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Dryseal pipe threads are based on the USA (American) pipe thread, however, they differ from the USA (American) pipe thread in that they are designed to seal pressuretight 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, see Figure 1. 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 the major and minor diameters tends to prevent spiral leakage and makes the joints pressuretight without the necessity of using sealing compounds, provided that the mating 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 L1 length. However, where not functionally objectionable, the use of a compatible lubricant or sealant may be used to minimize the possibility of galling. This is desirable in assembling Dryseal pipe threads in refrigeration and other systems to effect a pressuretight seal.

In order to obtain a pressure tight seal using Dryseal pipe threads without a sealer, it is necessary to hold crest and root truncation of both internal and external threads within the limits specified. Unless this is done by use of threading tools with the crest and root truncation controlled so as to assure reproduction on the product threads, it is necessary to use a system of measuring or a system of gaging and measuring to determine conformance.

Even without truncation gages, the standard practice of using two separate thread gages for Dryseal pipe threads, each with a difference in truncation, length of engagement and point of engagement, provides a more detailed check of angle, lead and taper deviations than is required for non-dryseal taper pipe threads.

One method of checking crest truncation is by using 6 step plain gages. It should be recognized that this method may give misleading results in that the crest truncation of the product threads is always less (flat narrower) than that indicated by the position of the gages, the degree of inconsistency depending on the angle, lead and taper deviations present in the product thread.

Another method employs both crest and root truncation check gages. The root check gage is made with a thread form having an included angle of 50 degrees and with a smaller gage crest flat than the root flat to be checked. The major diameter of this gage is controlled in relation to 6 gaging steps in a similar manner to the plain 6 step crest check gage. Like the crest check gage, the results may be some-what misleading except that in the case of the root check gage, the root truncation of the product thread is always more (flat wider) than is indicated by the position of the gages.

Unless lead, angle and taper of product threads are very well controlled, use of 6 step crest and root check gages will result in product threads with narrower crest flats and wider root flats than envisioned by the dryseal tolerances. Use of such a gaging system could result in rejection of threads which would actually conform to the dryseal tolerances specified. The only completely reliable referee method for determining whether crest and root truncation has been held within tolerance is direct measurement of the external thread flats and measurement of these flats on a cast or section of the internal thread.

This standard covers two classes of NPTF Dryseal pipe threads; Class 1 and Class 2. The classes differ only in inspection requirements, with Class 1 threads exactly those standardized in ANSI B2.2-1968 (inspection of root and crest truncation not specified). Class 2 threads are identical to Class 1 threads except that inspection of root and crest truncation is required. For Class 2 threads, direct measurement of crest and root truncation is the referee method. These classes do not apply to other than NPTF Dryseal pipe threads.

All dimensions in this standard including all tables are in inches unless otherwise noted.