1. Threaded Fittings
Threaded pipe joints are one of the most common options for fixing the connection of two threaded parts. One is the external thread, with the thread protruding outside the pipe fitting; the other is the internal thread with the thread inside the fitting. A connection is formed when the external thread is screwed into the internal thread and fixed by tightening the connection. There are two types of threaded joints:
- Tapered thread vs Parallel thread
When the external and internal threads are pulled together, the tapered thread seals. These fittings are at an angle to the centerline (in contrast, the straight thread is parallel to the centerline). Thread sealant or thread tape needs to be used to fill the gap between the top (crest) and the root (valley) of the thread to prevent leakage. Tapered threads are usually suitable for system pressures up to 15,000 psi.
Parallel threads, also known as straight threads, are sealed as the sides of the thread are pulled together. There is no interference between the sides, crest and root of the parallel thread. Installing them requires supplementary items, including washers, O-rings or metal-to-metal contact to form a leak-proof seal. Parallel threads are usually used in applications where the system pressure is 5000 psi or lower.
The choice of threaded pipe joints first selects tapered threads or parallel threads based on the system pressure. When the system pressure does not exceed 5000 psi, straight threads are usually used. Tapered threads are effective when the system pressure reaches 15,000 psi.
When not limited by pressure requirements, the choice between parallel and tapered threaded connections usually comes down to simple user preference.
2. Compression Fittings
Compression pipe joints are another common option and use ferrules to form a leak-proof seal. When the nut of the joint is tightened, the ferrule is compressed between the nut and the receiving joint. Due to this compression, the ferrule effectively bites into the pipe, thus forming a firm grip and a tight seal. The compression fittings of compression pipe joints are also easy to assemble and disassemble.
There are two main types of compression joints: single ferrule and double ferrule.
Single ferrule compression joint: Contains one ferrule to form a seal. When the nut is tightened, the ferrule is compressed between the nut and the receiving joint. Then the ferrule bites into the pipe to form the pipe clamp and the main body sealing effect.
Double ferrule compression joint: Includes the front ferrule and the rear ferrule. Each has its own function but can work with the other. The front ferrule forms a seal between the valve body and the pipe, while the rear ferrule clamps the pipe.
It is important to understand how the rear ferrule bites into the pipe because not all pipe joint designs are similar. The reliability of the compression joint pipe clamp is related to the degree to which the rear ferrule bites into the pipe.
There are some design differences among compression pipe joints, and these differences may affect performance. For example, snap-in ferrules, whether in a single or double ferrule design, will bend during assembly. The bending drives the leading edge of the ferrule into the pipe to indent the pipe surface, thereby forming its grip.
However, any vibration, pulsation, thermal shock or lateral load on the fitting may damage the minimum contact of a single clamping ferrule. In dynamic fluid systems, there may be the possibility of damaging the pipe or pulling it out. Fittings with a mechanical grip design with two ferrules can provide a stronger grip and seal.
