Heat Trace: Importance of Temperature Sensor Placement
Submitted by Jon Irvine, Featured in Automation.com Magazine
A temperature sensor is a key component of any process heating application as it provides temperature feedback about the process, which can be used to monitor or control the process. Whether the purpose is process maintenance or freeze protection, heat trace is a common process heating application where sensor placement is critical.
In general, for heating application, it is best to place the sensor in the area with the most extreme process condition. This will decrease risk of an interrupted process caused by unsatisfactory temperatures. Placing the temperature sensor in the coldest expected location of the system will help maintain a process above the desired minimum temperature, and a temperature sensor placement in the hottest expected location will assure the process never exceeds the maximum desired or allowed temperature. While every application is different, special care should always be used in determining the location for optimum process performance.
Heat Trace Freeze Protection and Process Maintenance
In terms of process maintenance, heat trace is used to keep product flowing through the pipe allowing efficient transfer. Failure to maintain process temperature in a pipe could result in poor product quality or premature failure of costly equipment like valves, pumps and compressors. Protecting against low ambient temperatures is another purpose for heat trace as it maintains fluid temperature in pipes to keep liquid from freezing and blocking or bursting the pipe. Heat trace is also used to keep instrumentation on the pipe functioning in cold weather as failure to do so could result in a facility shut down, costly repairs, or even safety issues. In both freeze protection and process maintenance heat cable applications, it is critical that a temperature sensor is properly located in the system.
One recent example where Valin® Corporation created a heat trace solution for freeze protection involved a large, coal-fired power plant in Arizona. This power plant received its coal by railcar, which was off-loaded, then crushed and sent to a storage yard. Before crushing the coal, rubberized conveyor belts transport the coal up through a coal chute and into storage silos. Since coal is dusty, it is common for mining and power plants to minimize the dust by spraying down the coal with a water-based chemical solution – causing the coal to freeze together in clumps during colder weather and stick to the cold walls of the coal chute. Once the coal reaches a point when it can no longer be transferred, the power plant can spend a full day of work clearing the chute.
Not only is clearing the coal chutes an inefficient use of the power plant’s time, but the method in which the chutes were cleared is dangerous and not compliant with OSHA standards. Workers would climb the four-story coal chutes to reach in and clear the chute, as well as bang on the sides to loosen the clumps. It is imperative that the walls of the chute be prevented from freezing and the coal from clumping, resulting in unnecessary downtime as a well as a hazardous situation for the plant workers. Thus, Valin® created a heat trace solution using the Chromalox SRM/E Self-Regulating Medium Temperature Heat Trace Cable for freeze protection that included heat tracing the coal chutes, insulation around coal chutes, as well as an electronic control system that will help extend the life of the heat trace cables.
Thermocouples and RTDs
The two most common sensors used in a heat trace application are thermocouples and Resistance Temperature Detectors (RTDs) – both have acceptable accuracy for heat trace applications. Thermocouples are more cost-effective with a lower upfront cost and more durable design. There are various thermocouple choices but type J is the most common for heat trace applications because of its temperature range.
Type J thermocouples consist of two dissimilar metals – iron and constantan – soldered together at one end creating a measurement junction at the tip of the probe. A Type J thermocouple can be identified by the color of its lead wires; positive lead is white and negative lead is red. Accuracy of the sensor is ensured by using a Type J thermocouple extension wire with a Type J thermocouple probe. If these are not appropriately matched or placed long distances from the control unit, additional costs will add up making the temperature senor placement much more expensive than necessary.
On the other hand, RTDs are available in a variety of designs; the most common for heat trace applications is a 3-wire 100 ohm platinum probe. RTDs also have a higher unit cost than a thermocouple and are not quite as rugged. Although one advantage of an RTD in a heat trace application is that the extension wire can be an ordinary copper conductor. If the probe is mounted a long distance from the control unit, extension wire is going to be more available and possibly more cost-effective overall than a thermocouple.
In a process maintenance heat trace application, the sensor should be placed directly on a pipe in the system, securely attached to the pipe 90-degrees for the heat trace to be effective. Mounting the sensor too close to the heat source can cause the sensor to read too high a pipe temperature and not maintain fluid temperature accurately. It’s also important to consider heat sinks – valves, supports, flanges – or un-insulated surfaces. These are areas that will be cooler than the rest of the system and a sensor placed here will read lower temperature. In addition, different process environments or design conditions such as indoor vs. outdoor installation, different insulation thickness and different pipe sizes, can also affect temperature sensed by the probe. In some cases where there is considerable difference, the system design should include multiple zones of control with separate sensors.
In a freeze protection application, the sensor is most commonly placed in the ambient air, but can also be mounted on the pipe. If mounted on the pipe, the probe should be placed, just as it would be for a process maintenance application – near the heat cable zone in the same environment. The sensor should be in the coldest expected area of the zone, particularly in a shaded area and away from any heat source to avoid any higher artificial temperature readings. A sensor placed in sunlight, for example, could read elevated temperatures potentially causing the heat trace to turn off and freeze the pipe in shaded areas of the facility. A sensor placed in a protected area next to an inside wall or discharge vent could see elevated temperatures and also turn off the heat trace.
Another consideration is physical damage of the sensor probe. A probe mounted in ambient air might stick out into high traffic areas where people or vehicles pass and could damage the sensor, making it important to protect the probe from any potential damage.
In conclusion, temperature sensor placement is crucial for a good process control result. It is important to determine your critical temperatures of the process and use the proper sensors to monitor those temperatures. The rule of thumb in a heating application like heat trace is to place the sensor in the most extreme location; the lowest expected location for minimum temperature control and the highest expected location for maximum temperature control. The temperature sensor can only process the information supplied to it and placement is everything.
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