Temperature Measurements: Going with the Heat Flow
Surface-mounted instruments read through pipe and vessel walls to provide accurate temperature measurement without process penetrations.
For most industrial temperature measurements, the sensing element is never directly exposed to the process. The electronic element such as a thermocouple (TC), resistance temperature detector (RTD) or thermistor is encased inside a metallic enclosure, typically a section of stainless steel tubing closed at one end. This provides protection for the delicate element and provides a practical way to insulate the wire leads.
This approach works because heat is conducted through the metal sheath and eventually reaches the actual sensing element. Often, heat has to flow through multiple barriers, including a thermowell and then the sensor sheath. Even though the enclosures have a slight value as an insulator, it is considered negligible, and the temperature reading will accurately reflect the process.
A pipe or vessel wall also transmits heat if the interior temperature is higher than the exterior ambient. Users may try to exploit this characteristic by seeing if it is practical to infer the interior temperature by measuring the external surface, clamping a sensor to the outside of the pipe. However, heat dissipation from the pipe’s surface keeps the external temperature from ever fully reaching the internal value. Adding insulation can help, but the two readings may never fully match, and changing external conditions will cause fluctuations in the difference.
Making Surface Readings Practical
The only way to make a surface reading practical is to provide dynamic compensation to correct the external reading to create a value closer to the pipe’s interior. This involves several critical steps:
- Determine the heat transmission characteristics of the pipe, based on its material and thickness.
- Mount the temperature sensor carefully and insulate it to ensure one-dimensional heat flow through the sensor assembly.
- Monitor ambient temperature conditions to determine their effect.
- Apply an algorithm that considers all these factors to create a final result capable of measuring changes in the process, regardless of ambient changes.
This is easier said than done, which is why successful and highly accurate external temperature monitoring applications are uncommon. Often, plants back into the correction factor by comparing known process temperature measurements with the external reading to derive a value for a specific situation, but this approach has serious limitations.
Some instruments use a different methodology (figure 1). It begins with an algorithm capable of making the kind of calculations just described. The user enters factors for the pipe material and thickness, and the instrument does the rest. When clamped on a pipe, an RTD is held in contact with the pipe surface to ensure predictable and consistent heat transfer. Once insulated, the reading from the surface RTD is compared to a second RTD in the transmitter housing, which monitors ambient conditions. These two readings are continuously run through the algorithm to infer the temperature inside the pipe even when ambient and other conditions change significantly (figure 2). The result is an accurate temperature reading without the need for a process penetration.
Applications for Surface RTDs in Process Applications
Surface-mount temperature sensors offers advantages that can help solve problems encountered with traditional instruments.
Small Line Sizes. One persistent problem is taking temperature readings where the line size is too small for conventional thermowells. Temperature sensors typically are inserted into a larger diameter pipe using a cross-mounted thermowell. For smaller line sizes, they have to be extended into the piping lengthwise at an elbow or at another specialized fitting, which may cause flow obstructions. Sensing through the pipe wall provides advantages.
A large pharmaceutical plant in Southeast Asia is using surface measurement to monitor the temperature of water coming from a specialized treatment facility prior to its introduction into a manufacturing process. The temperature reading is critical, but the small line size makes a conventional installation difficult. Adding another process penetration adds opportunities for contamination. The ability to mount a sensor on these small pipes (1.5” and smaller) at convenient locations makes the measurement easier to obtain and installation is quick.
Difficult Locations. Adding a temperature sensor in a hard-to-reach location or where equipment is densely packed can be expensive. Erecting scaffolding or a lift to support a welder can be difficult. If a device can be added using a pipe clamp and some insulation, installation is easier. When the instrument also can communicate via existing wireless networks running on a self-contained power module, getting the extra process variable becomes more practical.
A bauxite refiner in Louisiana is using surface measurement to monitor the temperature of a caustic return line mounted high up in existing piping racks. Due to the location and general equipment congestion around the critical pipes, adding a conventional thermowell is nearly impossible. Clamping on a wireless temperature instrument was more practical in this application. Because an individual technician can reach the mounting location, this approach worked and solved the problem without the need to add any wires to the device. Also, the instrument can be moved if another location needs to be monitored.
Difficult Wiring. Just as it can be problematic to mount a conventional temperature instrument in an awkward location, getting power and signal cables to a device also can create challenges and expense. As more facilities add wireless infrastructure to ease communication, these networks can be extended to field instruments, including those with built-in wireless communication capability using WirelessHART.
A Chinese chemical manufacturer has a pipeline about 0.62 mile (1 km) long that carries liquefied sodium. Over the distance, there are 20 measurement points because the temperature is critical from one end to the other. Any cold spots can cause solidification and a complete pipe obstruction. Wiring is problematic due to the distance, and thermowells interfere with critical flow characteristics. The surface-mount temperature-sensing technology has the ability to read externally, and wireless communication solved distance and wiring issues.
Uninterruptable Production. In operations where production cannot be stopped to provide to allow a thermowell to be installed, the ability to add a temperature sensor without a new process penetration is important. Because the surface-mount instruments can be added from the outside, mounting a unit and gaining a new temperature reading can take place with no required downtime.
A U.S. refiner in the Midwest used the surface-mount technology to monitor the performance of a hot-well oil pump. The wireless surface mount instrument was mounted while the plant was in full operation. With nothing to add beyond a pipe clamp and a small segment of insulation, it provides critical data using existing wireless infrastructure.
A natural gas pipeline operator on the West Coast uses surface measurement to monitor a pipeline compressor installation. In this application, a sensor was needed to measure the temperature of gas coming out of a compressor as close to the outlet as possible. Shutting down operation to add a thermowell was not possible, and it was not desirable to make a penetration into the pipe at the measurement point. Taking the reading externally provided a practical approach.
Problematic Piping. Some applications need temperature measurement in a location where complex piping makes conventional installation difficult. In situations where there are complex fittings, valves and other instrumentation installed near a pump, finding a place where a thermowell can be inserted can be a challenge, but there may be room for a reading taken from a surface.
A fine chemical manufacturer in Germany is keeping an eye on a reactor’s heating and cooling cycles by measuring the heat transfer oil circulating through the process. The measurement instrument is mounted on the outlet of the circulating pump immediately above the connecting flange. The clamp is covered by the normal insulation around the pump.
The ability to take an accurate temperature reading through a pipe or vessel wall without a process penetration using a self-contained instrument solves many temperature measurement problems. Adding wireless transmission of temperature and other data via a wireless network provides additional attributes able to address many process challenges.