In article 0,
"Lloyd E. Sponenburgh" lloydspinsidemindspring.com wrote:

a ShopSabre 4896 that has only ONE 2" pipe to each half of the table,
even though the pump has a 4" inlet!(duh!)

4" water pipe (sch 20-40-80) is slightly too large to fit in the space
provided, but 4" ASTM-D2729 sewer pipe will fit. We're only talking 11"
of vacuum (or -5.4psi). But I want some margin of safety, and can't find
a figure anywhere.

Two 3" pipes are slightly larger than one 4" pipe, so you could just
make the pipe to each half 3" Sch. 20, 40 or 80 and run to a 4" (via
Tee, Wyes, reducers etc in the smoothest transition manner possible) at
the pump/bower. Should be effective and means not trusting SDR to do too
much, which is usually a good idea.

If committed to using 4" sewer pipe, being a practical git and an old
vacuum hand, I'd rig up a test section and suck it down to 30" to see if
it collapsed or not. Use a regular roughing pump that can just pull that
without a sweat, not your blower/pump which probably can't. Leave it for
a good long while. Barring some sort of kink, defect, etc I would not be
too surprised if it didn't collapse. If it did, I'd pay attention to
when that happened (if during pump-down) and try another piece at an
inch or two less, until it didn't, to get a general idea of where it
usually failed. A trap/screen on the pump input would be wise for this
testing, as might a wire cage around the test pipe in case of shrapnel
leaving.

On the third hand this is the ugly cousin of using PVC for compressed
air; but I guess most of the shrapnel would end up in the pump/blower
if/when it failed.

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