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Tube fitting's ferrule gets a grip

Tube fitting's ferrule gets a grip


The top illustration shows a finger-tightened Swagelok fitting prior to engagement with the tube. As the fitting nut advances axially during tighteneing in the lower illustration, radial-gripping force is applied to the tube. The back ferrule drives the front ferrule, which provides the sealing action while the back ferrule provides the gripping action.

Solon, OH -Many leaks in fluid handling applications result from human error. A common problem is the failure to properly follow installation instructions recommended for tube fittings and, instead, go by what "feels" tight. When working with new tubing alloys such as super-duplex steel, which has about twice the Vickers hardness of 316 stainless steel, torquing down a fitting by what feels tight is insufficient for producing an adequate grip. Engineers at Swagelok Co. (Solon, OH) designed a new tube fitting with a patented back ferrule that uses radial force for gripping the tube and ensuring positive sealing on the front ferrule. "We used to harden only the tips of our back ferrules," says Sanjeev Moghe, an engineer and Swagelok's project manager. "But now we harden the entire ferrule for the harder alloy tubes, and that required a change in the ferrule's geometry."

A cross section of the back ferrule's patented geometry resembles a boot (see illustrations). The ferrule sits between the fitting nut and the front ferrule. Tightening the fitting drives the front ferrule between the body of the fitting and the tube, creating the seal. At the same time, the back ferrule hinges radially. Radial hinging re-vectors the axial force. As the angle of the middle of the ferrule becomes steeper, the ferrule's radial grip advances through an increasing load on the ferrule's toe.

The hinging action translates forward motion into radial swaging by forcing the toe of the boot-shaped back ferrule inward to grip the tubing. Installer torque is re-directed from the axial direction (advancement of the fitting nut) to the radial direction for gripping the tube.

For proper swaging and gripping of the tubing the back ferrule must be harder than the tubing, which is achieved through a process called case carburization. The case carburization process involves elevating the temperature of stainless steel and allowing free carbon to diffuse into the ferrule's entire surface. The result is a uniform hard layer on its metal surface. During pull-up, the new case-carburized back ferrule, with its new shape, hinges predictably, engaging the tube sooner in the sequence of tightening.

"Controlled laboratory tests, based on a limited range of 316 tubing samples with varied sizes and wall thicknesses, showed that the new fitting reduced the weighted mean average of assembly torque by 7% relative to the traditional Swagelok fitting for 316 stainless steel," says Pete Williams, Swagelok's chief scientist.

The radial action of the back ferrule isolates and protects the swaged area. It prevents the exposed vibration stress risers typical of bite-type fittings. The engineered hinge also absorbs excess torque inputs.

"The new fitting is more forgiving," says Sing Dang, a mechanical engineer who works at the Detroit Edison nuclear power plant (Newport, MI). He uses the new fittings on tubes leading to the plant's instrumentation and control systems. "We feel there is less possibility of a blow-out," says Dang.

Additional details

Contact: Michael Valentine, Swagelok Co., 29500 Solon Rd., Solon, OH 44139; Tel: (440) 248-4600; Fax: (440) 519-1085; E-mail; [email protected]

TAGS: Materials
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