Sensible Sensor Installation – Follow Up

Since readers of last week’s blog may be waiting with baited breath, I offer the following mints.

Thermowells inserted into the side of a pipeline instead of an elbow may be sensing the effect of the wall or flange temperature. If the pipe wall temperature was the same as the process fluid temperature you wouldn’t need a thermowell, just clamp the temperature sensor to the pipe wall or flange. In particular, side entry thermowells for jacketed polymer pipelines may be seeing the jacket (e.g. Therminol) temperature more than the polymer temperature. Similarly, side entry thermowells for un-insulated pipelines may be affected by the ambient temperature. There are those exceptional cases where the fluid velocity is high enough (heat transfer coefficient is large enough), the cross sectional temperature profile is flat enough, and/or the pipe size is big enough where side entry is good enough. A trend chart of ambient and heat transfer fluid temperatures along with process temperatures can help detect improper installations.

Electrodes tips near the top of a pipeline or vessel tend to be noisier due to gas bubbles momentarily attaching themselves. Electrode tips near the bottom of a pipeline or vessel tend to be slower due to fouling from solids that hang near the bottom. Electrodes on the suction of a pump are more likely to see both bubbles and solids and the occasional wrench or welding rod. These considerations apply to temperature sensors although the consequences are usually less.

Since the actual pH of the process depends upon process temperature through changes in the dissociation constants of the acids and bases, improper electrode location can also lead to incorrect process temperature correction of the pH measurement ( not to be confused with the Nernst electrode temperature correction).

The side entry of sensors into vessels whose tip doesn’t extend more than a couple of inches into the fluid may be reading stagnant fluid and may be seeing the effect of jacket temperature. The sensors may be hidden from circulation by baffles and wall effects. Bulk velocities in even highly agitated vessels rarely exceed 1 fps so be prepared for more fouling problems and slower sensor response for sensors in vessels rather than in pipelines. Ideally, the sensor should be near but not too close to the tip of the impeller.