RTD and thermocouple temperature sensors need to be in close proximity to the point of measurement. In most cases, however, they are not rugged enough to be inserted directly into the gas or liquid of interest. As a result, these sensors must be protected by thermowells or installed against a pipe or vessel to take a surface-temperature measurement.

Thermowells have been successfully used for decades in many applications, but they have certain drawbacks — mainly related to their intrusive nature. Inserting a thermowell into a pipe or vessel can pose significant installation challenges. Once installed, the thermowell is exposed to potential damage, and it can interfere with the flow of the medium under measurement.

Surface measurements are nonintrusive and can use a variety of mounting approaches to fit the pipe or vessel in which temperature needs to be sensed. While avoiding the intrusiveness of thermowells, surface-measurement devices post their own challenges related to accuracy, speed of response and other issues.
This article will compare thermowells to surface-measurement devices and will show where each technology provides a good fit.

Intrusive Temperature Measurement with Thermowells

Thermowells can be used to house whichever sensor type is appropriate to measure the temperature of almost any medium contained in industrial vessels or tanks. They also can be used to measure the temperature of liquids and gases flowing through pipes (figure 1).

Because a thermowell is immersed (in contact) with the process to be measured, installing an intrusive thermowell provides the best accuracy and response time. Additionally, thermowell installation in a pipe or vessel provides complete isolation from the outside environment, so the effects of external (ambient) temperature on the measurement are nonexistent. This type of installation for temperature measurement is the most widely used in the process industries, so there commonly exists a variety of thermowell types, sizes, materials, lengths and diameters to meet all types of application requirements.

A thermowell may present some disadvantages in certain applications. Installing a thermowell requires proper drilling, tapping or welding to the pipe or vessel. This means that prior to plant startup, all requirements for proper thermowell material and sizing need to be verified to ensure proper installation. A key requirement is adequate area to install the thermowell, which precludes use in pipes of less than 2” in diameter.

Because the thermowell requires penetration into the pipe, a joint is created. This becomes a potential leak point for the process media to flow out to the environment. The thermowell also may be exposed to extremely harsh process conditions such as extreme temperatures, high pressures and corrosive media. These conditions can cause the thermowell to fatigue and require replacement. In a pipe, the thermowell also will affect the flow of the media under measurement. This effect must be taken into account because it can be problematic under certain circumstances.

Every thermowell requires a wake-frequency calculation — a complex and time-consuming procedure — when it is specified. Thermowells also can cause issues after installation. Due to a thermowell’s intrusive nature, it may be difficult to clean the pipe or vessel during scheduled maintenance of the plant, particularly for pipe cleaning with a pig. If the thermowell needs to be replaced due to damage during cleaning or fatigue, it can require a plant shutdown.

Nonintrusive Surface-Temperature Measurement

A variety of designs exists for nonintrusive temperature measurement. Pipe-clamp devices can be used on pipe sizes ranging from 0.5 to 48” (figure 2). The process fluid heats or cools the pipe wall while the device uses an embedded RTD to track the temperature of the pipe exterior.

Contact pads contain a temperature sensor and are brass, copper or stainless steel. They are fastened to the surface by bolts/screws, clamps or welds. Magnetic-mount devices use one or more magnets to hold the tip of a spring-loaded temperature sensor tightly against a steel or other ferromagnetic surface. Metal contact blocks use a worm-drive hose clamp to press the temperature sensor against the surface. These are a good solution for round vessels.

All of these nonintrusive devices use different methods to keep the temperature sensor in contact with the surface, but all share common advantages and caveats. The main drawback is that these devices indicate surface temperature, which is related to — but not always equal to — the temperature of the product inside.
Nonintrusive measurement devices can be used on varying diameters of the pipe, down to the smallest sizes. Unlike a thermowell, installation is simple and quick. And because the devices are nonintrusive, plant shutdown is not required.

A particular advantage of using a magnetic sensor is its portability. A magnetic sensor assembled to a wireless transmitter can be used to get quick temperature measurements along the length of a pipe or at different points on a vessel. Because these nonintrusive devices are not in direct contact with the process, they generally have a longer service life than a thermowell.

Surface-measurement devices do have some drawbacks. Proper installation is required to ensure highest accuracy. Even with the best installation practices, measurements can vary within a few degrees of the actual process temperature. Depending on the pipe or vessel thickness, heat transfer can occur at a slower rate, causing the response time to be longer. Additionally, ambient temperature may affect the temperature-measurement accuracy if the device is not properly insulated.

Both intrusive and nonintrusive temperature-measurement devices have their place, with the particular application driving selection. In general, nonintrusive measurement should be considered whenever accuracy and response time requirements are not particularly stringent because these methods of measurement are lower cost, need less maintenance and do not require process shutdowns. When accuracy and response time are paramount, intrusive measurements are usually a good choice. They provide the highest level of performance — albeit with design, installation and maintenance issues that must be taken into account.