Thermowell

Thermowell

 Thermowells are typically constructed of solid drilled-out bar stock and are designed to protect a temperature sensor from flow, high pressure and harsh environments. Thermowells encase and protect temperature sensors from the harmful effects of the processes into which they are immersed without substantially insulating the temperature sensor (thermocouple, RTD, etc.) from the temperature of the process.

What are the most common types of thermowells?
Thermowells are commonly classified according to their connection to a process. The most common types of thermowells are (1) threaded, (2) socket weld, (3) weld-in, and (4) flanged.

As the names imply, a threaded thermowell is screwed into the process either directly into the wall of a tapped pipe or into a thermowell threadolet.  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.  A flanged thermowell has a flange collar which is attached to a mating flange on a pipe nozzle.

What are the components of thermowells?
Typically a thermowell consists of (1) a process connection, (2) shank construction, (3) a “Q dimension”, (4) bore size, (5) immersion (“U) length, and (6) lagging extension (“T”) length.

Thermowell process connections:Thermowells are inserted into and connected into a process in a pressure tight manner. The most common process connections for thermowells include threaded, socket weld, and flanged connections.

Thermowell shank construction: The most common shank constructions for thermowells are (1) straight, (2) step, and (3) tapered.  A straight shank Thermowell is the same size all along the immersion length of the Thermowell.  A step shank Thermowell has an outer diameter of ½” at the end of the thermowell immersion length to provide a quicker response time. In a tapered Thermowell the outside diameter of the Thermowell decreases gradually along the immersion length of the Thermowell. A heavy duty tapered thermowell is typically used for high velocity applications due to the specification of a tapered thermowell shank in the old ASME PTC 19.3 (1974) thermowell standard. However, where the nozzle inside diameter is a design constraint the straight shank thermowell design is often the most resistant to velocity induced resonance.

Thermowell root dimension (Q): The “Q” dimension of a thermowell is the thickest part of the shank of the thermowell that is on the hot side of the process connection or flange.  The size of a thermowell Q dimension is, of course, related to the bore size of the thermowell and the process connection size.

Thermowell Bore size: The inside diameter of a Thermowell.  Standard Thermowell bore sizes are .260” and .385”. These sizes are intended to accept a quarter or three eights inch diameter sensor.

Thermowell Immersion (“U”) Length: Thermowell immersion lengths are often called the “U” length.  The U length is the measurement of the Thermowell from the bottom of the process connection to the tip of the Thermowell.  The U length establishes the length of the Thermowell that is actually in the process being measured.

Thermowell Lagging Extension (“T”) Length: The lagging extension of a thermowell is often referred to as the thermowell’s “T” length.  The lagging extension or T length is located on the cold side of the process connection and is usually an extension of the hex length of the Thermowell. Typically, the T length enables the probe and thermowell to extend through insulation or walls.