Thermowells are containers inserted into an existing pipeline or process device that accept numerous types of temperature probes including RTDs and thermocouples. Thermowells are used to facilitate the repair of temperature sensing devices without interrupting the process being monitored. 

 

Measuring Skin Temperature of Tubes in Furnace Fire Zones

 

Thermowells are available in three main barrel or shank design types.  The barrel, or shank, is the container style apparatus, which is inserted into the process flow.  Since thermowells are inserted directly into the process flow, the goal is to allow for measurement while causing as little restriction of flow as possible.  Each of the three types of thermowells offer advantages and in some cases disadvantages in terms of reading and flow disruption.  Straight shank designs are the most common.  They are cost effective and require no extra machining.  However, they tend to involve a high degree of surface area, which can disrupt flow.  They are not recommended for use in high velocity flow systems.  Stepped shank thermowells have less surface area than straight shank types, and therefore allow smoother velocities around the barrel, as well as increased sensitivity for the inserted probe.  Tapered shank thermowells have the lowest degree of surface area.  Like stepped shank devices, they offer increased sensitivity, but with greater stiffness. They can be used in high velocity flow systems.

Selecting Thermowells

When selecting between the available types of thermowells, there are a number of important specifications to keep in mind.  Foremost among these are stem length and bore diameter of the thermowell, the temperature and viscosity of the media into which the thermowell will be seated, and lagging extensions through which the sensor will have to pass.  Stem length is defined as the length from bore to the end of the well.  One must consider the length required to pass through walls, pipe fittings, etc.  Bore diameter refers to the size of the thermowells’ aperture through which the probe will be inserted.  Obviously, the size of the probe must be known before this can be determined.  Typical values are .260 or .385.  Lagging extensions take into account the insulation, pipe fittings, walls etc, through which a sensor may pass.  The lag length is the distance from top of connection means (i.e., thread) to bore.