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 standard- weight (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. Taper- tapped 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. #Little_PEng