage or sensor short circuits are detected by the transmitter's analysis electronics and transmitted to the automation system. Devic- es that operate with a 4-20 mA current output do this in the form of a fault current (NAMUR 43) or HART data output, while smart transmitters send indications over their digital network. In addition to transmission of the measured signal, the HART protocol also enables the trans- mission of digital information superimposed on the 4-20 mA signal. This information can contain device status, maintenance requirements, sensor failure indication, sensor open circuit indication and much more.

Ravi Jethra is the Senior Market Segment Manager – Energy at Wika Instru- mentation, LP.