Improving Temperature Measurement in Power Plants

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  • 7/30/2019 Improving Temperature Measurement in Power Plants


    Improving Temperature Measurement in Power Plants


    By Ravi Jethra, Industry Business Manager - Power/Renewables, Endress+HauserTemperature is one of the most widely measured parameters in a power plant. No

    matter the type of plant, accurate and reliable temperature measurement is essential

    for operational excellence.

    Incorrect measurement because of electrical effects, nonlinearity or instability can

    result in damage to major equipment. Using advanced diagnostics, modern

    temperature instrumentation can inform a plant's maintenance department that a

    problem exists, where it is and what to do about it long before anyone in operations

    even suspects that an issue exists.

    This article covers some of the basics of temperature measurement in power plants

    and discusses technical advances that impart higher a degree of safety and reliability.

    These advances are based on innovative technologies that are being implemented in

    process instrumentation. Implementation of these new technologies can result in

    improved safety along with lower installation and maintenance costs.

    Thermocouples versus RTDs

    Although some specialty temperature measurements involve infrared sensors, the vast

    majority of measurements in a power plant are made with resistance temperature

    detectors (RTDs) or thermocouples (T/Cs). Both are electrical sensors that produce a

    mV signal in response to temperature changes.

    A modern temperature transmitter can be set up with triple

    redundancy for maximum reliability on critical processes,

    such as this steam header. All photos courtesy of


    RTDs consist of a length of wire wrapped around a ceramic or glass core placedinside a probe for protection. An RTD produces an electrical signal that changes

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    resistance as the temperature changes. RTD sensing elements can be made from

    platinum, nickel, copper and other materials and can have two, three or four wires

    connecting them to a transmitter. Ni120 (120 Ohm nickel) RTDs were commonly

    used in the power industry, particularly in coal-fired plants.

    Ni120 at one point was largely used by rotating machine suppliers on their equipment,such as pumps. Instead of buying separate Pt100 wires, these suppliers would use the

    same Ni120 wire to build their own RTDs in-house and provide these RTDs as part of

    their equipment.

    RTDs are commonly used in applications where accuracy and repeatability are

    important. RTDs have excellent accuracy of about 0.1C and a stable output for a long

    period of time, but a limited temperature range. The maximum temperature for an

    RTD is about 800F. RTDs are also expensive. 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. Common

    instrumentation wire is used to couple an RTD to the measurement and controlequipment, making them economical to install.

    A thermocouple sensor consists of two dissimilar metals joined together at one end.

    When the junction is heated, it produces a voltage that corresponds to temperature.

    T/Cs can be made of different combinations of metals and calibrations for various

    temperature ranges. The most common T/C type are J, K and N; for power industry

    applications, high-temperature versions include R and S.

    Types J, K and N are the most commonly used thermocouples due to their wide

    temperature range and ability to perform well in the harsh environments encountered

    in power plants.

    Thermocouples are selected according to the temperatures and conditions expected:

    For temperatures < 1,000F and mounting locations subject to vibration, as

    well as low-corrosion atmospheres: NiCr-Ni (Type K)

    For temperatures < 1,832F and corrosive atmospheres: NiCr-Ni (Type N)

    For temperatures > 1,832F: Pt Rh-Pt (Types R and S).

    A thermocouple can be used for temperatures as high as 3100F. T/Cs 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 measurement 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.

    RTDs and thermocouples are both used in power plant temperature measurement.

    Each has its advantages and disadvantages, with the application determining which

    sensing element is best suited.

    RTDs tend to be relatively fragile and generally not suitable for high temperatures or

    high vibration, so areas such as steam generators and pump monitoring tend to use

    thermocouples, but exceptions exist.

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    At the Ostroleka power plant in Poland, Endress+Hauser used a rugged RTD for the

    first time. Problems at Ostroleka involved vibration and electrical noise.

    Thermocouples could handle the vibration, but not the electrical noise.

    Endress+Hauser developed an RTD that had up to 60g vibration resistance and

    handled temperatures up to 812F. The construction of the RTD is far more robustthan other RTDs on the market, making it suitable for both high temperatures and

    extremely high vibration.

    With either RTDs or T/Cs, it's important to ensure that the temperature transmitters

    have the curves and linearization data built-in to the memory for the specific RTD or

    T/C without the need for custom programming.

    Transmitters Superior to Direct Wiring

    Most temperature applications in power plants involve directly wiring a temperature

    sensor to the control system. Often engineers wire direct because they mistakenly

    believe this is a cheaper and easier solution. Despite the large installed base of direct

    wired sensors, the trend is toward using transmitters in conjunction with temperaturesensors. Transmitters save time and money in installation, improve measurement

    reliability, reduce maintenance and increase uptime.

    A transmitter converts the mV signal from an RTD or T/C to a 4-20mA signal or to a

    digital fieldbus output such as HART, Foundation Fieldbus or Profibus PA in the case

    of a smart transmitter. Either of these outputs can be transmitted over a twisted pair

    wire for a considerable distance. Smart transmitters incorporate remote calibration,

    advanced diagnostics and built-in control capabilities and some are capable of

    wireless operation.

