Machinery's Handbook, 31st Edition
Dryseal Pipe Threads 2095 American National Standard Dryseal Pipe Threads for Pressure-Tight Joints.— Dryseal pipe threads are based on the US (American) pipe thread; however, they differ in that they are designed to seal pressure-tight joints without the necessity of using sealing com - pounds. 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 specifica - tions 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 as - sembling 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 accept - able 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/ASME B1.20.3-1976 (R2018)
Truncation
Height of Sharp V Thread ( H ) 0.03208 0.04811 0.06180 0.07531 0.10825
Minimum
Maximum
Threads Per Inch
At Crest
At Root
At Crest
At Root
Formula 0.047 p 0.047 p 0.036 p 0.040 p 0.042 p
Inch Formula 0.0017 0.094 p 0.0026 0.078 p 0.0026 0.060 p 0.0035 0.060 p 0.0052 0.055 p
Inch Formula
Inch
Formula
Inch
27 18 14
0.0035 0.094 p 0.0043 0.078 p 0.0043 0.060 p 0.0052 0.060 p 0.0069 0.055 p
0.0035 0.140 p 0.0043 0.109 p 0.0043 0.085 p 0.0052 0.090 p 0.0069 0.076 p
0.0052 0.0061 0.0061 0.0078 0.0095
11 1 ∕ 2
8
All dimensions are given in inches. In the formulas, p = pitch. Types of Dryseal Pipe Thread.— American National Standard ANSI/ASME B1.20.3-1976 (R2018) covers four types of standard Dryseal pipe threads: NPTF, Dryseal US (American) Standard Taper Pipe Thread PTF-SAE SHORT, Dryseal SAE Short Taper Pipe Thread NPSF, Dryseal US (American) Standard Fuel Internal Straight Pipe Thread NPSI, Dryseal US (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 PTF-SAE SHORT (tapered), int thd
2 a , b 3 a , c
1 NPTF (tapered), ext thd
NPSF (straight), int thd 4 a , c , d NPSI (straight), int thd
NPSI (straight), int thd
2 a , e PTF-SAE SHORT (tapered) ext thd 4 NPTF (tapered), int thd 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. 1
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