The thought of adding a new temperature measurement point to a process unit can cause dismay for instrumentation engineers. Installing a temperature sensor can be a complex and difficult task because it generally involves cutting into the process vessel or piping and adding a thermowell, which usually requires shutting down the process, draining the equipment, hot-work permits and welding.
Adding to the difficulties, the thermowell is an intrusion into the process, creating an opportunity for leakage and internal blockages along with ongoing maintenance issues. Unfortunately, until now, few reliable and practical alternatives were available to deliver accurate temperature measurements.
Other methods have been attempted that rely on the fact that the surface of the pipe or vessel under measurement reflects the internal temperature. With an appropriate correction, it should be possible to determine internal temperatures using the external surface measurement, but many problems are associated with surface measurements, most relating to the variability in process plant environments.
Changes in ambient conditions require altering the degree of correction to convert the surface reading. Even if the system temperature remains constant, the temperature measured on the surface will change with the ambient conditions. Users employing this method frequently wrap everything in insulation hoping to eliminate the environment’s influence. This can help but if overdone, creates its own headaches.
In some applications, a surface measurement may be the only option. Adding a thermowell may not even be possible in these and other situations:
- Small pipes that are unable to support the necessary thermowell size
- Pressure-rated vessels in which drilling a hole or welding are not permitted
- Processes which cannot be shut down
- Wake frequency calculations that predict a short service life for the thermowell
- If inaccessibility makes such work impractical
In these cases, the only choice is to make a surface reading and try to correct it in the best way possible. Fortunately, improved technologies make the correction effort much easier, delivering highly accurate measurements.
Characterizing heat flow
The technical basis for using a surface temperature reading to create an accurate picture of what is occurring inside the process is based on heat flow dynamics. The way heat moves through metal is well known and highly predictable. If the liquid inside a 304 stainless steel pipe with a wall thickness of 5 millimeters has a temperature of 150ºC, the outside surface of the pipe will have a temperature reflecting the external environment.
Heat flow depends on the difference between the internal and external temperatures. If the ambient temperature around the pipe is 25ºC, the internal temperature can be inferred based on known heatflow characteristics. In a steady-state environment, this is not difficult.
However, no environment is truly steady state because the ambient temperature will likely change, and the process temperature may fluctuate, too. To complicate matters, air movement strips heat away from a surface faster, even though the ambient temperature might not change. So, 25ºC with air moving 2 meters per second is effectively colder than quiescent air at the same temperature. Therefore, any correction must consider air movement.
A weather station in a transmitter
A reliable surface temperature measuring system must be programmed to reflect the application and its environment:
- The material and heat transfer characteristics of the pipe or vessel on which it is mounted
- The ambient temperature around the transmitter
If the primary temperature sensor is in direct contact with the relevant surface, obtaining a reliable reading is simple. Measuring the ambient temperature is simple as well. The difficulty is combining these two readings to produce an accurate process temperature measurement, an issue solved with new technology.
The stem that supports the transmitter has known heat-transmission characteristics. Since the process surface and ambient air temperature are known, the amount of heat flowing through the stem can be calculated, which in turn can be used to determine the internal process temperature.
Adding insulation helps create a one-dimensional heat flow path from the surface sensor to the transmitter while minimizing the environmental effects. All these factors taken together and processed mathematically can yield an effective correction to infer the temperature inside the pipe or vessel based on a measurement of the outside surface temperature. This is the principle behind Rosemount’s new 648 Wireless Temperature Transmitter with Rosemount X-well technology.
The instrument and its integral pipe clamp include a resistance temperature detector to measure the process temperature, plus a second sensor in the transmitter to measure ambient conditions for correction of the process reading. The corrected measurement is then transmitted via a WirelessHART signal.
Figure 1 shows how the instrument operates. T1 is the ambient temperature. T2 is the surface temperature, and T3 is the calculated process temperature. Sensor assembly thickness and thermal conductivity are represented by x1 and k1, respectively, and the process pipe wall thickness and thermal conductivity are represented by x2 and k2, respectively. T3 is calculated within the instrument by accounting for all these factors.
The measurement technology in action
A number of field deployments using the measurement technology have been made in many industries (see Image 1). Some applications are:
- A pharmaceutical plant monitors water line temperature as part of compliance requirements. The lines are too small for conventional thermowells.
- An alumina processor monitors overhead piping in which its location precludes conventional measurement methods. A wireless transmitter eliminates wiring challenges.
- An oil refinery installs a sensor adjacent to a hot oil pump. Adding the measurement is
critical, but shutting down the process to add a thermowell is not possible.
- A chemical processor checks heating and cooling cycles on a reactor by measuring heat-transfer oil moving through a reactor vessel. As part of the installation, the plant checks the new device against a traditionally mounted sensor and is able to duplicate measurements within ±1 percent.
- A natural gas pipeline company reports compressor temperature in a remote installation.
Ryan Leino is the Rosemount Temperature Global Product Manager for Emerson in Minnesota. He holds a bachelor’s degree in chemical engineering from the University of Wisconsin-Madison. Leino has 11 years of experience with Emerson in the areas of Inside Sales, Product Marketing and Product Management.