Tame a Troublesome Thermowell

Readers suggest solutions for a fluid-bed corrosion issue.

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THIS MONTH’S PUZZLER
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?

CHECK WALL THICKNESS
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


IT’S VIBRATION
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.


THERE’S MORE TO IT
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.


THINGS AREN’T AS THEY SEEM
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:
1) a ceramic coating;
2) and, by traditional tempering, called precipitation hardening. Common ceramic coatings include titanium nitride, tungsten carbide and diamond-like carbon.)
Kevin Burke, marketing manager
Parker Hannifin Corp., Huntsville, Ala.


OCTOBER’S PUZZLER
Our plant relies on an old batch fluid bed dryer (See Figure 1.) We can’t afford buying a new dryer, let alone its floor space. The dryer is difficult to operate because of loss of fluidization during the early part of the drying process. The baghouse employs a timer-controlled shaker that’s mechanically unreliable. When the shaker operates, the discharge valve to the blower momentarily closes. This is all that’s needed for solid to accumulate in the bowl above the screen.

Operators often beat the bowl to keep the solid fluid. They have another trick: To fluidize the solid early in the batch, they open the door of the bowl — when the discharge valve opens air is drawn into the bowl, creating a vacuum to lift the wet solid: the door closes. When it was installed, the dryer ran without so much operator attention. Since then two steam heaters have corroded and have been abandoned in-place (the air flows through them). Can you suggest some ways to avoid dryer problems and make the dryer easier to operate?

Send us your comments, suggestions or solutions for this question by September 11, 2009. We’ll include as many of them as possible in the October 2009 issue and all on ChemicalProcessing.com. Send visuals — a sketch is fine. E-mail us at ProcessPuzzler@putman.net or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you’d like to pose to our readers, send it along and we’ll be pleased to consider it for publication.
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