Hydrocarbon Processing - February 2022 - 15

Reliability
HEINZ P. BLOCH, RELIABILITY/EQUIPMENT EDITOR
Heinz.Bloch@HydrocarbonProcessing.com
Resourcefulness solves problems
In the now almost 32 yr since this
monthly column was first published, we
have had more than a few occasions to
highlight the pitfalls of trial-and-error solutions
in plants that process toxic, flammable
or explosive materials. So, why
not mention two fine examples where resourceful
action by subject matter experts
(SMEs) has solved problems?
A bumper sticker becomes a coupling
stretch monitor. Forty-seven years
ago, in late 1975, an SME with a well-balanced
combination background that was
both practical and theoretical participated
in commissioning an important centrifugal
compressor train at a facility in France. He
knew that the long span between the respective
shaft ends of the large centrifugal
compressor and its steam turbine driver
was bridged by a diaphragm coupling. In
modern turbomachines, the taper-bored
hubs of diaphragm couplings-virtually
identical to FIG. 1-engage the tapered
shaft ends with a strong interference fit,
and keys are no longer used. Although
shown with a keyed fit to shafts identified
as #28 and #30 on the patent drawing reproduced
in FIG. 1, the machinery train in
France was one of the first that dispensed
with keys. The two coupling hub bores
were dilated by hydraulic means and forced
up on their respective shaft ends until an
interference fit of 1.5 mm/m-2 mm/m
(0.0015 in./in.-0.002 in./in.) was reached.
However, compressor shafts tend to
grow when they operate surrounded by
hot process gases. Steam turbine shafts
will grow even more because steam is
usually extremely hot. At standstill, and
before gases and vapors are admitted to
driver and driven machines, both shafts
are at stable ambient conditions. The distance
between shaft ends (DBSE) will be
at its maximum and the two diaphragms
in FIG. 1 will be " stretched " a small amount.
Once the compressor is brought up to
speed and is performing its compression
duty, the shafts of both driver and driven
machine will have thermally grown, and
the DBSE will be at its minimum. Hopefully,
and under stable operating conditions,
the two diaphragms will be " normal
and relaxed, " meaning neither stretched
nor pushed towards each other.
But how did the SME verify that " normal
and relaxed " had been achieved as
pre-calculated and anticipated? He took
two shiny automotive bumper stickers-
auto insurance advertisement stickers-
(2 decals, items " 24 " in FIG. 1) and with a
fine-point felt-tip marker drew progressive
steps " a/b/c/d " 0.040 in. (1.0 mm) apart.
With a pen knife, he cut these steps before
bonding each decal to the coupling spacer
right under the guard plates " 20. " At " 22, "
each of the two guard plates has a radial
clearance of 0.5 mm (0.020 in.) relative to
spacer tube " 12. " At normal speed, observation
with a stroboscopic light showed how
many steps were covered (or uncovered) in
the plus or minus directions. The coupling
diaphragms were doing fine with the machine
at stable load and temperature.
How remembering Archimedes
solved a leakage problem. During
commissioning of several pusher centrifuges
(FIG. 2), it became immediately evident
that there was excessive and intolerable
leakage from the annular opening
between the stationary inlet elbow and
the rotating centrifuge basket. The courteous
Japanese startup managers asked if the
normally U.S.-based SME (who had visited
the Swiss centrifuge manufacturer during
the mechanical run test phase) would
please design a mechanical seal that would
solve the problem.
Designing and fabricating a special mechanical
seal would take weeks, the SME
thought, but declining the request would
have been a disappointing refusal. The
SME began by sketching a mechanical
seal with parts made of bronze, polytetrafluoroethylene
(PTFE) and stainless-steel
springs. The refinery's machine shop fabricated
the components overnight, and
FIG. 1. A page from a long-expired coupling
decal patent.
a machinist crew was asked to install the
parts in time for a pre-noon restart attempt
on the following day. Unfortunately, the
FIG. 2. While an Archimedes screw has the
helix on the outside, this " Archimedes bushing "
has the helical screw flight on the inside.
Hydrocarbon Processing | FEBRUARY 2022 15
Hydrocarbon Processing - February 2022

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