On 5/11/2013 4:37 PM, nestork wrote:

DD_BobK;3060537 Wrote:

I was wondering if they were only looking at stress induced from
tightening& doing their tightening stress approximation as merely an E
* radial
strain based wedging action?
Would that not be independent of wall thickness?

Total fitting load is tightening load + pressure load.

Of course the system pressure capacity would be higher with Sch 80 vs
sch 40 but perhaps they were trying to emphasize the point that sch 80
was
not more over-tightening resistant than sch 40?

Am I missing something here?

Bob


They're just saying that using Schedule 80 fittings is treating the
symptom, not the disease. They're saying that the disease of plastic
female threaded fittings splitting when tightened can be cured by using
a proper thread sealant instead of using pipe dope or teflon tape on
plastic threads.

They're saying that the use of either teflon tape or pipe dope on
plastic threaded fittings will increase the wedging action of the male
pipe threads, thereby increasing the liklihood that the female fitting
will split.

So, the admonition against switching to Schedule 80 female threaded
fittings is that it's not addressing the real problem. If people used a
thread sealant instead of pipe dope or teflon tape, there wouldn't be
the increased splitting force on the female fitting, and there wouldn't
be any splitting problem. So, switching to Schedule 80 female plastic
fittings to avoid splitting is not really solving the problem. The real
solution is found in using thread sealant instead of pipe dope and/or
teflon tape to avoid the splitting problem altogether, rather than buy
stronger fittings that will stand up to the greater splitting force.

Basically, if the square peg won't go into the round hole, you need to
realize that you need a round peg, not a bigger hammer. In this case,
the analogy is that if your female plastic fittings are splitting, you
need to realize that the cause is the pipe dope or teflon tape your
using, and you need to use thread sealant instead, not use a stronger
female fitting.

or they're being over-torqued if not using tape or metal pipe dope.

That's a better summary of the point they're trying to make than
theirs...

Now granted the _strain_ is the same as it's the dimensionless ratio of
the distance moved/turn and the geometry of a Sch 40 or Sch 60 or 80
fitting is the same so the geometries do cancel out. But the _stress_
in psi induced by that same strain is less for the fitting with more
material in it--if that weren't so it wouldn't be possible to design for
higher pressures by adding material because it wouldn't matter is the
conclusion that statement leads to. That clearly just isn't so so
they've confused the writeup in that regard.

Granted the weak point is as they state w/ the notching effect of the
threading but there's still significantly more material left behind in a
heavier fitting than the lighter and that does make a difference.

Certainly it shouldn't be/isn't necessary to use heavier fittings if
properly assembled (or the matching fittings are under-designed) is
their main point and that anything that actually causes deformation of
the fitting is detrimental. Those are certainly common sense
conclusions as well.

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