PIPE THREAD SIZES - MAKING SENSE OF PIPE THREADS
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The most common global pipe thread forms are:
Pipe Thread Sizes Pipe thread sizes are based on an inside diameter (ID) or flow size. For example, “1/2–14 NPT” identifies a pipe thread with a nominal inside diameter of 1/2 inch and 14 threads to the inch, made according to the NPT standard. If “LH” is added, the pipe has a left hand thread.
The word “tapered” in several of the above names points to the big difference between many pipe threads and those on bolts and screws. Many pipe threads must make not only a mechanical joint but also a leakproof hydraulic seal. This is accomplished by the tapered thread form of the male matching the thread form of the female tapered thread and the use of pipe sealant to fill any voids between the two threads which could cause a spiral leak. The bottoms of the threads aren't on a cylinder, but a cone; they taper. The taper is 1/16 inch in an inch, which is the same as 3/4 inch in a foot. Because of the taper, a pipe thread can only screw into a fitting a certain distance before it jams.
The standard specifies this distance as the length of hand tight engagement, the distance the pipe thread can be screwed in by hand. It also specifies another distance – the effective thread, this is the length of the thread which makes the seal on a conventional machined pipe thread. For workers, instead of these distances, it is more convenient to know how many turns to make by hand and how many with a wrench. A simple rule of thumb for installing tapered pipe threads, both metal and plastic, is finger tight plus one to two turns with a wrench. Torque installation values can be determined per application, but due to the variations involved in pipe joints such as disimiliar materials of male and female threads, type of sealants used, and internal variations in product wall thickness, a standard torque specification cannot be generically applied.
The following table shows the distances and number of turns called for in the standard. A tolerance of plus or minus one turn is allowed, and in practice threads are often routinely cut shorter than the standard specifies. All dimensions are in inches.
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American Standard Taper Pipe External Thread Nominal size Actual OD Threads per inch Length of engagement (tightened by hand) Length of effective thread 1/8 0.407 27 0.124 ≈ 3.3 turns 0.260 1⁄4 0.546 18 0.172 ≈ 3.1 turns 0.401 3/8 0.681 18 0.184 ≈ 3.3 turns 0.408 1/2 0.850 14 0.248 ≈ 3.4 turns 0.534 3/4 1.060 14 0.267 ≈ 3.7 turns 0.546 1 1.327 11.5 0.313 ≈ 3.6 turns 0.682
Taper/Parallel Threaded Joints Despite the standards created to maintain uniform fittings, tapered pipe threads are inexact and during the course of use and repair the threads can become damaged and susceptible to leakage. The area where the crest and the root of the thread meet can form a spiral leak path no amount of tightening will eliminate. A pressure tight joint is achieved by the compression in the threads resulting from tightening. This compression and sealing occurs in the first few turns of the internal thread. As wrenching takes place, material from both the male and female threads deform into each other. This ensures full thread contact which minimizes spiral leakages.
Variations between injection-molded plastic and machined metal thread forms can occur due to different manufacturing processes. Pipe threads were originally designed as machined thread forms. With the use of thermoplastics and plastic injection molding in the manufacture of plastic pipe thread forms, mold shrinkage and plastic sink make it difficult to insure leak free joints. For this reason, the use of a Teflon based sealant is recommended on all plastic pipe threads. The most common form of sealant is Teflon tape wrapped 2 to 3 turns around the male thread before assembly. Liquid Teflon based sealants are also used successfully to ensure a pressure tight seal. It is always important to use care when applying sealants to avoid introducing the sealant material into the system flow path.
The following sections show examples of how different threads are used and issues that can arise in attempting to create a leak free connection.