The traditional way to measure process temperature involves installing a metal or ceramic thermowell though the wall of the vessel or pipe, with a temperature sensor mounted inside. This provides good accuracy because it puts the sensor far enough into the process fluid to minimize the effect of vessel wall temperature. In addition, response time is fast and well defined. However, as we'll discuss, intrusive measurements have drawbacks.
There's a much simpler non-intrusive alternative when just measuring the vessel wall temperature will suffice. This involves attaching a sensor to the surface. Here, we'll explore the advantages and limitations of surface temperature measurement, explain how it's done, and give some examples of successful use.
DRAWBACKS TO INTRUSIVE MEASUREMENT
Difficulties with intrusive temperature measurement start with installation. Putting in a thermowell requires a process shutdown and use of a fair amount of equipment. First, a hole must be drilled in the pipe or vessel wall; then this must be tapped so the thermowell can be screwed into place. Alternatively, the thermowell can be welded in, which means bringing out welding equipment and a trained welder.
There's also a lower limit on pipe size. It can be very difficult to put a thermowell into a pipe that's only an inch or two in diameter.
Maintenance also poses problems. Replacing a failed thermowell usually means a process shutdown. For welded units, replacement requires first cutting and grinding away the remains of the old thermowell before putting in the new one.
In many processes, material tends to accumulate on the inside of the piping. It's common in these cases to use a pig to clean the line. However, a pig can't go through a pipe with thermowells protruding into it. Therefore, pigging the pipe requires first shutting everything down to allow removal of the thermowells. Missing one thermowell can lead to real problems when a pig is sent through. So, some companies don't put thermowells in lines that need frequent cleaning even if knowing the temperatures of those lines would be useful.
A thermowell made of the wrong material can cause corrosion problems.
Some of the biggest difficulties with thermowells are mechanical, mostly having to do with vibration. Fluid passing over the thermowell generates a series of vortices that are shed alternately from the two sides of the thermowell (which is the principle behind a vortex-shedding flow meter). These vortices cause the thermowell to vibrate in two directions: parallel and perpendicular to the flow. The rate at which the vortices are shed (the wake or Strouhal frequency) is the product of the Strouhal number (a function of the shape of the body and the fluid's Reynolds number) and the flow velocity divided by the diameter of the object in the flow. If either of the wake frequencies gets too close to a natural vibration frequency of the thermowell a resonance condition can occur in which the amplitude of the vibration becomes very large and causes the thermowell to fatigue and break. The calculations needed to check the thermowell doesn't face this condition aren't easy. For one thing, many thermowells are conical or stepped-conical in shape rather than cylindrical, which complicates computations.
Viscous process materials present their own issues; they may foul the thermowell and interfere with the ability to control the process.
LIMITATIONS OF NON-INTRUSIVE MEASUREMENT
Non-intrusive measurements have their own restictions. Ambient temperature and the amount of pipe insulation affect accuracy while the time needed for the pipe wall to respond to a temperature change of the medium slows down response.
That said, many applications don't require the last possible digit of accuracy; even a relatively slow response still is fast enough for practical use.
The sensors used for non-intrusive temperature measurement generally are thermocouples or resistance temperature detectors (RTDs), the latter either 2-, 3- or 4-wire types. A variety of cabling is available, including mineral insulated (MI), with or without an expansion loop to accommodate wide swings in temperature or temperature-induced movement between parts.
The main differences in surface measurement are in the ways the sensor is mounted:
Contact block/pad. This simple design (Figure 1) suits all surfaces and provides good immunity to the ambient environment. The sensing element is built into a brass, copper or stainless steel pad that is fastened to the surface by bolts/screws, clamps or welds. Mounting blocks come in a variety of shapes to fit different surface profiles — flat, V-block, radius and custom. The sensing element can be a 2- or 4-wire RTD or a thermocouple. The design can handle harsh conditions, e.g., high temperatures, high humidity or dirt; temperatures can range from 0° to 200°C (32° to 392°F) with conventional cable and from -196° to 600°C (-320° to 1,112°F) with MI cable, depending on the sensor type used.