THREADED JOINTS ON PIPE

Threaded joints are normally used in low-pressure small-bore, nonflammable service, although threaded iron pipe is commonly used in domestic gas piping and threaded joints up to NPS 12 (DN300) have been used in low-pressure liquid service. For quality joints, it is essential to have smooth, clean threads. A proper form for a pipe threading die is shown in Fig. A6.27. Because cut-thread surfaces are somewhat imperfect, thread sealants (pipe dope) and lubricants are often used to ensure a leak-tight joint. Lubricants such as linseed oil or a compound containing powdered zinc or nickel are sufficient to produce a leak-tight joint in well-made threads. Imperfect threads may require white lead or plumber’s tape to provide a good seal. In high-pressure piping where leakage cannot be tolerated, the threaded joints may be seal-welded. Where seal welding is employed, all exposed threads should be covered to prevent cracking in the weld.

Dimensional Standards. 

Dimensional standards for threads are established in ASME Standard B1.20.1. This standard specifies dimensions, tolerances, and gauging for tape and straight pipe threads, including certain special applications. The normal type of pipe joint employs a tapered external and tapered internal thread. But straight pipe threads are used to advantage for certain types of pipe couplings, grease cup, fuel and oil fittings, mechanical joints for fixtures, and conduit and hose couplings.

Pressure-Tight Joints
Pressure-tight joints for low-pressure service are sometimes made with straight internal threads and the American standard taper external threads. The ductility of the coupling enables the straight thread to conform to the taper of the pipe thread. In commercial practice,