|
Spin flanges are a two piece component sold
as a single unit. Inner pipe (Spin) portion
can be made in nearly any corrosion resistant
cold workable alloy. The flange can be supplied
in a variety of materials and pressure ratings
with commercial or Navy drillings.
Spin flanges ARE beautiful
thing. Let us count the ways:
- Lower installed cost where flange bolt hole
alignment is critical (and faster, too)
- More flexible than other pipe flange designs
resulting in lower bending moments transmitted
to component flanges as a result of ship motion,
thermal motion, vibration, component motion,
or whatever.
First and foremost of all concepts is the realization
that no component built entity with significant
size is EVER exactly shown
by the drawings intended to represent it. This
goes for houses, buildings, ships, manufacturing
facilities and everything else. Pertinent to
shipboard piping systems, even in areas of no
shop fabricated piece errors, pipe piece fabrication
tolerances and ship fabrication tolerances will
accumulate to require the occasional custom
fabricated spool piece to make up for differences.
Fabrication tolerances can be categorized as
those of offset, length and rotation. Small
differences in offset are often jacked or sprung
into place. Small differences in length can
be trimmed off or made up by repair couplings.
However, the orientation of a pipe flange connecting
to a pump or other piece of equipment - especially
a pump - can seldom be reliably taken from drawing
representations - especially when in an area
of tight arrangement. The limits of forgiveness
are very small. Often, a one degree difference
in bolt hole alignment will make a pipe fabrication
useless - especially if it is flanged on both
ends.
Three fabrication choices in order of decreasing
cost:
- After the pump, or other component is installed,
build a mock-up pipe piece from whatever.
It could be plywood, scrap flanges and angle
iron, or anything else that is relatively
stiff. Carry the mock-up back to the shop.
Using the mock-up, mount target mating flanges
on the fab floor and make the pipe piece to
fit between the target flanges. The total
time for such a fabrication can extend into
the second week.
- After the pump, or other component is installed,
build the pipe piece in place with pieces,
tack them and carry the pipe piece back to
the shop for final welding. The total time
for such a fabrication can take most of a
week.
- Fabricate the pipe piece in the shop from
drawings using the spin flange. Rotational
tolerances will be of no consequence and small
offset and length tolerances can often be
accommodated by other means. A comparable
fabrication time will be a day.
In addition to a lower installed cost, spin
flanges are much more flexible than any other
pipe flange design. Accelerated fatigue tests,
those tests with large angular deflections calculated
to fail a normal flange in a 1000 cycles, showed
that spin flanges had fatigue lives of two to
four times as many cycles. Although this property
of superior fatigue capability is seldom meaningful
to a shipyard, the collateral benefit is that
spin flanges transmit much lower bending moments
to pump nozzles, heat exchanger nozzles or any
other equipment nozzles.
This property of lower bending moments is, or
should be, very meaningful to shipyards because
a loaded pump nozzle will distort the pump casing
and change the alignment of the rotating parts.
Many unexplained pump failures have been caused
by pump nozzles loaded in bending. Moreover,
a pipe piece not loading a pump nozzle when
the ship is being built, will load the pump
nozzle after launching, or in a seaway, or with
temperature changes, or any other of many changes.
The effects of many of these changes are additive.
Many manufacturers of pumps and other components
will publish nozzle load limits for their equipment.
However, these limits are generally ignored
because there is no way to predict what the
as-built loads will be. Worse yet, engineers
often don't think to look for the effects of
nozzle loads when analyzing premature pump failures.
And, the pump internals failures from a distorted
casing can be easily confused with those of
motor misalignment.
|
