Monday, September 26, 2016

Industrial Thermowell Styles: Straight, Tapered, and Stepped Shanks

industrial thermowells
Industrial thermowell styles courtesy of REOTEMP
A thermowell is a machined tube intended to protect or seal off a sensing element from the process media. They are available in many materials and process connections.  There are three common body styles (referred to as shanks) used on thermowells: the straight shank, the taper shank, and the step shank. This post will explain the difference in styles.

Straight Shank
The straight shank has the same diameter throughout its entire immersion length. Because of its greater diameter and thicker tip, it usually has the slowest response time compared to other shank styles. However, the extra metal allows it to have a high corrosion and high abrasion resistance. Common installations for a straight well are tanks or pipes to have low pressure, low-velocity and/or a high abrasion process. Although the straight shank has the best mechanical strength properties, it should not be used in high-velocity flow systems. Its larger surface area can be overly disruptive to flow, or it may fail due to vortex shedding.

Tapered Shank
The second design is the tapered shank, which has an outside diameter that gradually decreases from the point just under the process connection, down to the tip. This taper allows for a very high mechanical strength, with a faster response time than a straight well. This design has very good vibration resistance and is commonly used and high-velocity flow applications. Common installations for a taper well are applications with a very high flow rate, high vibration, high-pressure and are high temperature.

Stepped Shank
The final type of shank is the stepped shank. Usually this type of well is a straight shank from the process connection down to about two and a half inches from the tip, with the final two and a half inches a smaller diameter straight shank. The reduction in diameter allows for a faster response - usually the fastest response out at three different shank designs. There is however a reduction in strength because of the diameter change, and less mechanically rugged as a straight or tapered shank style. Common installations for a step well are where rapid response is needed, but without high-velocity flow or where there is a possibility of being physically damaged from the process media.