As an important component of a temperature measuring location, the thermometer thermowell, also known as protection tube or pipewell, is responsible for sealing the process from the environment. Alongside the protection of the temperature sensors, thermowells also offer protection for the environment and the operating personnel. Aggressive media as well as high pressures and flow rates are kept away from the actual temperature probe, which enables the measuring insert to be changed without interrupting the process and thus during ongoing operation. Due to the wide range of possible applications for protection tubes, they are available in different designs and materials. A very decisive criterion when choosing a thermowell is the nature and type of the process connection. The thermowells and protection tubes can either be welded in, screwed in with a thread or connected to the process by means of a flange. In order to ensure accurate temperature measurement, it is necessary that the thermometer and thermowell or protection tube fit together perfectly.
In addition to the type of process connection, the manufacturing method is another distinguishing feature. Here one chooses between one-piece thermowell or fabricated protection tubes. Protection tubes are constructed from a tube, that is closed at the tip by a welded solid tip. Thermowells, on the other hand, are made from a single piece of solid bar stock material (round or hexagonal).
Depending on the design, thermowells are used for high process-side loads and high chemical stresses. Thus internationally or in the petrochemical industry, thermowells are now used almost exclusively. Furthermore, flanged, weld-in and threaded thermowells are also used in process technology and in equipment manufacturing. Thermowells with flange or for welding in are also used in the petrochemical industry, onshore/offshore and in plant construction.
Protection tubes are generally recommended for low to medium process loads. Protection tubes with threads and flanges or for inserting/welding in are used in the chemical industry, process technology and also in equipment manufacturing. Protection tubes with flanges are used for high chemical loads. Certain protection tubes can additionally be used in the chemical industry, petrochemical industry and offshore. Here they are found in boilers, incinerators, furnaces and reactor vessels.
It must be ensured that the medium flows against the sensor of the thermometer. This generally occurs when the thermowell tip is located in the middle third of the pipe. The insertion length of a thermowell or protection tube depends on the thermometer used. For mechanical thermometers, a minimum insertion length of 60 ... 100 mm is assumed, whereas electrical thermometers only require an installation length of at least 35 ... 50 mm. The maximum insertion length for protection tubes is limited by the manufacturing length of the tubes (approx. 5 to 6 metres) and for thermowells by the manufacturing length of the gun drill machine (between 1,000 and 2,000 millimetres, depending on the model). However, longer thermowells can also be produced by welding individual pieces together.
Most thermowells and protection tubes are made from stainless steel. Special materials such as Hastelloy® and Monel®, amongst others, are used with especially demanding conditions. Depending on the application and material, a thermowell or protection tube can either be made completely of the special material or only those parts which are in contact with the process medium can be coated, e.g. with corrosion-resistant plastics.
As a further distinguishing feature, there are also different types of thermowell stems. A thermowell can be either tapered, straight or stepped. The tapered stem shape is the one most commonly used, guaranteeing a short response time and high strength at the same time. Protection tubes usually have the straight stem shape and thus a particularly cost-effective construction. For applications with high process-side loads with strong vibrations, we recommend the WIKA thermowell in a ScrutonWell® design.