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Spin Flanges by Bestweld

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.

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