- 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.