E+C August 2018

TEMPERATURE MEASUREMENT + INSTRUMENTATION

ference (RFI) can affect the signal. The electrical output from a thermocouple is only a few mV and can be completely overshadowed by RFI/EMI, de- pending on the installation. This can result in false alarms and occasional trips. A typical power plant has many sources of EMI and RFI. On-site power generation and transmis- sion equipment are major sources of electrical noise, but plants also have numerous large rotat- ing machines with huge electrical fields. By using transmitters that comply with the IEC61326 stand- ard, temperature measurement can be made im- mune to EMI/RFI problems, even in electrically noisy environments. Temperature transmitters are available that accept more than two dozen differ- ent types of RTDs or thermocouples, and RTD inputs with two, three or four wires. These sen- sors can be connected to a transmitter without the need for special programming. Advanced transmitter functions Today's smart transmitters offer functions that were unheard of 20 years ago and the extra cost is justified by the reduction in maintenance time and the prevention of failures. For critical measurements, it's also possible to set up a triple redundant system. In this case, three temperature sensors in a steam pipe to the middle-steam header are set up with a two-out-of- three voting scheme for increased reliability and safety. reacting to failures after they occur. Because of its physical construc- tion, measurement points recorded by thermocouples tend to drift. One of the main reasons for this is the ‘migration’ of material from one leg of the measurement element to the other. The time span during which a thermocouple will measure accurately tends to vary from just a few days to a number of years. To determine the availability and accuracy of a thermocouple, it's very important to recognise drift when it occurs. With two connected thermo- couples, the transmitter constantly compares the two measured values and, should the result ex- Smart transmitters also detect prob- lems such as thermocouple drift and low voltage, allowing maintenance technicians to perform planned and proactive maintenance instead of just

ceed the prescribed difference, will issue an alarm. Modern temperature transmitters also have the ability to provide a low voltage warning if the poten- tial drops below a threshold value. In applications where fast response time is needed, customers use grounded thermocouples, but this thermocou- ple type may cause a ground loop. This is avoided by using transmitters with superior galvanic isola- tion, up to 2 kV. Galvanically-isolated transmitters in general also provide superior noise rejection as well as protection from electrical transients and surges in electrically noisy environments or during weather extremes. Curing maintenance headaches Smart transmitters diagnose many common prob- lems that might take several days for a mainte- nance technician to find, diagnose and repair. For example, it may be very difficult to diagnose if a temperature loop is suffering from ground loops, noise, bad connections, cable breakage or many other problems. Process sensors and instruments in the power industry frequently work in very aggressive envi- ronments. Cable glands are rarely 100% sealed, and eventually corrosion on the terminals or even the connection wire becomes a reality. Corrosion on the sensor connection system can lead to er- rors in measurement. Although the atmosphere in a power plant may not have as many corrosive materials as a chemi- cal plant, dust and other materials can cause corro- sion over a period of time. Because the terminals in a transmitter and the lead wires are made of different materials, corrosion can occur. In power plants, a manual check of all the sen- sor connections is virtually impossible. Tempera- ture transmitters, on the other hand, continuously monitor resistances of the sensor connection ca- bles, and give a warning so that preventive mainte- nance measures can be carried out with no meas- urement degradation. Electronic devices can fail when exposed to extreme temperatures. Smart transmitters have a built-in RTD at the electronics module that moni- tors ambient temperature. When temperature ex- ceeds the limits the unit is specified for, it gives a warning indication. The mechanical, thermal and electrical pres- sures in power plants are, in many cases, enor- mous. This stress on sensors can quite often lead

36 Electricity + Control

AUGUST 2018