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Screw Joints – Do’s and Don’ts

asked Sep 2, 2016 in Engineering by Normandy (21,000 points)

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answered Sep 2, 2016 by Normandy (21,000 points)

Do’s

Don’ts

  • Take care when unscrewing a joint, even though the system may have been depressurized:
    • The seal on a union type joint breaks as soon as the ring nut is slackened.
    • Threaded joints continue to seal until the threads finally release, giving no warning of internal pressure. It is there­fore particularly important not to stand in front of plugs or caps when slackening them.
  • Prior to making a joint, check for thread damage, correct profile and correct engagement. Check with thread gauge if neces­sary.
  • Before applying any thread compound, a threaded joint should be made up dry by hand. For pipe sizes up to 1 1/2 inch NB, ensure that at least 4 to 5 threads engage.
  • Check if there are any restrictions on the use of PTFE tape. There may be a total ban on PTFE tape, or it may be restricted to low pressure non-hazardous duties or just to non-hydro­carbon duties.
    • If using PTFE tape, ensure that you are using Heavy Duty PTFE tape (0.2 mm thick) on the mechanical (as opposed to instrument) threaded joints. Wrap the tape in a clockwise direction, no more than two layers thick and starting at the end of the pipe.
    • If using a liquid thread compound, follow the manufac­turer’s recommendations. Some compounds have restricted use and others require a setting time.
  • Thread tape should not be used on Bull Plugs, i.e. testing points on exchanger nozzles.
  • When wrenching up screwed joints from hand-tight, for pipe sizes up to 1 1/2 inch NB at least 3 more threads should be engaged (at least 7 to 8 threads engaged in total). For larger pipe sizes, thread engagement should be checked from ANSI BI.20.1 - Pipe Threads, General Purpose.
  • It may be necessary to use a thread locking compound on the lock nut of union type joints. This prevents them from loosening due to vibrations.
  • Do not cross threads, force screwed joints together or attempt to mate unmatched threads. A seal will not be made.
  • Do not make a seal by applying excessive PTFE tape to threaded joints. If a seal cannot be made, the threads are prob­ably damaged and new threads should be cut on a fresh section of pipe.
  • PTFE tape should never extend beyond or overhang the first thread on a joint as tape could shred and enter the fluid system.
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