Ultra high purity dual ferrule stainless steel connectors

In certain industries such as the semiconductor industry, processes involving high purity reactive gases require the use of stainless steel piping and connectors. This article focuses on the highly specialized ultra high purity connectors and the sophisticated manufacturing capabilities that are required to make them.

These high purity stainless steel connectors are highly engineered compression fittings. A standard compression fitting for attaching tubing consists of a connector body, a conical ferrule or “collet”, which fits over the tubing and nut that connects to the body and compresses the ferrule into the body and onto the pipe to form a seal. For several reasons this kind of fitting is insufficient for the needs of high purity processing, and in these cases a two ferrule design, manufactured from hardened high purity stainless steel is typically used.

In a two ferrule tube fitting system, the two ferrules are designed to both fit over the pipe and to fit inside each other. As the nut is tightened the back ferrule pushes against the front ferrule, which is driven against the fitting body to create the seal. The nut is further tightened which springs loads the seal and causes the back ferrule to deform in such a way that it grips the piping to keep the connection firmly in place.

Double-Ferrule Compression Tube Fittings provide leak-proof, torque-free seals at all tubing connections. They eliminate potentially hazardous and expensive leaks in instrumentation, process, pneumatic, hydraulic, gas and other tubing systems

The components of dual ferrule connectors

Every stainless steel dual ferrule connector consists of four components: the connector body, the front ferrules, the rear ferrules and tightening nuts. When installed the five piece connection (including the pipe) provides a solid leak free joint.

The body of the connector

There are several machined sections within the body of the connector, all of which are important to correct function. Externally the body consists of a number of hexagonal sections sized to take a standard wrench, with a standard thread section next to it. The number of sections will be determined by the number of pipes that are going to be connected using the connector.

Internally each section is machined as a conical section, smoothed and hardened to match the outside of the front ferrule. The conical section is followed by a section that is as wide as the outside diameter of the pipe, until it reaches a “shoulder”, where the end of the piping sits. The rest of the body has an inside diameter the same as the inside of the pipe.

For applications in the semiconductor industry, the body will typically be machined out of 316L stainless steel, will be citric passivated to prevent corrosion.

The front ferrule

This conical sectional shaped component that is precisely machined and finished on the outside to match the inside of body section. The front or narrow end of the cone is designed to match a standard pipe diameter.

As the nut is tightened onto the connection body the front ferrule moves forward to burnish and seal with the fitting body, and create the primary tubing seal.

The back ferrule

The back ferrule is considerably shorter that the front ferrule and is manufactured so that as it slides it moves the front ferrule forward to form the seal against the body of the connector, and then as the nut tightened further the front of the back ferrule grips or “swages” the tube – the back ferrule has to be harder than the tubing to do this. At the same time the tightening force causes the front of the ferrule to distort inward to increase the grip on the tube and improve the seal.

The fitting nut

To complete the connector the stainless steel tube is simply inserted into the fitting until it reaches the shoulder within the body, then the nut is tightened down onto the screw thread. The tightening of the nut provides the force that presses both ferrules, and creates the leak tight seal.

Manufacturing dual ferrule connectors

These connectors are typically manufactured from forged and machined stainless steel and require a combination of several high specialized capabilities including citric passivation, electropolishing and abrasive flow machining

There are many different versions that this type of connector that are manufactured including elbows, t-junctions, and multiple connections of 4 or more pipes. The connectors vary in diameter from 1/16” to 2” and can provide a leak proof seal up to 6000psi

The double ferrule Tube Fitting relies both on the design and the capabilities of the manufacturer to meet exacting standards for the component manufacture. Inside the fitting it is an axial movement along the tube that creates the joints, not rotational pressure. This axial movement prevents transmission of any torque from a fitting to the tubing. Since there is no initial strain in the tubing, the making of the joint does not weaken the tubing. However if the components of the fitting are not manufactured to very high standards then the connect will not function satisfactorily

Summary

Double-Ferrule Tube Fittings are easy to install and require no special tools. They are reusable several times and can withstand heavy impulse and vibration in both vacuum and pressure systems.

WS Associates represents experienced manufacturers of double ferrule tube fittings. Those manufacturers meet the exacting standards of the semiconductor industry, both in the US and worldwide. They work with ultra-high purity metals used in the semiconductor and medical instrumentation industries.