Thermowells are the most uncomplicated devices employed for measurement of temperature. However, these devices haven’t gained much popularity in industrial applications. Thermowells are basically brought into use for creating isolation between temperature sensing element and its immediate environment which can be either in the form of liquid or gas.
“The American Society for Testing and Materials (ASTM) has defined the term thermowell as a closed-end reentrant tube designed for insertion of a temperature-sensing element, and provided with means for a pressure-tight attachment to a vessel.”1
Thermowells are generally constructed in such a way that they effectively cover and guard the temperature sensing elements from the high pressure, flow and other detrimental properties of the processes into which they are inserted. Thus, with the help of a thermowell, temperature sensors like thermocouples and RTDs can be safely applied (without any insulation) for measuring the temperature of a process. They enable the removal and replacement of temperature sensor from the process without making any compromise with the process and ambient surroundings.
Components of Thermowells
Major components of a thermowell are listed below:
- Thermowell process connections: A thermowell is normally introduced and connected to the process containing liquid or gas media via a pressure- tight approach.
- Thermowell Shank construction: Various types of thermowell shank constructions are available in the industry. Some of them are mentioned in a section below.
- Q Dimension: The thickest part of the thermowell’s shank happens to fall into the hot region of the process connection and is generally referred to as Q dimension. Thermowell’s Q dimension size is associated to two other sizes. One is thermowell’s Bore size and the other one is process connection size.
- Bore size: The internal diameter of a thermowell is referred to as thermowell’s bore size. Typical bore sizes of a thermowell are .260″ and .385″.
- Thermowell Immersion Length: It is usually represented as the “U” length. It is the real length of the thermowell which is inserted into the process under measurement. It is determined by measuring from the base of the process connection to the thermowell’s tip.
- Lagging Extension Length: It is usually denoted by “T”. This length is an extended version of the hex length of the thermowell and is typically positioned on the cold region of the process connections. Via this extension length, the temperature sensing probe and thermowell goes beyond the insulation or wall boundaries.
Types of Thermowells
Principally, there are two types of thermowells i.e. low pressure thermowells and high pressure thermowells. Various designs exist for these two thermocouple types. The most widely used thermowell designs depending upon their process connections are classified below:
- Socket weld
- Flanged welded
“A threaded Thermowell is screwed into the process. A socket weld Thermowell is welded into a weldalet and a weld in Thermowell is welded directly into the process. A flanged Thermowell has a flange collar which is attached to a mating flange.” Universally available thermowell types are shown in the figures below.
Threaded-Straight Socket-Tapered Flanged-Tapered Weldable-Tapered
Following are the materials which have been considered suitable for the construction of thermowells all around the world:
- Carbon steels: It is the most inexpensive material used in thermowell construction. Owing to its low corrosion resistance its use is limited only to applications where low temperature and stress are involved.
- Chrom/Moly steels: These are high strength steels used in pressure vessels and industrial boiler plant.
- Stainless steel: It is also a cost effective material for thermowell construction which is mainly used in areas requiring high corrosion and heat resistance. A whole range of stainless steel materials is available in the industry.
- Incoloy: It is an alloy made up of nickel, iron and chromium.
- Inconel: It is an alloy made up of nickel and chromium.
- Monel: It is an alloy made up of nickel and copper.
- Hastelloy: It is an alloy made up of nickel, chromium, molybdenum and tungsten which is extremely corrosion resistant.
- Haynes alloy: It is a multipurpose alloy which offers high quality resistance to sulphidising, carburising and chlorine bearing environments.
- Titanium: This material is very light in weight and offers brilliant strength.
Thermowell Shank Construction
The most widely available thermowell shank designs include:
- Straight shank Thermowell: It maintains the same size throughout the entire thermowell immersion length.
- Step shank Thermowell: Its external diameter happens to be ½” near the end of the thermowell immersion length which results in fast time of response.
- Tapered Thermowell: Its external diameter drops off steadily all over the thermowell immersion length. In general, a heavy duty tapered shank thermowell finds its use in applications involving high velocity.
- Thermowells constructed from drilled molybdenum rods and high purity alumina sheaths are considered to be very costly and intricate type of thermowells.
- One of the most popularly used thermowell variant which mainly finds its application in low pressure and moderate to high temperature settings, (for example industrial furnaces) is referred to as protection tube. These protection tubes are generally formed from metal or high temperature glass or ceramic.
- Thermowells are primarily used to work with temperature measurement devices like thermocouples and RTDs,
- Thermowells can be employed both at high temperatures as well as low temperatures. High temperature applications include process furnaces like glass melting tanks whereas lower temperature applications involve the use of inexpensive radiation thermometer and fiber optic meters.
- Thermowells are generally applied in processes where there is a danger of corrosion, high pressure, abrasion, or shear forces to the existence of temperature sensing element.
- Use of thermowells makes it possible to take away and replace a defective instrument from the process without affecting it.
Following points must be considered while selecting a thermowell for a particular application:
- Thermowell Immersion Length: The immersion (U) length of a thermowell plays a crucial role in deciding the accuracy of temperature measurements. In general, “U” length of a thermowell is limited to five times the thermowell’s external diameter for getting accurate results. This is the immersion length maintained in case of processes involving liquids whereas in case of gas or air associated processes, the required minimum U length is set to ten times the external diameter of the Thermowell.
- Resistance to vibration: Sometimes, the performance of thermowells gets adversely affected by turbulent vibrations. These vibrations are created by gas or liquid flowing in the process area which is subjected to temperature measurement. The intensity of these vibrations depends upon the thermowell’s diameter and the liquid (or gas) flow. To prevent or reduce the occurrence of these vibrations, thermowells must be designed with excellent stiffness. Most preferred type of thermowell design which can stand against vibrations is the tapered thermowell design since it offers superb stiffness without even compromising the temperature sensitivity of a thermowell.
- Thermowell material: Appropriate material must be chosen for construction of an effective thermowell. In case the thermowell is made up of unsuitable material, there will always be risk of thermowell failure. Hence, thermowell must always be chosen keeping in view the process temperature along with the corrosion and erosive conditions which may possibly be allied with the process.
In spite of all the benefits offered by thermowells, they suffer from some serious limitations which are mentioned below:
- Their use for temperature measurement leads to additional acquisition and installation costs.
- They offer slow and sluggish response to variations in temperature.
- Loss of heat along the entire thermowell length may result in inaccurate temperature measurements.