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Improving Temperature Measurement in Power Plants
03/01/2013
By Ravi Jethra, Industry Business Manager - Power/Renewables, Endress+Hauser 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 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
Endress+Hauser
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 0.1ºC and a stable output for a long
period of time, but a limited temperature range. The maximum temperature for an
RTD is about 800ºF. 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 < 1,000°F and mounting locations subject to vibration, as
well as low-corrosion atmospheres: NiCr-Ni (Type K)
For temperatures < 1,832°F and corrosive atmospheres: NiCr-Ni (Type N)
For temperatures > 1,832°F: Pt Rh-Pt (Types R and S).
A thermocouple can be used for temperatures as high as 3100°F. 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 812°F. 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
wires.
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
thermowell.
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
indication.
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 of specification." 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 the
older devices. When the technician needs an instruction manual, schematic or other
support material, these days he or she can just call it up on a cell phone app or a tablet
browser.
Thermowells Provide Protection
RTDs and T/Cs can be surface mounted, installed in a probe or inserted into a
thermowell. In severe power plant environments, a thermowell acts as a barrier
between the process and sensing element. It provides protection from corrosive
processes and abrasives, and it also provides protection when placed in applications
where there is high pressure and/or flowing media.
A thermowell will allow the sensing element to be removed without interrupting the
measurement as the sensing element is inserted into the thermowell from outside the
pipe or vessel.
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Thermowells at a power plant in Poland were breaking
because of the Von Karman Trail effect. The solution was a
stronger thermowell and a vibration-resistant RTD sensor
insert.
A thermowell adds considerable cost to the measurement point because the
thermowell has to be inserted into the pipe, furnace or vessel. This often entails
cutting into the pipe and welding a fixture. Because the thermowell adds a layer of
protective metal, it slows down the response time of the sensor. Thermowells are
subject to failure, especially in the severe environments found in power plants. Excess pressure, vibration, temperature and corrosion are major causes of thermowell failure.
The four main failures of thermowells are:
Mechanical - Bending or breakage caused by an applied force which is beyond
the limits of the thermowell's yield strength. High-pressure steam is a likely
culprit.
Corrosion - Induced by chemicals and/or elevated temperatures.
Erosion - Resulting from high-speed particle impingement on the thermowell.
Vibration/Fatigue - Failure due to Von Karman Trail Effect (vortex shedding
around the thermowell).
Proper diagnosis can identify all but the most unusual thermowell failures. For
example, at the Ostroleka power plant in Poland, a thermowell on the outlet of the
boiler feedwater pump was constantly breaking because of the effects of the Von
Karman Trail. The thermowell broke five times because of high frequency vibrations.
The solution was to replace the thermowell with a stronger Endress+Hauser
Omnigrad M TR10 thermowell, fitted with an ITHERM StrongSens vibration
resistant RTD sensor.
Handling High Temperatures
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Power plants can generate extremely high temperatures that often cause measurement
problems. For example, in energy-from-waste plants, furnace temperature is a critical
measurement. Burning the waste at high temperatures minimizes the release of
harmful emissions. To accurately record the temperature in the furnace, three or more
temperature thermowells are inserted into the furnace directly above the flame.Because of the very harsh conditions in the furnace, conventional probes made from
Incoly 800HT alloy will typically fail after three or four months of service. Because
the probes are sited in an elevated position, changing them can be difficult. In addition,
each time the furnace is opened there is the possibility that cooler air will enter or that
hot gases will escape, both of which can decrease the efficiency of the process and
cause health and safety concerns.
A recent trial showed that Endress+Hauser temperature probes with thermowells
made from SD75 alloy can withstand the extreme temperatures up to 3,000ºF
typically found in the furnace of an energy-from-waste facility. During the 12-month
trial, two probes made from the new alloy were used alongside standard thermowells.In a like-for-like comparison, the new probes lasted three times longer than their
Incoly 800HT counterparts.
The high chromium and silicon content of the alloy increases the stability of the
instrument and makes it highly resistant to corrosion at high temperatures. The
presence of these elements promotes the formation of a protective oxide scale, making
the alloy resistant to attacks from sulfur, vanadium, chlorides and other salt deposits.
Summary
Advanced instrumentation is greatly improving temperature measurement in the
power industry. The benefits of using smart transmitters instead of direct wiring
includes installation cost savings, reduced downtime and proactive maintenance
through the use of advanced diagnostics. When a power plant has to be shut down
because of a failed sensor, the cost could run into the millions of dollars. Smart
transmitters can tell a plant that a problem exists, where it is and how to fix it —
anticipating failures before they occur.
Author
Ravi Jethra is the Program Manager - Power Industry at Endress+Hauser. He has over
two decades of experience with application engineering and projects on
instrumentation in power plants worldwide. He holds a bachelor's degree in
instrumentation engineering from Bombay Univ. and an MBA from Arizona StateUniv. He is a senior member of ISA and ASME.
Achieving Success in Bottom Ash Spray Valve Control
The Rotork K-TORK vane type valve actuator has solved a difficult flow control
application found in many coal-fired power plants – high-pressure bottom ash spray
valve control.
High-pressure spray water is used to sluice bottom ash and pyrites from the boilers'
hopper bottoms and to carry the ash out of the plant. The valves used are typically
ANSI Class 300 double-offset high-performance butterfly designs ranging in size
from 3" to 12", automated with double-acting actuators. They cycle from four to 10
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times per day and discharge to atmosphere, creating a very high pressure drop. The
flow media is recirculated ash water that is abrasive and flows at pressures between
400 and 500psi.
Rotork K-TORK actuators installed on two units at the AEP
Pirkey Power Station in east Texas. All photos courtesy of
Rotork
In plants owned by AEP, Duke Energy, Luminant Generating and other utilitycompanies around the world, K-TORK actuators have provided over 10 years of
maintenance-free service while preserving the life of the valves and valve seats in
these arduous duties. Forty actuators were installed in 2001 (20 per boiler unit) and
have provided 12 years of operation with no downtime or maintenance required. All
still have the original, durable lip seals
Among the challenges, it is imperative that the valves close fully and with zero
leakage in a high pressure drop state. If the valve disc moves even slightly from the
seat, the abrasive, high-pressure water will "wire-draw" or cut the butterfly valve seat.
Traditionally, rack-and-pinion or scotch-yoke actuators have been used in this
application, but "slop" or hysteresis in the rotary-to-linear conversion allows for the
pressure in the pipe to move the disc from the seat, often causing premature failure of
the valve after a period of only three to 12 months.
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The K-TORK double-opposed lip seal is forgiving to dirty or
contaminated air.
The problem becomes more acute when multiple valves are leaking, lowering the
available back-pressure at the header, which makes it difficult or impossible to move
the ash from the boiler.
When assembled to the valve with a 'No-Play' coupling, the K-TORK actuator has
zero lost motion, "slop" or hysteresis. The one-piece vane and drive shaft cannot be
back-driven and will hold the disc of the valve firmly in place.
Additional challenges include the location of the valves on a manifold at the bottom
of the boiler where space is critical and plant air can be poor quality. K-TORK
provides the smallest torque-to-size ratio and the double-opposed lip seal design isforgiving to dirty or contaminated air.
Also, the low-friction performance of K-TORK provides a speed-controlled, smooth
valve operation, eliminating the risk of water hammer created by the high pressure
drop.
Finally, longer run time between shutdowns demands increased reliability from the
equipment in these critical applications. In particular, as the number of plant
maintenance personnel has decreased, actuators that reduce maintenance (seal
replacement) time and work orders have a direct payback to the owner, especially
when valve life can be significantly increased through improved actuator
performance.
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