Coupling Pipe Fittings

Classification Of Coupling Pipe Fittings

Threaded Pipe Fittings

Threaded pipe fittings are one of the most common options for securing the connection between two threaded parts. One is a male thread with threads protruding outside the pipe fitting; the other is a female thread with threads inside the fitting. The connection is formed when the male thread is screwed into the female thread and secured by tightening the connection.

Tapered Threads

Tapered threads seal when the male and female threads are drawn together. These fittings are at an angle to the centerline (in contrast, straight threads are parallel to the centerline). Thread sealants or thread tape are needed to fill the gap between the top (peak) and root (valley) of the thread to prevent leaks. Tapered threads are generally suitable for system pressures up to 15,000 psi.

Parallel Threads

Also known as straight threads, they seal because the sides of the threads are drawn together. There is no interference between the sides, crests, and roots of parallel threads, and installing them requires supplementary items, including washers, O-rings, or metal-to-metal contact to form a leak-proof seal. Parallel threads are typically used in applications where system pressure is 5,000 psi or less.

The choice of threaded pipe fittings, first based on the system pressure, choose tapered threads or parallel threads, when the system pressure does not exceed 5,000 psi, straight threads are usually used. Tapered threads are effective when the system pressure reaches 15,000 psi.

Compression Pipe Fittings

Compression pipe fittings are another common choice, using ferrules to form a leak-proof seal. When the nut of the fitting is tightened, the ferrule is compressed between the nut and the receiving fitting. Due to this compression, the ferrule effectively bites into the tube, forming a secure grip and a tight seal. Compression fittings for compression pipe fittings are also easy to assemble and disassemble.

Compression fittings come in two main types: single ferrule and double ferrule. Single ferrule compression fittings utilize a ferrule to create a strong seal. The ferrule is compressed between the nut and the receiving fitting by tightening the nut, creating a secure grip on the tube and ensuring a dependable seal.

Dual-ferrule Compression Fittings

Includes a front ferrule and a back ferrule. Each has its own function but can work in conjunction with the other. The front ferrule creates a seal between the body and the tube, while the back ferrule grips the tube.

Understanding how the back ferrule bites into the tube is important because not all fitting designs are alike. The performance of a compression-fitting tube grip hinges on the effectiveness of the back ferrule in securing the tube.

Some design differences between coupling pipe fittings can affect performance. For example, bite-type ferrules flex during assembly, whether single or dual-ferrule designs. The flex firmly drives the leading edge of the ferrule into the tube, ensuring a secure grip by indenting the tube surface. However, it’s essential to note that any vibration, pulsation, thermal shock, or side loads on the fitting can potentially compromise the minimal contact of a single gripping ferrule.In dynamic fluid systems, there is the potential for damage to the tube or pullout. Fittings with a mechanical grip design with two ferrules provide a more robust grip and seal.

Cone And Thread Fittings

Cone and thread fittings are another widely used fitting type that is suitable for medium and high-pressure applications. For many years, cone and thread fittings have been the standard choice for demanding high-pressure applications.

A cone and thread fitting will have a gland, collar, female port and weep holes that allow technicians to detect leaks and verify proper installation.

Installing cone and thread fittings is complex and requires a cone and thread tool to create the connection, as well as a lubricant to reduce friction during the cutting process. The pipe must be properly tapered and threaded before being connected with the fitting. It is critical to ensure that no burrs, gouges or scratches are introduced during this process. Once the pipe preparation is complete, the collar is threaded onto the pipe and the gland nut is inserted into the fitting body for final tightening. For systems that are subject to frequent shock or vibration, anti-vibration components are recommended to help extend the life of the tubing connection. If done correctly, this can provide long-term reliability.