What are Tube Fittings?

If you ever plan to connect your instrumentation to your process, more than likely you will need to use tubing rather than a pipe. To make the connection between your process piping and your tubing, you will need to use a tube fitting, which is a type of compression fitting.

How much do you know about these important components? Find out how tube fittings work and how to get the most from the fittings in your system.

Tube Fittings Basics

A tube fitting is a type of compression fitting. Tube fittings originated in the 1950s as a replacement for flare-type fittings. With a flared fitting, you need a special flaring tool to shape the tubing in order to create the seal when the fitting is tightened. Flared fittings
are difficult to make and are certainly more time-consuming. This is especially true for applications in the field.

Tube fittings consist of a body, front ferrule, back ferrule, and nut. In a traditional 2 ferrule tube fitting the front ferrule is designed to drive forward into the body of the fitting when the fitting is “made up” or tightened. This creates the process seal between the
body and the tubing. The back or rear ferrule is smaller and designed to “bite” the tubing providing some stabilization to the entire connection. Single ferrule fittings are designed a bit differently, with a single ferrule doing the job of both the front and rear ferrules in a 2-ferrule fitting. We will discuss these in more detail shortly.

The body of the tube fitting can have various types of end connections to accommodate different piping configurations. Thread x Tube or Tube x Tube connections are common, but special threads with O-Ring seals and metal seals x Tube are used in various industries as well.

Where to Use Tube Fittings

Most major fluid lines in any processing facility are constructed using Pipe. When attaching any mechanical or electrical measuring device to the process using only piping components can be expensive and lead to multiple leak paths. That’s where tubing fits in.

Tubing allows a technician several advantages over piping components alone. Tubing can be bent to locate equipment at exact locations or bent to allow for connection to a tap location that is perhaps difficult to access. Tubing can be easily cut to exact lengths required and is easily connected to other piping or devices by using a tube fitting.

Tube fittings are very common and are available in a wide range of materials. Most process piping is made of steel, so 316SS is a common tubing material of construction. However, other materials such as Hastelloy, Steel, Titanium, and Aluminum are also available.

How Does a Tube Fitting Work?

The original design for a compression tube fitting was a double ferrule design. Since the early 1980s, other types have entered the market. The basic working principles are the same, but each type has some distinguishing features.

Double-Ferrule Design

To make up a tube fitting, insert the tubing into the body of the fitting until it reaches the fitting shoulder. The shoulder in the rear ferrule bites into the tubing axially around its full diameter. This gives you a large surface area used to stabilize the connection.

When you tighten the nut, the front ferrule drives down into the body of the fitting. The angled shape of the body compresses the end of the ferrule onto the outer diameter of the tubing. The action of the ferrule against the body and tubing simultaneously forms a tight seal.

Single-Ferrule Design

A single-ferrule tube fitting works much like a double-ferrule fitting. However, a single- ferrule compression fitting has a single ferrule that does the work of both ferrules in a double ferrule design. Single-ferrule fittings are intended to provide better performance
in applications with higher levels of vibration. Many major manufacturers offer single and double ferrule designs. You can choose the
style you prefer. We suggest consulting with a person knowledgeable about the entire tube fitting market before making a decision.

Other Design Features

Several other features can be available depending on the manufacturer. Compression fittings can have elongated front or rear ferrules. The fitting nut can have different lubrication methods incorporated into it. You’ll also find different methods of hardening the rear ferrule. The rear ferrule needs to be especially hard so it bites the tubing firmly.

Pros and Cons of Compression Fittings

Compression tube fittings have many advantages. You don’t need special tools to make up a compression tube fitting. Installing tube fittings is simple and fast. You can disassemble or partially remove them for maintenance. You can break and remake the joint without damaging its integrity.

Tube fittings also have a few disadvantages. Larger sized tubing and tube fittings (typically ¾” and above) require special tools to bend the tube and tighten the fittings. Ferrules can be lost easily if the nut is removed accidentally on a new fitting.

Can I Mix and Match Tube Fitting Components?

Many manufacturers offer different design features built into their tube fittings. You may wonder if you can use the ferrules from one manufacturer with the body and nut from another manufacturer, or if parts are interchangeable between manufacturers.

Usually, you should avoid this kind of mixing and matching. It can lead to leaks and other safety issues. In fact, many manufacturers specify that you should not interchange or intermix their components. That said, if you have a clear understanding of the fittings you are using and their design characteristics, you will be able to determine if components from an alternative manufacturer are appropriate. We suggest discussing
your options with a tube fitting professional.

Other Considerations

Following best practices when making up a tube fitting helps ensure you get the best possible performance. First, use a tubing cutter with a sharp wheel. This gives you a straighter cut. Deburring the tubing is easier too. Deburring the tubing is a critical step in making up the fitting. You should debur the interior and exterior of the tube before you insert it into the body. This step is important because the tubing needs to rest on the shoulder inside the fitting. Burrs can prevent this from happening, which will compromise the seal. When available use a go/no-go gauge to check if you made up the fitting correctly. You want the gauge to fit snugly between the body and the nut. If it’s loose, you may not have tightened the nut enough. If the gauge won’t fit, you probably tightened the nut too much and over made the fitting. When you take a tube fitting apart and remake it, the go/no-go gauge won’t work. Use caution when you reapply pressure to the line to be sure that it doesn’t leak. The manufacturer will typically specify how many times you can remake a fitting. A good rule of thumb is to replace the compression fitting after you’ve remade it 4 times.

Getting the Most Out of Your Tube Fittings

Tube fittings are a critical component of most process systems. Whether you need to replace a component or redesign your system Sagacity All-Stream Fabrication Engineering has what you’re looking for. We have tube fittings available for immediate delivery. Our service technicians are ready to help keep your facility running smoothly and safely. Contact us for more information, and let’s get started today.

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