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Machinery's Handbook 27th Edition 1866 DRYSEAL PIPE THREADS American National Standard Dryseal Pipe Threads for Pressure-Tight Joints.— Dryseal pipe threads are based on the USA (American) pipe thread; however, they differ in that they are designed to seal pressure-tight joints without the necessity of using sealing compounds. To accomplish this, some modification of thread form and greater accuracy in manufacture is required. The roots of both the external and internal threads are truncated slightly more than the crests, i.e., roots have wider flats than crests so that metal-to-metal contact occurs at the crests and roots coincident with, or prior to, flank contact. Thus, as the threads are assembled by wrenching, the roots of the threads crush the sharper crests of the mating threads. This sealing action at both major and minor diameters tends to prevent spiral leakage and makes the joints pressure-tight without the necessity of using sealing compounds, provided that the threads are in accordance with standard specifications and tolerances and are not damaged by galling in assembly. The control of crest and root truncation is simplified by the use of properly designed threading tools. Also, it is desirable that both external and internal threads have full thread height for the length of hand engagement. Where not functionally objectionable, the use of a compatible lubricant or sealant is permissible to minimize the possibility of galling. This is desirable in assembling Dryseal pipe threads in refrigeration and other systems to effect a pressure-tight seal. The crest and root of Dryseal pipe threads may be slightly rounded, but are acceptable if they lie within the truncation limits given in Table 6. Table 6. American National Standard Dryseal Pipe Threads—Limits on Crest and Root Truncation ANSI B1.20.3-1976 (R1998) Truncation Height of Minimum Maximum Threads Per Sharp V Thread At Crest At Root At Crest At Root Inch (H) Formula Inch Formula Inch Formula Inch Formula Inch 27 0.03208 0.047p 0.0017 0.094p 0.0035 0.094p 0.0035 0.140p 0.0052 18 0.04811 0.047p 0.0026 0.078p 0.0043 0.078p 0.0043 0.109p 0.0061 14 0.06180 0.036p 0.0026 0.060p 0.0043 0.060p 0.0043 0.085p 0.0061 111 ⁄ 2 0.07531 0.040p 0.0035 0.060p 0.0052 0.060p 0.0052 0.090p 0.0078 8 0.10825 0.042p 0.0052 0.055p 0.0069 0.055p 0.0069 0.076p 0.0095 All dimensions are given in inches. In the formulas, p = pitch. Types of Dryseal Pipe Thread.—American National Standard ANSI B1.20.3-1976 (R1998) covers four types of standard Dryseal pipe threads: NPTF, Dryseal USA (American) Standard Taper Pipe Thread PTF-SAE SHORT, Dryseal SAE Short Taper Pipe Thread NPSF, Dryseal USA (American) Standard Fuel Internal Straight Pipe Thread NPSI, Dryseal USA (American) Standard Intermediate Internal Straight Pipe Thread Table 7. Recommended Limitation of Assembly among the Various Types of Dryseal Threads External Dryseal Thread For Assembly with Internal Dryseal Thread Type Description Type Description 1 NPTF (tapered), int thd 2 1 NPTF (tapered), ext thd a,b PTF-SAE SHORT (tapered), int thd 3 a Pressure-tight joints without the use of a sealant can best be ensured where both components are threaded with NPTF (full length threads), since theoretically interference (sealing) occurs at all threads, but there are two less threads engaged than for NPTF assemblies. When straight internal threads are used, there is interference only at one thread depending on ductility of materials. a,c NPSF (straight), int thd 4a,c,d NPSI (straight), int thd 2a,e 4 NPSI (straight), int thd PTF-SAE SHORT (tapered) ext thd 1 NPTF (tapered), int thd Copyright 2004, Industrial Press, Inc., New York, NY
DRYSEAL PIPE THREADS 1867 b PTF-SAE SHORT internal threads are primarily intended for assembly with type 1-NPTF external threads. They are not designed for, and at extreme tolerance limits may not assemble with, type 2-PTF- SAE SHORT external threads. c There is no external straight Dryseal thread. d NPSI internal threads are primarily intended for assembly with type 2-PTF-SAE SHORT external threads but will also assemble with full length type 1 NPTF external threads. e PTF-SAE SHORT external threads are primarily intended for assembly with type 4-NPSI internal threads but can also be used with type 1-NPTF internal threads. They are not designed for, and at extreme tolerance limits may not assemble with, type 2-PTF-SAE SHORT internal threads or type 3- NPSF internal threads. Machinery's Handbook 27th Edition An assembly with straight internal pipe threads and taper external pipe threads is frequently more advantageous than an all taper thread assembly, particularly in automotive and other allied industries where economy and rapid production are major considerations. Dryseal threads are not used in assemblies in which both components have straight pipe threads. NPTF Threads: This type applies to both external and internal threads and is suitable for pipe joints in practically every type of service. Of all Dryseal pipe threads, NPTF external and internal threads mated are generally conceded to be superior for strength and seal since they have the longest length of thread and, theoretically, interference (sealing) occurs at every engaged thread root and crest. Use of tapered internal threads, such as NPTF or PTF- SAE SHORT in hard or brittle materials having thin sections will minimize the possibility of fracture. There are two classes of NTPF threads. Class 1 threads are made to interfere (seal) at root and crest when mated, but inspection of crest and root truncation is not required. Consequently, Class 1 threads are intended for applications where close control of tooling is required for conformance of truncation or where sealing is accomplished by means of a sealant applied to the threads. Class 2 threads are theoretically identical to those made to Class 1, however, inspection of root and crest truncation is required. Consequently, where a sealant is not used, there is more assurance of a pressure-tight seal for Class 2 threads than for Class 1 threads. PTF-SAE SHORT Threads: External threads of this type conform in all respects with NPTF threads except that the thread length has been shortened by eliminating one thread from the small (entering) end. These threads are designed for applications where clearance is not sufficient for the full length of the NPTF threads or for economy of material where the full thread length is not necessary. Internal threads of this type conform in all respects with NPTF threads, except that the thread length has been shortened by eliminating one thread from the large (entry) end. These threads are designed for thin materials where thickness is not sufficient for the full thread length of the NPTF threads or for economy in tapping where the full thread length is not necessary. Pressure-tight joints without the use of lubricant or sealer can best be ensured where mating components are both threaded with NPTF threads. This should be considered before specifying PTF-SAE SHORT external or internal threads. NPSF Threads: Threads of this type are straight (cylindrical) instead of tapered and are internal only. They are more economical to produce than tapered internal threads, but when assembled do not offer as strong a guarantee of sealing since root and crest interference will not occur for all threads. NPSF threads are generally used with soft or ductile materials which will tend to adjust at assembly to the taper of external threads, but may be used in hard or brittle materials where the section is thick. NPSI Threads: Threads of this type are straight (cylindrical) instead of tapered, are internal only and are slightly larger in diameter than NPSF threads but have the same tolerance and thread length. They are more economical to produce than tapered threads and may be used in hard or brittle materials where the section is thick or where there is little expansion at assembly with external taper threads. As with NPSF threads, NPSI threads when assembled do not offer as strong a guarantee of sealing as do tapered internal threads. Copyright 2004, Industrial Press, Inc., New York, NY
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