E+C August 2018

TEMPERATURE MEASUREMENT + INSTRUMENTATION

Improving temperature measurement in power plants

By Ravi Jethra, Wika Instrumentation

Temperature is one of the most widely measured parameters in a power plant. No matter the type of plant, accurate and reliable temperature measurement is essen- tial for operational excellence.

Take Note!

The advantages of a thermo- couple include: Can be used for tempera- tures as high as 1700°C. Faster response to tem- perature changes. Increased durability ena- bles use in high vibration and shock applications. 1

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I ncorrect measurement because of electrical effects, nonlinearity or instability can result in damage to major equipment. Using advanced diagnostics, modern temperature instrumenta- tion can inform a plant's maintenance department that there is a problem, where it is and what to do about it long before anyone in operations even suspects that an issue exists. Thermocouples versus RTDs Although some specialty temperature measure- ments involve infrared sensors, the vast majority of measurements in a power plant are made with resistance temperature detectors (RTDs) or ther- mocouples (T/Cs). Both are electrical sensors that produce an mV signal in response to temperature changes. RTDs consist of a length of wire wrapped around a ceramic or glass core placed inside a probe for protection. An RTD produces an electri- cal signal that changes resistance as the tempera- ture changes. RTD sensing elements can be made from platinum, nickel, copper and other materials and can have two, three or four wires connect- ing them to a transmitter. Ni120 (120 Ohm nickel) RTDs were commonly used in the power industry, particularly in coal-fired plants. RTDs are used in applications where accuracy and repeatability are important. They have an ex- cellent accuracy of about 0.1ºC and a stable output for a long period of time, but a limited temperature range up to approximately 425°C. With regards to cost, an RTD in the same physical configuration as

a thermocouple will typically be about five times more expensive. RTDs are also more sensitive to vibration and shock than a thermocouple. Com- mon instrumentation wire is used to couple an RTD to the measurement and control equipment, making them economical to install. A thermocouple sensor consists of two dis- similar metals joined together at one end. Upon being heated, the junction produces a voltage that corresponds to temperature. Thermocouples can be made of different combinations of metals and calibrations for various temperature ranges. A thermocouple can be used for temperatures as high as 1700°C. Thermocouples will respond faster to temperature changes than an RTD and are more durable, allowing use in high vibration and shock applications. Thermocouples are less stable than RTDs when exposed to moderate or high temperature conditions. Thermocouple extension wire must be used to connect thermocouple sensors to meas- urement instruments.The extension wire is similar to the composition of the thermocouple itself and is considerably more expensive than the standard instrumentation wire used with RTDs. Although RTDs and thermocouples are both used in power plant temperature measurement, each has its advantages and disadvantages, with the application determining which sensing ele- ment is best suited. RTDs tend to be relatively fragile and generally not suitable for high temperatures or high vibra- tion, so areas such as steam generators and pump

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34 Electricity + Control

AUGUST 2018