Magnetic mount. In this design one or more magnets hold the tip of a spring-loaded thermocouple sensor tightly against a steel or other ferromagnetic surface (Figure 2). It suits surfaces of varying shapes (flat, curved or irregular) and temperatures from –50° to 150°C (-58° to 302°F). Because the unit can be moved, it's useful for applications such as hot spot detection.
Pipe clamp RTD sensor. This design holds the tip of a spring-loaded RTD sensor against the surface using a bolt-tightened clamp (Figure 3). Mechanically rugged, it can support a temperature transmitter — often a wireless one for an installation where running cable would be difficult or expensive. It is suitable for pipelines, pipe heating systems, flow lines and well heads. Sensors can measure temperatures from –200° to 300°C (-328° to 572°F). Sensor tips are available in silver or nickel for better response time and accuracy. The mounting clamp is made of stainless steel. Insulation patches can be installed between the pipe surface and the clamp itself if there are concerns about contact corrosion.
Metal contact block design and worm-drive hose clamp. This design (Figure 4) is a good solution for round surfaces. It can handle harsh environments with high temperatures, high humidity and dirt. Equipped with an RTD or thermocouple, it can be used at temperatures from –196°C to 600°C (-321° to 1,112°F). The contact block usually is stainless steel or copper.
Surface temperature sensors already have proven themselves in a variety of plant duties, including:
Reactor surface temperature measurement. This application involves a gasification reactor. The unit operates at high temperature; it's very important to detect any damage to the expensive and fragile refractory lining because damaged refractory can lead to a complete reactor failure. Refractory damage causes hot spots on the outside surface of the reactor that waste energy but also warn of possible future failure. The problem is to detect these hot spots.
The solution was a magnetic mount sensor that could be moved from place to place to check suspicious spots. The sensor has a single 3-mm Type K thermocouple and is connected via 10,000 mm of armored cable.
This sensor has enabled early detection of hot spots and reduced both operating and maintenance expenses.
Pipe heating control. A large intermediate chemical manufacturer in Eastern Europe uses pipes ranging in diameter from 50 to 200 mm (2 to 7.8 in.) to carry plastics intermediate compound methylene diphenylene diisocyanate (MDI). If the compound becomes too hot it will disintegrate; if too cold it will crystallize and plug the pipes, requiring a shutdown to clean it out. So, the plant uses a pipe heating system to keep the MDI in a viscous state; it should hold the temperature at 50°C (122°F) with a tight tolerance of ±1.5°C (±2.7°F). Measurements must be made at multiple points along 70 m (230 ft) of pipe.
Conventional measurement using surface sensors with strap-on clamps didn't provide the needed accuracy and was slow to respond to temperature changes. It was tough to maintain the proper temperature without over- and undershoot. In addition, the sensors were difficult and time-consuming to replace, which increased the risk of product loss or a process shutdown if a sensor failed.
The answer was to use pipe clamp sensors with RTD elements and spring-loaded silver tips for good thermal contact and fast response. These are connected to high density temperature transmitters, each with eight inputs. Installation was both easy and cost effective; the sensors provide the needed measurements more reliably and accurately than the conventional method. In addition, the sensors' replaceable measuring inserts help ensure process integrity.
High pressure steam boiler surface temperature monitoring. Measurement was a challenge because of very high mechanical vibration levels, high density measurements with longer length cables, an operating temperature of 600°C (1,112°F), a total temperature range of 0° to 800°C (32° to 1,472°F), and the need for metal-to-metal contact on boiler tubes.
The sensor used is a stainless steel contact block design equipped with a 3.2-mm Type K thermocouple. This is mounted with clips to a boiler tube (Figure 5); a MI expansion loop handles issues of expansion and contraction without damaging the sensor. The sensors are connected to a high density temperature transmitter.
Benefits include accurate boiler fatigue calculation, early detection of boiler tube failures and hot spots, reduced boiler operating and maintenance costs, increased sensor life under harsh conditions and lower cabling costs.
GUIDELINES FOR NON-INTRUSIVE MEASUREMENT
Several simple precautions can help ensure the best accuracy with surface temperature measurement. First, confirm the temperature being measured is actually that of the surface of the vessel or pipe by minimizing the effects of the surroundings — by making certain that thermal contact between the sensor and the surface is good and stays that way over time. Choose a sensor body material that's compatible with the surface to be measured and won't cause galvanic action that'll build up a layer of corrosion, increasing thermal resistance. For magnetic mount sensors it's a good idea to minimize the actual area of contact between the sensor tip and the surface.
For welded sensors differential expansion can pose the biggest challenge. Large differences between the expansion coefficients of the sensor contact block and the surface can cause severe stresses. Where large temperature changes occur over time — as, for example, during startup of a furnace — these stresses (perhaps aggravated by vibration) actually can tear the sensor off the surface.
DANJIN ZULIC is a marketing engineer, global temperature marketing, for Rosemount, Emerson Process Management, Chanhassen, Minn. E-mail him at Danjin.Zulic@emerson.com.