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General and Details of High-pressure Threaded Fittings(ASME B16.11)
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General and Details of High-pressure Threaded Fittings(ASME B16.11)
* From : * Author : Ma Junqui * AddTime : 2010-11-13 * Clicks : 312
Threaded Fittings general
Threaded joints probably represent the oldest method of joining piping systems. Like Socket Weld fittings, threaded fittings are mainly used for small pipe diameters (Small Bore Piping); generally for piping whose nominal diameter is NPS 2 or smaller.
The dimensional standards for taper pipe threads are given in ASME B1.20.1. That document gives all required dimensions including number of threads per inch, pitch diameter, and normal engagement lengths for all pipe diameters.
·                     Threaded piping is commonly used in low-cost, noncritical applications such as domestic water, fire protection, and industrial cooling water systems.
·                     Threaded fittings are normally made of cast gray or malleable iron, cast brass or bronze, or forged alloy and carbon steel.
·                     They are available in three pressure ratings: 2000lbs, 3000lbs and 6000lbs.
 
NPT Thread
All fittings on this page are described, are provided with NPT thread, ASME B1.20.1. The American National Pipe Thread Tapered, is the best known and most widely used connection where the pipe thread provides both the mechanical joint and the hydraulic seal. NPT has a tapered male and female thread which seals with Teflon® tape or jointing compound.
Dimensions of American Taper Pipe Thread, with Sealant Compound

DN
NPT
Major Diameter
mm
Tapping
Drill Size mm
TPI
Pitch
mm
1/16"
7.895
6.00
27
0.941
1/8"
10.242
8.25
27
0.941
1/4"
13.616
10.70
18
1.411
3/8"
17.055
14.10
18
1.411
1/2"
21.223
17.40
14
1.814
3/4"
26.568
22.60
14
1.814
1"
33.228
28.50
11.5
2.209
1 1/4"
41.985
37.00
11.5
2.209
1 1/2"
48.054
43.50
11.5
2.209
2"
60.092
55.00
11.5
2.209
2 1/2"
72.699
65.50
8
3.175
3"
88.608
81.50
8
3.175
3 1/2"
101.316
94.30
8
3.175
4"
113.973
107.00
8
3.175
5"
141.300
134.384
8
3.175
6"
168.275
161.191
8
3.175
8"
219.075
211.673
8
3.175
10"
273.050
265.311
8
3.175
12"
323.850
315.793
8
3.175
 
 

 
Types of Threaded Fittings by Class and Size

Description
Class Designation
2000
3000
6000
Elbows 45 and 90 degrees
Tees, Crosses, Coupling
Half-Coupling, Cap
1/2 - 4
1/2 - 2
1/2 - 2
1/2 - 4
1/2 - 2
1/2 - 2
1/2 - 4
1/2 - 2
1/2 - 2
Pipe Wall
SCH 80 and XS
SCH 160
XXS

Plugs and Bushings are not identified. They may be used up through Class 6000 NPS
 
Fittings for Threaded Systems
Elbow 90°:This Elbows make 90° changes of direction in the run of pipe.
Tee:This Tee makes 90° branch from the main run of pipe.
Cross:Crosses makes 90° branch from the main run of pipe.
Elbow 45°:This Elbows make 45° changes of direction in the run of pipe.
Full-coupling:Termed Coupling, joins pipe two pipe or to a nipple etc..
Cap (End Cap):Seals the threaded end of pipe.
Half-coupling:The Half Coupling can be directly welded to the run pipe, to make a branch connection.
Square head plug:Seals the threaded end of fitting.
Hex head plug:Seals the threaded end of fitting.
Round head plug:Seals the threaded end of fitting.
Hex head bushing:Can be used to reduce a threaded fitting.
Union (MSS SP-83):Unions are primarily used for maintenance and installation purposes. It is a screwed joint design and it consists of three interconnected pieces. Two internally threads and a centerpiece that draws the ends together when rotated.
 
Advantages and Disadvantages of Threaded Fittings
Advantages
·                     Installation productivity is moderately high, and specialized installation skill requirements are not extensive.
·                     Leakage integrity is good for low-pressure, low-temperature installations where vibration is not encountered.
Disadvantages
·                     Rapid temperature changes may lead to leaks due to differential thermal expansion between the pipe and fittings.
·                     Vibration can result in fatigue failures of screwed pipe joints due to the high stress intensification effects caused by the sharp notches at the base of the threads.
·                     Socket welds are not acceptable in piping systems involving nuclear or radioactive service or corrosive service with solutions which promote stress corrosion cracking or concentration cell action. Generally require butt welds in all pipe sizes with complete weld penetration to the inside of the piping.
·                     In hazardous piping systems threaded connections should be avoided, if possible. Their vulnerability to fatigue damage is significant, especially where exposed threads are subject to corrosion.
 
Note: At the smallest sizes, the amount of wall lost during threading actually equals approximately 55% of the original pipe wall.