Pipe and pipe components such as fittings are typically joined together (fitted) by threads which are cut on the OD (outside diameter) of the pipe.
Pipe threads are of two basic types, namely
- Tapered Threads
- Straight (or Parallel) Threads
The Americans call the latter "straight," whereas the British call them "parallel." For all intents and purposes, Straight threads are Parallel, and Parallel threads are Straight.
This is the default type of thread on a pipe. If it's a "pipe thread," it's most likely a tapered pipe thread, unless specified otherwise.
The tapered thread was an ingenious invention: they engage easily, go in a number of turns, and then bind (the male thread "expanding" out against the female thread) so as to create a fluid-tight seal.
Traditionally, tapered threads require a sealing medium such as "pipe dope" in order to actually become fluid-tight. More modern systems, such as the "dryseal" thread (see NPTF) involve very tight manufacturing tolerances such that the male and female threads fit together so closely that no sealant is required.
I don't think I'm alone, however, in that I don't buy it—I lack the faith in dryseal threads to believe that they will not leak. Therefore, I almost always use a sealant, regardless of whether the thread is NPTF or just NPT. Half the time you never really know whether both fittings are NPTF anyway, so why not just add some sealant and be done with it? Who needs the headache of a leak, when it's so easy to avoid? —$.02
There are, however, cases where the use of a sealant should be avoided, due to incompatibility with the working fluid, and the subsequent contamination issues that might result from its use. These cases are typically found in the manufacture of new systems or devices, where careful control of all components assures thread compatibility, etc. But if I'm in the field, replacing a fuel pump on a Crummy, I'm gonna dope the thing up, that's all.
Straight (Parallel) Threads
Straight or parallel threads do not seal on the thread. Therefore, these threads require an additional accessory in order to effect a seal. Said accessory is usually an o-ring or a gasket. The most familiar example is a garden hose, with which we are all familiar. Who hasn't experienced the old, dried-up, cracked and malfunctioned hose gasket? The water just runs down your wrist, or sprays out onto your arms and legs. This experience teaches us that straight threads don't seal.
We define two basic configurations of straight thread sealing strategies, namely
- Base Seat Thread
- Tip Seat Thread
These types are referenced to the male fitment. In other words, the seal is affected either at the base of the male threads, or at the tip of the male threads. The garden hose is an example of a Tip Seat Thread fitment. An example of a Base Seat Thread fitment is the ORB (O-Ring Boss) which is an SAE specification.