by Freeman Newton
The following is an unbiased study into material selection for hard chrome tanks and ventilation systems. The author, Freeman Newton, recently retired after 40 odd years in thermoplastic fabrication but now consulting, built the very first North American inertial hard chrome scrubbers in the late 60's (& coined the word 'chromic dry scrubber') and designed the inertial S-LIMITS to meet the EPA emission standards. He has presented many papers to the A.E.S. & other organizations on suitable plastics for chemical environments as well as papers on Assay and Perchloric fail-safe ventilation systems. He has designed mega long fume hoods (having only one duct outlet & no turning vanes in direct contradiction to the ventilation manuals' specifications!) that are very efficient and far less costly to make & service. Generally speaking, PVC is not recommended for hard chrome hoods nor ducting but is acceptable for inertial (horizontal) scrubbers which are roof mounted and not too close to the tanks being exhausted.
Two U.S. hard chrome platers were approached. DELTA AIRLINES in Georgia, David Wood, Mgr of Facilities and B.F. Goodrich (ex CLEVELAND PNEUMATIC), Tom Wolfe, Senior electroplating engineer. Four Canadian Companies were contacted. EBC0 Industries, Hugo Eppich, owner. FINNING TRACTOR (Pacific Fluid Power Div., in Alberta), Bentley Barr, Plating Mnager. MOORE's Machine Shop, Alberta. Ron Moore, owner. Also COLCO in British Columbia, a world wide designer and supplier of complete hard chrome plants. Tom Easton, electrical engineer.
Both U.S. Companies reported deterioration, embrittlement and difficulty, after a time lapse, in doing any rewelding of PVC. Tom Wolfe said he had noticed immediate welding problems with PVC and replaced it with CPVC. David noticed some problems after 3-1/2 years and has replaced most of the PVC with CPVC. Both say that the CPVC is excellent. Tom stated that any rework, ie. welding, on the CPVC was easily done unlike the PVC. David is apparently using l2 foot deep tanks using Halar fluorocarbon. For the record, the CPVC resin happens to be made by B.F. Goodrich!
FINNING TRACTOR. They have had PVC hoods & ducting since l994. Initially one hood failed (stress cracked around the welds) shortly after being installed. This was replaced using a higher quality PVC but, 3-l/2 years later, that needed replacing. Some weld cracks in the ducting were fibreglassed over and are still standing up well. There is no problem with the PVC scrubber but then other hard chrome inertial scrubbers of similar design are still OK after 20 years or so.
However, Bentley says that PVC is OK and ideal for the the chrome etching tank as the power is only applied to it 8 times a day in 30 second bursts. Problems with the true plating ventilation is due to l0,000 amps creating an ionized (CR06) which attacks the PVC. This action is accelerated, he said, if catalysts are used. That was a critical and interesting observation by him.
EBCO. All hoods and ducting are from mild steel. Minor replacement problems. Excessive heat from mega amperage when plating very large diameter rolls in a narrowish tank was solved in l979 or l980 by spraying the hot liquid into a CT-120/2 inertial scrubber and achieving evaporative cooling. That very simple PVC scrubber unit is still being used, says Hugo. However, the first blade bank now needs replacing due to some embrittlement & deterioration of the PVC blades. That piece of equipment has had a very successful life span and the equipment costs were very quickly amortized. Emission standards, he says, are still being met.
MOORE'S machine shop. All hoods and ducting are from heavy gauge mild steel, welded construction. Reasonably long life, too, said Ron. Some minor problems were due to not cleaning down ducting occasionally. Reasonably large operations. All scrubbers are now S-LMITS, PVC construction, of course!
COLCO. They have always recommended or supplied all the hoods and ducting from heavy gauge welded mild steel. Reasonably long life, too, says Tom Easton. All scrubbers are made of PVC. For Peru and offshore, where the emissions standards are not so onerous, often the simpler eliminator scrubbers (eg. CT-120/2s) are used. For Canada, the USA, Ireland and Australia, they chose to use the S-LMITS (recent tests by them were measured at 0.0000307 Mg/M3).
CONSIDERATIONS! For potential hoist impact problems, always use steel and not plastics for the fume hoods! Ensure that hoods have low level drain-offs & that horizontal ducting has a slight slope towards the hoods or to the scrubber to facilitate drainage and prevent build-up from occurring.
TANKS Liners from rubber last 3 years according to one source. PVC liners (i.e., professionally made such as by Flexi-Liner & other specialist Companies, not thin gauge swimming pool liners!) should last over five years depending upon not only the tank temperature but also on radiating heat sources such as nearby bus bars and the number of anodes being activated. Premature liner failure is often due to tearing during loading parts. An inexpensive method of giving some protection against liner tearing is to bolt on a min. 12" thick high density Polyethylene plate to the top of the tank flange and having it overlap inwards by around 34". Hd.Pe. being excellent for impact & abrasion.
COSTING VIEWPOINTS. Assuming just the plain material costs, then if PVC = l, mild steel = l/2, CPVC would be 4 and HALAR 30 times as much. From a totally installed ducting viewpoint, if PVC = l, then mild steel would also be l, CPVC might be at least 2 and HALAR 4 or 5.
OTHER MATERIALS for hoods & ducting? Stainless should be OK but is costly. Fibreglass, using the best resins should only be OK for some years and then should fail (eg. FRP medium term failure is quite common in car battery mfg ventilation systems handling only 35% sulfuric acid fumes). Hastelloy C should be better than stainless, ditto for Carpenter 20. Titanium should be excellent as should be all the other weldable fluorocarbons but these latter options are also very expensive!
RECOMMENDATIONS. For showcase areas you could consider some of the plastics. Otherwise use welded heavy gauge steel (eg l0 ga) for fume hoods and ducting. Although requested, B.F. did not confirm that their CPVC materials are A.0K for ducting and hoods & re-welding. If CPVC pipe is used, do not directly weld in place but cement join to an injection moulded fitting, which welds well to the sheet, as there have been problems with the welding of CPVC pipe to itself. PVC has proven itself to be OK in the long term for inertial (horizontal) scrubbers which are normally not close to the fume hoods and close to the source of ionization & apparently PVC is l00% OK for chrome etching systems. Never, use galvanized steel as the run back will contaminate the chrome. For fans use ordinary mild steel centrifugal paddle wheel designs and not axial blowers. Where there is any chance of hoist impact onto fume hoods, always use steel!
Issued first on December 7th, l998
May 2001: minor updating was done.
A.V.I.D Acid Ventilation Industrial Design. White Rock, British Columbia Canada
Freeman Newton
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