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Iron Pipe Fittings Jointing Using Hemp

Malleable Iron Pipe Fittings Jointing procedure using hemp

The effectiveness and reliability of many popular jointing compounds and sealants reduces as the thread size increases. For example, PTFE is not often recommended for threads above 11/2.

On large thread sizes, up to 4, the traditional method of hemp and paste is often the preferred and most reliable way of obtaining a pressure tight joint. Of course, hemp is not permitted on many installations and a substitute should be used.

 

Method

  1. Ensure both threads are clean, free from oil, swarf and other particles.
  2. Check that the machined pipe thread is clean cut and free from tears and chatter marks.
  3. Check the pipe thread is within the permitted length and diameter tolerances
  4. Using clean, dry hemp, wrap the hemp around the pipe thread in a clockwise direction (when viewing the front of the thread). Start at the front of the thread and work towards the back. Once complete, the crests of the pipe thread should still be visible.
  5. Use a wire brush to brush the hemp in a clockwise direction around the thread. This is an important and essential step because it:
  1. Tightens the hemp and ensures that it is correctly located in the root of the pipe thread
  2. Removes any excess hemp from the pipe thread
  1. Apply the paste evenly along the whole length of the pipe thread
  2. Assemble pipe and fitting and screw together until ‘hand tight’
  3. A pressure tight seal is obtained by further tightening the joint up to the wrenching allowance. This is up to 11/2 turns for sizes up to 11/2 and 3 turns for size 4.

 

Notes

  • the hemp is used as a filler and holds the paste in position during the jointing process
  • when correctly assembled, the seal of a pressure tight threaded joint is made between the metallic faces of the thread flanks.
  • The paste provides lubrication during the tightening process and enables a seal between the metal faces. These faces will appear uneven at a microscopic level. Poor quality of pipe thread cutting and any torn threads will make it very difficult to obtain a seal.
  • Hemp and paste act as a filler to fill the larger gaps between the roots and crests of the pipe threads.
  • After wrenching, a correctly made joint with paste and hemp may be turned back up to 45o while still maintaining a pressure tight joint. This allows easier positioning of fitting outlets and gives a small amount of axial adjustment. Turn back is a feature of taper/parallel (R/Rp) threaded joints.