What is a Thermowell?
A thermowell is a pressure-tight sheath which surrounds a temperature measuring device (e.g. thermocouple. RTD or thermistors) and protects temperature measuring device from undesireable aspects of the process such as corrosion, errosion, high velocities etc.
It can be thought of as a thermally conductive protrusion into a process vessel or pipe allowing a temperature-sensitive instrument to detect process temperature without opening a hole in the vessel or pipe.
Why use a Thermowell?
It is exceptionally rare to find a temperature element being used in the process industries without a thermowell, but why use a thermowell?
Primarily, thermowells are used to protect temperature sensors against mechanical damage from excessive pressure, material velocity and corrosion.
Secondly, though just as importantly, thermowells are also used to maintain the integrity of pressure boundaries in systems i.e. thermowells allow a temperature sensor to be replaced without the need to depressurise the process system. This makes maintenance safer, quicker and cheaper.
Thermowell Material of Construction
They may be ubiquitos within the process industries, but what are thermowells commonly made from?
Although thermowells may be made out of any material that is thermally conductive, pressure-tight, and not chemically reactive with the process selecting the right material is crucial to the longevity of a thermowell.
It is worth noting that the corrosive effects of chemicals are increased at higher concentrations and temperatures. Also, particles suspended in a fluid can cause erosion so this should be considered in selecting the thermowell material of construction.
Most thermowells in the oil and gas, and petrochemical industries are formed out of either metal (stainless steel or other alloy) or ceramic materials e.g. SS304, SS316, SS316L, SS310, Inconel® 600, Incoloy® 800, Monel®, Hastelloy®.
Thermowell Stem Design
The thermowell stem, or as it is often called – the thermowell shank, of most thermowells used in the process industries tend to be tapered i.e. their diameter decreases gradually over the length of their insertion length. A tapered design offers superior strength and a faster response time to temperature changes compared with cylindrical thermowells.
Common Types of Thermowell
Thermowells are commonly classified according to their process connection i.e. how they are connected to the process pipe or vessel. The three most common types of thermowells are flanged, threaded or welded.
A threaded thermowell is screwed into the wall of a tapped pipe or more commonly into a thermowell threadolet.
A flanged thermowell has a flange collar which is attached to a mating flange on a pipe nozzle.
There are two variants of a welded thermowell;, namely a socket weld thermowell, and a weld-in thermowell. A socket weld thermowell is typically welded into a weldolet socket, but the thermowell may be welded directly into the pipe wall. A weld in thermowell is welded directly into the process vessel or piping.
Thermowell Insertion Length
Thermowell insertion length is one of the more critical dimensions that needs to be specified when ordering. But what is thermowell insertion length?
Thermowell insertion length, or as it is often referred to – the U length, is the total unsupported length from the edge of connection (flange or threaded) up to thermowell tip.
This is important because for best accuracy, the insertion length should be long enough to allow the entire temperature sensitive portion of the measurement device to extend into the medium being measured. Often this is achieved by adherence to the rule that a thermowell length should be based on the degree of insertion relative to pipe wall diameter, e.g. half to 2/3 across. Using this approach allows a degree of standardisaition to thermowell length throughout a process plant rather than specifying each thermowell with its own specific insertion length.
However, all good intrument engineers will check that adherence to this rule will satisfy the requirements for the sensitive length of the temperature device to be completely within the fluid. Things to bear in mind when checking insertion lengths are that the type of measurment sensor, and the state of process fluid will have has an impact.
When measuring the temperature of liquids, the sensitive portion of the temperature sensor should be extended into the process fluid the length of the temperature sensitive portion plus at least an inch (25mm).
For gas applications, the sensitive portion should be immersed the length of the temperature sensitive segment plus an additional three inches (75mm).
The temperature sensitive section of a thermocouple or thermistor is short; therefore, a thermowell with a relatively short insertion length can be used.
The temperature sensitive section of bimetal thermometers, RTDs and liquid in glass thermometers is usually between 1 and 2".
The requirement for sufficient exposure of the sensitive section of temperature sensors to the fluid can cause a problem in small diameter pipes, therefore it is very common for thermowell insertions in 2" pipes that the pipe will be swaged up to 4" where the thermowell will be inserted, and then swaged back to 2".
Thermowell Lagging Extension
Another important, and sometimes misunderstood thermowell dimension is lagging extension. A thermowell is specified with a lagging extension if the thermowell will be installed in a vessel or piping system that is covered in insulation. The extension length (T-length) is the distance between the instrument connection and process connection of the thermowell.
Typical Thermowell Dimensions
Thermowell Standards
The primary codes for thermowells are:
ASME 19.3TW (2016). This Standard applies to thermowells machined from bar stock and includes those welded to or threaded into a flange as well as those welded into a process vessel or pipe with or without a weld adaptor. Thermowells manufactured from pipe are outside the scope of this Standard.
IEC 61520:2000 Metal thermowells for thermometer sensors - Functional dimensions. This standard specifies functional dimensions for metal thermowells for thermo-meter sensors of nominal diameters ranging from 3 mm to 8 mm for use in process control.
ASME B40.9 Thermowells for Thermometers and Elastic Temperature Sensors. This standard provides terminology and definitions, dimensions, safety, construction and installation issues, test procedures and general recommendations.
