Why do people often specify cast iron or ductile iron fittings with pvc pipes? What is the advantage? For example, if I specify C900 pvc pipe, why not just specify C900 pvc fittings? Thanks.
Twenty or more years ago, when PVC became acceptable for municipal systems, pipe was often deflected more than the manufacturers recommendations, generally at bends, so as to make a little more or less bend. This led to a rash of failures, generally at the fittings and a lot of us in the Pennsylvania area started to spec DI fittings. I guess once burned is enough. The problems were workmanship related but the designer has little control of this, even if his own employees are inspecting.
Steve
Further to the good answer from SteveWag, you may be interested in some history regarding pvc fittings e.g. from a vendor who supplied both DI and pvc fittings at
I believe these applications involved predominantly small diameter pvc fittings, and I suspect most of these integrally molded. As fitting requirements got larger and in other applications, and bends innately higher stressed etc. by whatever occurred at such locations in larger pipelines, various schemes of fabricating pvc fittings including fusion and solvent cement gluing segments etc. together also encountered some substantial problems. (The AWWA pvc water pipe standards sub-committee had incidentally removed solvent-cement pipe joining from the earlier AWWA C900 standard editions copy via the 1989 version, with the first note in the Foreword, "Provisions for solvent-welded joints throughout the standard were deleted." However, several years later C90X pvc standards sort of resurrected "solvent-welding", though only for gluing together larger fabricated pvc fittings together e.g. from miter-cut pipe sections in factory conditions/with factory control, I believe due to cheaper manufacturing/equipment cost than integral molding for very limited production of such items.)
Some manufacturers may now be attempting to reinforce pvc fittings with somehow overlayed composite wraps, I guess hoping for more successful performance than prior offerings.
PVC-U was made to DICL pipe diameters and it was too costly to produce another range of PVC fittings to go with the pipe. Also the performance of large diameter solvent welded PVC pipe fittings was considered to be questionnable given the quality of trades who are used to install such pipes.
Local authorities like rubber ring joints as they perceive it to be a simple install that can be managed for quality.
Current generations of PVC-M and PVC-O are thinner wall than the PVC-U. It is questionnable if they could be joined using solvent cement. Using RRJs and thrust blocks avoids all that expensive testing of the joint to demonstrate adequacy. In addition the prospect of even more large diameter fittings injection moulds involves considerable investment.
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