Tame a Troublesome Thermowell

Readers suggest solutions for a fluid-bed corrosion issue.

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We replaced a thermocouple in a continuous hydrocarbon fluid-bed reactor during a turnaround. It’s located in a duct where high erosive flow occurs because of high velocity catalyst beads. We also replaced the carbon-steel thermowell with a high-nickel-alloy one because of a report of a slight corrosion problem. Since then, the thermocouple has failed every six months when previous thermowells lasted a year until the turnaround. What do you think the cause is and what can we do about it?

It may be that the wall of the high alloy thermowell is thinner than the carbon steel well it replaced. I am assuming that the alloy is chemically resistant, and no corrosion allowance was added (as it would be for carbon steel). The thinner wall may also be the manufacturer’s attempt to compensate for the worse thermal conductivity of the alloy.
Verify that the wall is thinner. Then, evaluate the option of going to a thermowell with a thicker wall even if it means making it from bar stock.
Alexander E. Smith Jr., process engineer
Parsons Engineering Corp., Boston

After considering this problem, I have developed a maintenance checklist. Consider the following:
1. If feasible, reduce the insertion length of the thermowell.
2. It appears that the failure may be vibration-induced. Thermowells need adequate stiffness to withstand vibration. Adjust the length of the wells. A four-to-six-inch insertion length may be adequate, based on experience.
3. Some solutions are available because this is a common problem. Try velocity collars to support thermowells.
4. Consider changing location or orientation of the thermowell so that the Fluid Catalytic Converter (FCC) flow is parallel or near-parallel (and not crossways) to the thermowell.
5. Consider installing a protective barrier for the thermowell — so that the FCC reaction products and FCC catalyst do not impinge directly on the thermowell. This may slow response.
6. Increase the diameter of the thermowell and go with a tapered design.
G. C. Shah, HSE project manager
Mustang Engineers, Sunnyvale, Calif.

Let’s consider the problem by using change analysis. What’s changed since the corrosion presented itself: the thermowell material and the installation itself. However, it is also possible that the process may have been altered, e.g., particle size or the ceramic bead distribution.
Assuming the maintenance was done correctly we have only one question: has the orientation of the thermowell changed? Corrosion is probably less of a problem than erosion. An old relationship used to estimate the erosion rate, E, of carbon steel in fluid beds could be used to show the effect of gas bulk velocity, V, and orientation from the wall tangent, the impingement angle, Θ :
E2/E1 = (V2/V1)4 [sin (1.6 Θ 2)/sin (1.6 Θ 1)]
If the original angle was changed from 30° to 60° with the same velocity, the erosion (of carbon steel) would be 30% worse (E2). This change can’t explain the erosion. For this reason, something may have changed in the process. Operations sometimes does not come clean with maintenance.
Dirk Willard, senior process engineer
Swenson Technologies, Monee, Ill.

High-nickel-based alloys are not nearly as abrasion resistant as carbon steel, thus the reduced life. A less expensive, practical solution would be to use a lower-nickel-content alloy that is readily available, i.e., T-316 S/S and then harden the thermowell exterior. (Stainless steels are softer with a lower modulus than carbon steel, 10% as a rule of thumb. Two methods that could make the steel tougher and improve abrasion resistance are:
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