Environmental regulations and the need for better control are driving increased use of temperature profiling in many process heating applications. For temperature profiling, multipoint thermometers allow you to measure multiple locations, either horizontally or vertically, within the process. An example of horizontal profiling is reading multiple temperature points at some level in a batch reactor or process vessel. A typical vertical temperature profile would be top-to-bottom measurements in a tank. These types of instruments typically are referred to as rigid or inline multipoint temperature units. A long thermowell can have multiple thermocouples positioned at various points (figure 1).

Most rigid multipoint temperature assemblies have thermowells that are 4 to 6’ long. Specialized thermowells have been assembled that are more than 100’ long with as many as 40 separate temperature sensors.

Rigid multipoint thermowells can be mounted across a vessel, vertically from top to bottom, or even at angle to obtain a temperature profile. When mounting horizontally, the thermowell often completely spans the vessel’s width. To prevent damage to the extended thermowell in these situations, a mounting bracket can be installed on the opposite side of the vessel to anchor the thermowell against forces such as agitation.

In other applications, a multipoint temperature assembly can have multiple sensors spread across the process in an octopus-like array. Each sensing device is attached to the end of a flexible cable. In both cases (rigid and array), a single multipoint instrument can be used to provide multiple temperature measurements from a single process connection or access point in the system.

Getting to the Sensors

In a rigid-design thermowell, the unit bolts to the process vessel via a flange. Wiring from the temperature sensors passes through a safety chamber (to prevent process fluids or gases from escaping) and into a junction box. The thermowell is designed so that individual sensors and wiring can be removed and replaced at the flange (figure 2) without removing the thermowell. The sensing elements are encased in a guiding tube that remains in the thermowell. This allows a faulty insert to be exchanged easily for a new one, thus minimizing maintenance and increasing productivity.

Multiple sensors can be positioned within each thermowell. For example, in a 100’ tall reactor, a user could have 10 sensors evenly spaced every 10’. For redundancy purposes, the user also could have 20 total sensors, with two thermocouples every 10’. If one of the two thermocouples at any given location were to fail, the remaining sensor still would be able to measure the temperature while the other was replaced.

For a horizontally installed multipoint, the maximum immersion length of each individual multipoint thermometer is (theoretically) the reactor’s internal diameter. Custom assemblies can be made to accommodate various vessels.

High Temperatures and Harsh Environments

Applications such as gasifiers, Claus units or steam reformers often have high process temperatures — up to 2900°F (1600°C ). Such applications require special, high temperature thermowells and thermocouple temperature probes.

Depending upon the application, a number of metallic and nonmetallic choices are available for the thermo-well. Ceramic (nonmetallic) thermowells and protection sheaths (figure 3) are resistant to temperatures in excess of 2900°F and offer good protection from abrasion. However, ceramic is brittle and possesses less mechanical load strength than most metals.

Ceramic thermowells can add a layer of protection against gases because they are not porous. As a result, they are highly resistant to letting gases pass through and damage the internal metallic thermowell (in the case of a ceramic protection sheath) or temperature sensor. Typical industries or applications where ceramic thermowells are employed include steel, cement, brick kilns, and ceramic and glass production.

Metallic thermowells provide better mechanical strength when compared to ceramic, but they have lower temperature and abrasion resistance. Examples of the exotic metal compounds that can be employed are:

• Inconel 601, which resists temperatures to 2147°F (1175°C).
• Incoloy 800HT, which resists temperatures to 2012°F (1100°C).
• Hastelloy X, which resists temperatures to 2102°F (1150°C).

Metal thermowells often are used in cement plants, steel treatment, waste incineration and industrial furnaces.

After long-term use in demanding process conditions such as high temperatures, pressures and corrosion rates, a crack in the thermowell wall might occur. Known and unpredictable process phenomenon such as turbulence and highly exothermic local chemical reactions also can cause a thermowell to crack. This can allow the process media to fill the internal volume of the thermowell. In such a case, the process media can be contained by additional barriers built into the thermowell.

Octopus-Array Sensors

New environmental standards for low sulfur fuels are driving the need for temperature profiling in the oil, gas and chemical industries. These standards and regulations require plants to reduce air pollution emissions, particularly NO_X, and are leading to equipment retrofits and upgrades.

In hydrofiners and fixed-bed reactors, temperature measurements often must be made in an array. Instead of profiling in a straight line — as with a long thermowell — the sensors must be arranged to measure temperatures at various points in a layer such as a catalyst bed.

For example, the sulfur content in mineral-oil products must be limited; typically, this is achieved by catalytic desulfurization in a hydrofiner. Heated to 572 to 752°F (300 to 400°C) and raised to a pressure of 362 to 870 psi (2.5 to 6 MPa), the oil is mixed with hydrogen and reacts with the catalyst. The sulfur molecular connection then is converted to H₂S and hydrogen carbides.

One of the issues with this process is performing temperature-profile monitoring at the different catalyst layers of a hydrofiner. With a limited number of access points available in a hydrofiner, multiple sensors must be inserted at each access point. Using an octopus array, the sensors can be arranged so they spread out across the vessel much like octopus tentacles (figure 4).

Individual thermocouples are mounted in a single nozzle. The unit’s “tentacles” can be positioned within the reactor as needed to monitor the layer. Using data from multiple sensors, the control-and-monitoring software can construct a 3-D image of the catalyst layer’s temperature profile.

Temperature profiling with a sensor array also is useful in fixed-bed reactors. Due to the solid state of the catalyst, it may not be possible to achieve a homogeneous reaction mixture. Hot and cold spots may form, and coke formation can quickly lead to deactivation of the catalyst. With multiple measuring points per process connection, sensors can be positioned freely in the reactor to detect problems. As a result, operations can become aware of a degrading process situation quickly and take appropriate action.

Temperature sensors typically are routed directly to multipoint transmitters or to a terminal-block assembly that can accept multiple sensors. When transmitters are chosen, each transmitter sends a 4 to 20mA signal to the control system (one per sensor). The terminal-block assembly is used to route the thermocouple cables to the control system’s analog temperature inputs. Either the transmitters or the terminal-block assembly typically is housed in an enclosure.

For difficult temperature-measurement applications, specialized sensors and solutions are available. To deal with difficult sensing problems, it is best to consult with an instrument company that has experience in your specific application.