Straight thread

IP-9.14(c) Typical Threaded Joints Using Straight Threads. 109... 

Pressure design of straight-threaded joints shall be based on calculations consistent with design requirements of this Code. These calculations shall be substantiated by testing in accordance with to-be-determined procedures and protocols. The testing shall consider such factors as assembly and disassembly, cyclic loading, vibration, shock, hydrogen embrittlement, thermal expansion and contraction, and other factors to be determined. 

When the molecules have reached a certain distance from the wall the shear reaches a level at which the molecules dont get elongated any more but where they will align. The direction of the aligned molecules is at a specific angle to the shear profile. This has been shown by HINCH (1967) for molecules behaving as flexible inextensible, nearly straight threads in a shearing flow. 

Recent studies by electron microscopy (L4) have shown that the macromolecules of the gel agarose (a major constituent of agar) exist as long and relatively straight threads. This suggests a gel structure of loosely interwoven, extensively hydrogen-bonded polysaccharide macromolecules. 

An example of leak prevention with a rubber seal in a thread fitting that is gaining popularity is the Society of Automotive Engineers (SAE) straight thread. The SAE straight threads are... 

Figure 1.1 SAE straight-thread fittings - straight threads are mechanical types they do not provide a seal...

Two basic types of threads are used in the United States and Canada straight thread and tapered thread. The straight thread requires use of an 0-ring or gasket to accomplish a gas-tight seal. (See Fig. 9-7.) This type of thread requires less installation torque than a tapered thread and is mostly used with aluminum cylinders. Common sizes currently in use are 0.750-16UNF and 1.125-12UNF. Most commonly used are the... 

Fig. 9-7. Typical straight thread valve inlet connection. Oversize threads...

The handtight plus 3 turns for wrenching identified in both CGA V-1 and NBS Handbook H28, Federal Screw Thread Standards, provides a method for engaging the valve to a steel cylinder that is not affected by manufacturing tolerances on the valve and cylinder threads. [3] This engagement is illustrated by the dimension (L3) in Fig. 8-7. For aluminum cylinders and/or straight threads, consult the manufacturer. 

Except for valves used in liquefied gas service, the inlet hole diameter should be restricted to 0.300 inch (7.6 mm) maximum beyond the length (Li) + (L3). For straight threads, this depth should not exceed the minimum full thread length on the cylinder valve. This recommendation applies to cylinders having a service pressure over 500 psig (3447 kPa) and approximately 125 lb water capacity. 

For further information on cylinder valve inlet connections and other thread types presently in use, that is. Special Gas Taper Threads (SGT) and National Gas Straight Threads (NGS), refer to CGA V-1, American National, Canadian, and Compressed Gas Association Standard for Compressed Gas Cylinder Valve Outlet and Inlet Connections, [1]... 

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