THIS MONTH'S PUZZLER
We have two brick-lined vessels with an expansion joint installed between them. The joint consists of a protective outside layer of high-density nylon fiber, two layers of polytetrafluoroethylene (PTFE) and then an inside layer of carbon-impregnated PTFE. The process fluid is a combustion stream consisting of hydrochloric acid, flue gases and steam at a bulk temperature of about 180°C and a vacuum of about 2 psia. Our commissioning team is concerned about the installation because the joint accumulates water. Can you see any potential problems with this expansion joint?
MAKE A SHIELD
If water is a concern, a shield of a suitable material could be anchored and sealed at the high side brick only, leaving clearance at the other sides to allow for the expansion. The shield should be raised above the brick and joints to insure it does not bind the system, particularly during the expansion situation. If the flue gas has particulate in it, a compressible elastomer "rope" could be installed on the open sides to keep the particulate from packing underneath the shield against the expansion joint.
Charles P. Etling, principal mechanical engineer
Hansen Engineering Group, St. Louis
LOOK AT OTHER POLYMERS
My concern would not be the water accumulation but the vacuum and temperature conditions that PTFE might not be able to withstand. Air pockets in the joint material will expand above 180°C for PTFE, a problem over time. Why not use a two component formaldehyde-based resin? From my experience (corrosion) resistance is good, even at that high temperature.
Rolf Weber, manager director
Pen Tsao Chemical Industry Ltd., Shanghai, China
The problem is water-hammer.
Romeo Jabonillo, process engineer
Fluor, Irvin, Texas
DON'T USE NYLON
I would have used high temperature silicone for the outer layer. We've used this in ducts in a power plant. Nylon is too porous and will allow the buildup of moisture underneath.
Mark Rothstein, forensic consultant
Location not provided
TEST THE RELAXATION MODULUS
The expansion joint could collect water. HCl, like chlorine gas, is relatively inert. It only needs the presence of water to make it corrosive. Water will cause the carbon impregnated layer to swell. The water will penetrate the Teflon layer and then the nylon layer and cause swelling. Water will conduct heat further out to the Teflon and then nylon layer. Eventually, the Teflon will soften and flow and so will the nylon. If these polymers contained a cross-linker they would resist flow but might eventually burn because of the chemical nature of linkers.
There is another danger. Introducing a liquid into a polymer plasticizes it, causing reduced strength and modulus. Most likely, this material will suck in from the vacuum and fail much sooner than expected.
I would be prepared to consider replacing the existing pipe with a Y. Another alternative might be to design the venting to relieve its own stress. Expansion joints are a poor substitute for expansion. To win support for this solution, you might consider taking a few strips of the expansion joint material and stretching them with a fixed weight, or stress, in an acid/humidity bath raised to a high temperature. A lab with a hood will be required. The worst case would be a horizontal fixture that would allow the water to accumulate. Watch the bath test for a few weeks. The material will either fail during stretching or it will show signs of wear that make your case for pipe alterations.
Dirk Willard, contract process engineer
Organic Technologies, Coshocton, Ohio
USE A REFORMULATED FLUOROPOLYMER
May I suggest using one of the reformulated fluoropolymers on the market as the choice: a terpolymer of tetrafluorethylene (TFE). (They have a usable thermal range of -45°C to +275°C.), can be formulated for resistance to steam, acids, and resist attack from chemicals and fluids, etc. They are used in Aerospace and most other industries.
I am not in the industry, but I am sure there are a couple of suppliers that can formulate the most appropriate polymer material for the purpose.
Richard Ashley, Associate Dir., QC
Teva Pharmaceuticals, Pomona, N.Y.
We use a pyrophoric catalyst in a polymerization reactor. The ZrCl2 catalyst reacts violently with air and water. According to the catalyst manufacturer, the reactor, monomer feed tank and other tanks must be purged to 10 ppm-volume (ppmv) oxygen and 10 ppm-mass water. Monomer comes by railcar; the data sheet says the monomer meets a 10-ppmv oxygen standard; there's no measurable water present. The monomer goes through a molecular sieve dryer then it is fed to the reactor feed tank. After hydrotesting, air-drying and a nitrogen purge, we still can't get the feed tank oxygen down below 100 ppmv. However, we still can't get the monomer oxygen down to 100 ppmv in the feed tank. After 30 additional hours of purging, we can't get below 200 ppmv. The water in the tank is about 50 ppm. What can we do? What could be the source of additional oxygen and water?
Send us your comments, suggestions or solutions for this question by July 7, 2010. We'll include as many of them as possible in the August 2010 issue and all on CP.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.
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