Thermowell Location in Fractionation Columns

1. Purpose

This procedure explains the philosophy used to locate thermowells in fractionation columns. In addition it provides guidelines for needed action when a column is fabricated with the thermowells incorrectly located.

2. Thermowell Location Philosophy

Generally, Inflection Point Engineering philosophy is to place the thermowell six inches above a tray floor in the bubbling area. By placing it in the bubbling area rather than the downcomer it will respond to changes in vapor composition more rapidly than if it was in the downcomer. By placing it close to the tray floor (six inches above) it should always be in the active vapor-liquid contact area. Because of the vapor and liquid activity here, the heat transfer coefficient will be larger than if the thermowell was higher. If the thermowell was near the tray above, the thermowell could possibly be only in contact with vapor and, therefore, have a lower heat transfer coefficient which will result in a slower response time to changes in temperatures.

Some other companies prefer to place the thermowell in the downcomer so as to measure the liquid temperature. While this yields a slower response time, it naturally reflects the composition of the liquid better than measuring the temperature of a two-phase mixture or just the gas phase. As controlling on a tray liquid composition is generally no better than controlling on a tray vapor composition, Inflection Point Engineering normally puts the thermowell in the bubbling area.

Thermowells are normally specified to be long enough so that they protrude into the column one-foot or more and are to not be in contact with other internals. As the column nozzle length is normally six inches, the thermowell is normally one foot six inches in length. The one-foot distance from the column wall is to avoid wall effects. Other column internals may transfer heat to the column wall or to/from other locations in the column. If these internals are touching the thermowell they will affect the measured temperature.

3. Thermowell in a Downcomer

If the thermowell is in a downcomer, clearly indicate that it is not built to Inflection Point Engineering specifications. However, in general it is very expensive and hence very difficult to get a new nozzle put into a vessel that is already erected. Tell the customer that although they did not get what they paid for, the column will still work.

4. Thermowell in a Downcomer and Blocked by the Downcomer Wall

A shorter thermowell shall be put in the column provided that it can protrude into the column at least six inches. If this is not possible then a hole shall be cut into the downcomer wall to allow the thermowell to pass through. It is important that the hole in the downcomer wall be just large enough for the thermowell. Vapor from the tray may enter the downcomer through the clearance between the downcomer wall and the thermowell. This vapor will interfere with the liquid coming down the downcomer.