Posted by Mark Kw
on June 21, 2009, 7:34:52, in reply to "Re: Shark Bite Plumbing Fittings"
71.48.249.178
These types of fittings are not "new" by any means, we've been using similar fittings shown below on air brake tubing for more than twenty years but they have their limitations.
These types of fittings using spring-teeth or compression teeth are very susceptible to vibration, un-even expansion/contraction, corrosion, dirt & build-up. Any application that is subject to vibration where the fitting and/or either piece of pipe/tube does vibrate at the exact same frequency and amplitude will eventually fail because the teeth will either cut or wear on the exterior of the pipe/tube allowing the lock to be lost and in some cases the vibration will reduce the wall thickness to a point where strength is lost and the pipe/tube will separate.
Corrosion is another issue especially with non-ferrous metals. Cu, AL & brass corrosion creates a compound problem of both structural degradation and expanding growth. When the corrosion starts, it will start first where the normal oxide layer on the pipe/tube has been damaged; in other words, right where the teeth bite into it. The corrosion not only removes the solid metal the teeth are dependent upon but also it "grows" causing expansion pressure that will cause the teeth to disengage - similar disengagement failure is seen in application where common environmental exposures as lime, chlorides and salt penetrate the fitting from the outside.
Common dirt is a big problem too because it also penetrates the fitting from the outside and with a little help from expansion & contraction, it will work under the teeth causing both abrasion wear and lock disengagement.
I used a lot of Victaulic fittings in commercial & industrial applications. The have two basic styles, groove-lock and teeth-lock. While the Victaulic teeth-lock does not rely upon spring pressure and thus eliminates many of the common disengagement failures, they are still susceptible to vibration, exterior corrosion (outside of the pipe/tube) and thermal cycling issues. Depending on the application, the non-spring locking system for plastic pipe used by Victaulic can still fail within days/weeks. While these devices are commonly accepted by engineers, you'll find anyone with field experience knows more about where they will work and where they won't. All the more reason why engineers should be required to have at least 25 years field experience before being allowed to design anything!
Plastic embitterment & chalking are two more issues you won't see addressed yet they will come back to bite you in the butt. PVC is the biggest offender because while the mfg's kick the temperature up on the last extrusion die zone to get that "pretty shine" on the outside, this often causes thermal degradation of the material that will result in premature embitterment and/or chalking. Chalking is when the surface becomes chalky like someone was dusting it with chalk or talcum powder; it is the pipe/tube breaking down just as an iron pipe sheds layers of rust and looses mechanical strength.
Thermal issues are always present no matter what the application. Every material expands & contracts in relation to both the environmental and containment material's temperature; along with this comes the fact that every material expands & contracts at a different rate and to a different extent as well. Plastics & most rubbers are thermal insulators while most metals are good thermal conductors. The problem that arises here is that when you shove warm or cold water through the plastic pipe/tube, its size changes as does the size of the fitting and these changes take place in all directions. In a perfect world, everything would move in conjunction to these changes but here in the real world we now have the issue of copper tube on one side and plastic on the other. The Cu expands rapidly and to a much greater extent than the plastic within common thermal operating ranges. This differential not creates additional mechanical stress on the fitting and locking mechanism and since the fitting is most commonly brass or iron, it too responds to thermal changes at a completely different rate & extent as well. All this combines to cause a constant state of wear & stress on the locking mechanism as well as the sealing mechanism. Corrosion & contamination particles get into the fitting and when they work their way around & under the sealing surface, the thermal movement properties just speed things along.
The only long-term reliable method of connecting plastic pipe to metal pipe/tube proven in industry is via a properly supported flanged joint. There are a multitude of other issues involved as well, not the least of which is allowing for thermal movement and vibration isolation. The common residential & light commercial building industries are about a century behind the industrial building industry when it comes to reliable components and design-engineering practices IF THE KNOWLEDGE IS EMPLOYED! I've seen more waste and problems created by piss-poor engineering than anything else and 99% of that is related to engineers who have little or no field experience. I mean, c'mon, we're still putting on shingle roofs and burying pipes & wires in walls without a single fore-thought given to expansion or repair.
Cross-linked polyethylene tubing, via the catalyst t-butyl peroxide, is commonly and generically referred to as "PEX". PEX is NOT approved for potable water systems because it breaks down forming and releasing into the water t-butanol and MTBE, two highly toxic chemicals. MTBE was mandated for use in gasoline as a way to increase the O2 content of the exhaust pipe (thank you californicator's). It's use in gasoline was short-lived but via cross contamination, 41 states have dangerous levels of MTBE contamination in their ground water supplies and no, the government isn't telling people nor is it tested for in a common well water test (pay the extra money and send it to a private non-political lab!)
Houses plumbed with PEX potable water tubing have shown contamination levels in the water exceeding 20,000ppb when some states have a MCL (maximum contamination level) of just 13ppb to 18ppb. Hydronic systems plumbed with PEX have shown 1 year contamination levels of MTBE high enough to cause the contamination of three square miles of ground water.
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