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Tubeskin Thermocouples

Tubeskin Thermocouples
ExL Thermocouples Stop Virtual Junction Errors
 

 Tubeskin thermocouples present special problems with temperature measurement, especially if there is flame impingement along the sensor.  AccuTru's ExL tubeskins are made with MI-Dry.   This unique insulating material virtually eliminates virtual junction errors.

How AccuTru's ExL Tube Skin Thermocouples benefit your company......

Measuring temperatures in tubes 

Tube skin thermocouples are specifically designed to measure the temperature of a material flowing through a tube from the outside of the tube.
 
In order to get the most accurate temperature reading from the sensor, positive metal to metal contact between the sensor and the tube is essential. Many customers prefer the weld pad be bent to fit the same curvature of the tube O.D. to ensure efficient temperature transfer to the sensor. Additionally, insulating the sensor tip in order to isolate environmental temperatures from effecting the reading is important.
 

Design Issues

Careful consideration should be give to design configuration and the sensor components. 
 
Most methods require welding the sensor to the tube via a “weld pad’ or some similar type device. The primary design function of the weld pad is to offer additional material, other than the sheath itself, to weld the sheath to the tube.
 
The weld pad material should be metallurgically compatible to the sheath (material) and to the heater tube (material). The dimensional criteria of the weld pad should be considered to be sure the welder has sufficient distance from the sheath material to prevent burning a hole in the sheath.
 
The thermocouple tip is welded to the surface that is close to the flame then curved around the tube O.D. to the backside (cold) of the tube and run
along the tube until the sheath exits through wall of the furnace or boiler so
 
 
 the remainder of the sheath is protected from impingement from the flame. The thermocouple sheath is fixed to the tube at predetermined intervals with “weld clips”. 
 
The path of the sheath is critical to sensor performance, especially in MgO sensors. Every attempt should be made to keep the sensor away from hot zones in the furnace. Failure to properly design the path increases the probability of virtual junction error, resulting in the temperature readings from the outside of the tube instead of the inside of the tube. If you are experiencing virtual junction errors, consider using AccuTru’s ExL tube skin thermocouples.
 
Tubes inside a process heater or boiler, may elongate or “swing” due to high temperature conditions. It is advisable to increase the length of the sensor and form “expansion” loops to compensate for these conditions. It is common practice to fabricate the expansion loops between the furnace/boiler exit and the first weld clip.
 
Many designs include a “heat shield” filled with high temperature insulation over the sensor tip for optimal protection. If it is desirable that the heat shield is to be welded to the tube the wall thickness of the shield and the material should be considered.

Where possible, the surface to which the weld pad is welded, the weld pad material, and the sensor sheath should all be manufactured out of the same materials. 

Installation

A major cause of failure is improper welding technique. This is often caused by a rush to get the sensor installed; failure to adequately clean the surfaces to be welded, and difficulty in reaching locations within confined spaces, making it hard for the welder to gain adequate access to the pad in order to weld. As much care as possible should be given to the preparation of the tube by removing dirt and particulates from the tube prior to welding the weld pad, heat shield and the weld clips. A wire brush or wire wheel is recommended for the tube surface preparation.
 
The thermocouple should be checked after the pad is welded to the tube, prior to welding the weld clips, to ensure the sensor is working properly.
 
An inspector may recommend that all components that require welding be covered with a “insulation blanket” to minimize heat stress to the tube.
 

Choice of Weld Pads

A variety of weld pads are available. The choices include standard flat weld pads, curved weld pads, “V” pads, “quick disconnect spring pads” and the “Lance” pads. 
 
Welding flat pads to pipes should be avoided if the diameter of the pipe is so small that a flat pad will not fit properly on the pad. Filling a gap in an improperly fitted pad with weld material significantly increases the risk of weld failure and the sensor separating from the tube.
 
Quick disconnect spring pads are prone to failure over time as the spring weakens and allows the sensor to pop out of the pad.
 
“V” pads decrease the risk of separation of the sensor from the tube, but welding is more difficult and improper welding can result in separation from the tube. In addition, this type of pad uses an exposed junction. If the junction begins to pick up contaminants from the tube, the sensor will begin to drift.
 
AccuTru’s Lance pad is an improvement to the V pad design.   The Lance pad offers the benefits of the V pad design but allows the use of a sealed sensor end, thus reducing the probability of drift.
 
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