    Direct wiring requires sensor extension wires from the sensor to the automation

    system input modules. For thermocouples, these wires are expensive and sometimes

    fragile. RTDs can use inexpensive copper wires, but some RTDs have up to four


    In a power plant, the automation system can be a few hundred feet to even thousands

    of feet from the temperature sensors. This can amount to a large amount of money for

    installation depending on the number of sensors and the distances involved.

    Aditionally, over long wiring distances, Electromagnetic Interference (EMI) and

    Radio Frequency Interference (RFI) can affect the signal. The electrical output from a

    T/C is only a few mV and can be completely overshadowed by RFI/EMI, depending

    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 transmission equipment are major sources of electrical noise, but plants also have

    numerous large rotating machines with huge electrical fields. By using transmitters

    with that comply with the IEC61326 standard, temperature measurement can be made

    immune to EMI/RFI problems, even in electrically noisy environments. Temperature

    transmitters are available that accept more than two dozen different types of RTDs or

    thermocouples, and RTD inputs with two, three or four wires. These sensors can be

    connected to a transmitter without the need for special programming.

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    Endress+Hauser TMT 162

    temperature transmitter with

    big display, mounted on a


    Advanced Transmitter Functions

    Today's smart transmitters offer functions that were unheard of 20 years ago. The

    extra cost of a smart transmitter is more than paid back with functions that reduce

    maintenance time and prevent failures that can shut down a power plant.

    For example, most transmitters have a back-up function so that critical and safetyrelevant temperature measurement points can be constructed in a redundant manner.

    Here, two sensors are connected to the transmitter. If one sensor fails, the transmitter

    automatically switches to the second sensor.

    The failure of the first sensor is transmitted and is simultaneously shown on the

    transmitter display. By using the back-up function of the transmitter, the temperature

    measurement point down time is reduced by up to 80 percent. When this feature

    prevents a process shut down, it more than pays for the cost of the transmitter and the

    redundant sensor.

    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.

    Smart transmitters also detect problems such as T/C drift and low voltage, allowing

    maintenance technicians to perform planned and proactive maintenance instead of just

    reacting to failures after they occur.

    Because of its physical construction, 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.

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    To determine the availability and accuracy of a thermocouple, it's very important to

    recognize drift when it occurs. With two connected thermocouples, the transmitter

    constantly compares the two measured values and, should the result exceed the

    prescribed difference, will issue an alarm.

    Modern temperature transmitters also have the ability to provide a low voltagewarning if the potential drops below a threshold value. With older technology

    transmitters, when voltage drops, the unit continues to send a signal, although it could

    be off by as much as 25 percent or more from the actual value.

    In applications where fast response time is needed, customers use grounded

    thermocouples, but this thermocouple type may cause a ground loop. This is avoided

    by using transmitters with superior galvanic isolation, up to 2kV galvanic isolation on

    most commercially available transmitters.

    Galvanically-isolated transmitters in general also provide superior noise rejection as

    well as protection from electrical transients and surges in electrically noisy

    environment or during weather extremes such as lightning or thunderstorms. Thecurrent generation of temperature transmitters has a galvanic isolation that is about

    three to five times better than previous transmitters.

    Curing Maintenance Headaches

    Smart transmitters diagnose many common problems that might take several days for

    a maintenance 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. Without a smart transmitter, a

    technician just has to plod through the sensor and its electronics, step by step.

    It's not just mechanical components that undergo wear and tear in a power plant; the

    electrical parts also see aging and corrosion. Process sensors and instruments in the

    power industry frequently work in very aggressive environments. Cable glands are

    rarely 100 percent sealed, and eventually corrosion on the terminals or even the

    connection wire becomes a reality. Corrosion on the sensor connection system (sensor

    element, field wiring and transmitter terminals) can lead to errors in measurement.

    Although the atmosphere in a power plant may not have as many corrosive materials

    as a chemical plant, dust and other materials can cause corrosion 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 sensor connections is virtually impossible.Temperature transmitters, on the other hand, continuously monitor resistances of the

    sensor connection cables,and give a warning so that preventive maintenance measures

    can be carried out with no measurement degradation.

    Electronic devices can fail when exposed to extreme temperatures. Smart transmitters

    have a built-in RTD at the electronics module that monitors ambient temperature.

    When temperature exceeds the limits the unit is specified for, it gives a warning


    The mechanical, thermal and electrical pressures in power plants are, in many cases,

    enormous. This stress on sensors can quite often lead to damage such as cable/sensor

    breakages or sensor short circuits, the natural result of which is failure of the

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    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. Devices that operate with a

    4-20mA current output do this in the form of a fault current (NAMUR 43) or HARTdata output, while smart transmitters send indications over their digital network.

    In addition to transmission of the measured signal, the HART protocol also enables

    the transmission of digital information superimposed on the 4-20mA signal. This

    information can contain device status, maintenance requirements, sensor failure

    indication, sensor open circuit indication and much more.

    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 categorized using DIP switches, and then transmitted as

    simple on-off discrete signals to the automation system. The four categories are

    "Failure detected," "Service mode," "Maintenance required" and "Out ofspecification." In short, smart transmitters can detect, identify and report small

    problems before they become large problems.

    When the technician arrives at the transmitter to effect repairs, he or she sees a large

    and brilliant blue back-lit display that provides a clear reading from a distance of 8 to

    10 feet. The digits on a new transmitter display are at least twice the size of any of...


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