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,

straight-tapped couplings are furnished for standardweight (schedule 40) pipe NPS 2 (DN 50) and smaller. If taper-tapped couplings are required for standard-weight pipe sizes NPS 2 (DN 50) and smaller, line pipe in accordance with API 5L should be ordered. The thread lengths should be in accordance with the American Standard for Pipe Threads, ASME B1.20.1. Tapertapped couplings are furnished on extra-strong (schedule 80) pipe in all sizes and on standard-weight NPS 21⁄₂ (DN 65) and larger.

Dry-seal pipe threads machined in accordance with ASME B1.20.3 are also employed for pressure-tight joints, particularly where the presence of a lubricant or sealer would contaminate the flow medium. Threads are similar to the pipe threads covered by ASME B1.20.1; the essential difference is that, in dry-seal pipe threads, the truncation of the crest and root is controlled to ensure metal-to-metal contact coincident with or prior to flank contact, thus eliminating spiral leakage paths. Dry-seal pipe threads are used in refrigerant systems and for fuel and hydraulic control lines in aircraft, automotive, and marine service. Thread sizes up to NPS 3 (DN 75) are covered by ASME B1.20.3.

Hose Nipples and Couplings.

Hose coupling joints are ordinarily used with a gasket and made with straight internal and external loose-fitting threads. There are several standards of hose threads having various diameters and pitches, one of which is based on the American standard pipe thread. With this thread series, it is possible to join small hose sizes 1/2 to 2 in, inclusive, to ends of standard pipe having American standard external taper pipe threads, by using a gasket to seal the joint. ASME B1.20.7 applies to the threaded parts of hose couplings, valves, nozzles, and all other fittings used in direct connection with hose intended for fire protection or for domestic and industrial general services. However, fire hose coupling dimensions and threads vary with fire districts, and the local fire authority must be consulted. Figure A2.15 illustrates a typical fire hose coupling.

FIGURE A2.15 Typical fire hose coupling

I think that would be enough for now, and in the next post we will continue this Threaded Joints on Pipe System only on Piping & Fabrication.