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Compression Fittings – Do’s and Don’ts

asked Jun 5, 2016 in Engineering by Normandy (21,000 points)

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answered Jun 5, 2016 by Normandy (21,000 points)

Do’s

Don’ts

  • Ensure that the compression fitting is free of dirt or any foreign material. The tube to be fitted must also be clean. The tube end must be cut square and any burrs should be removed.
  • Follow the manufacturer’s recommended procedure for tight­ening up the fitting when new. Swagelok compression fittings generally require 11/4 turns of the nut from finger-tight to obtain an effective seal. (Small tubing - 2, 3 and 4 mm - only requires a 3/4 turn.)
  • Once tightened, check the gap between the nut and the body of the fitting with a Swagelok Inspection Gauge. If the gauge fits, then additional tightening of the nut is required.
  • When retightening a disconnected fitting, the nut will not require as much tightening as when making the connection from new. Swagelok fittings require about 1/4 of a turn from hand-tight to regain the seal. Use the inspection gauge to check.
  • Metric and Imperial compression fittings are incompatible, therefore check which is being used. Swagelok metric com­pression fittings have a stepped shoulder machined on the nut hex and the body hex. The letters MM will also be stamped on shaped body fittings.
  • Do not combine or mix parts from various compression fitting manufacturers. The components may have different dimensions and tolerances and a seal will be difficult to achieve.
  • Never turn the fitting body to make up the joint. The body should be held and the nut rotated.
  • Do not use stainless steel tubing with brass fittings. The metal tubing material should either be the same or softer than the fitting material.
  • Do not use tubing which is scratched, depressed or deformed. Similarly, do not force oval tube into a fitting. The tube should be cut back to sound material.
  • Do not use PTFE tape of any other thread sealing compound between the nut and the body of the compression fitting.
  • Never bleed down a system by loosening the compression fitting nut.

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