Clsoing the Gap on Hydraulic Leaks

June 15, 2000
Environmental, cost, and safety issues are making leak-free hydraulics critical.

Dick Medvick
Senior Engineer
Swagelok Co.
Solon, Ohio

Two-ferrule tube fittings can be installed with a hand wrench and maintain a leak-free seal beyond the burst strength of the tubing. This tube burst in a pressure test at 13,400 psi and the fitting showed no damage. The 316 stainless-steel tube has a 3 /8-in. OD, 0.38-in. wall thickness and is rated to a working pressure of 3,300 psi.

Swagelok two-ferrule tube fittings create a metal-to-metal seal that stands up to a wide variety of fluids under high pressure. The fittings are manufactured in various materials, including 316 stainless steel.

This shows the components and principle of how two-ferrule tube fittings operate. As the fitting nut is tightened, the front ferrule presses into the fitting body and tube to make a leak-free primary seal. The rear ferrule hinges downward, swages, and grips the tube. The nut only needs to be tightened through 1 1 /4 turns to create a leak-proof metal-to-metal seal.

It used to be common to walk around hydraulic equipment and notice a leak here and there or a drip pan underneath a machine to catch stray oil. But stricter regulations on environmental protection and safety in the workplace, as well as tighter cost controls, have made the hydraulics industry painfully aware that it can no longer look the other way around leaks.

The goals of cleaner machinery and lower-cost operations have led to several developments within the industry, as well as competition from other technologies. For instance, manufacturers of electromechanical power-transmission components have touted high-accuracy equipment "without messy hydraulics." But the hydraulics industry has proposed several solutions of its own.

One way to solve the problems is by changing fluids. Some manufacturers are using oils that are more environ-mentally friendly than previous ones. There has also been a recent surge of interest in water hydraulics.

The concern about leaks is also a hot topic among seal and fitting manufacturers. The companies have developed a variety of solutions, many of which improve upon existing technologies, such as flare and compression fittings. Designers have developed elastomeric O-ring seals that install at the interface of mating metal fittings. Another type, called two-ferrule tube fittings, provides a leak-proof metal-to-metal seal in industrial applications without adding components, such as O-rings.

Two-ferrule fittings have been used successfully in instrumentation applications for over 50 years. The fittings reliably convey dangerous fluids in critical applications where leaks would be disastrous. However, even though these fittings are widely used in instrumentation, designers don't realize that two-ferrule fittings can also be used in industrial applications. The fittings are available to fit tubing ODs up to 2 in. and offer leak-free performance beyond the burst strength of the tubing.

Another misunderstanding is that two-ferrule fittings are prohibitively expensive for many industrial applications, where cost is often the deciding factor. Although the fittings may have a higher initial cost than other options, their total cost is low because they are inexpensive to install. Unlike other fittings, they do not require expensive flaring, flanging, welding, or brazing. Two-ferrule fittings are installed by simply tightening a nut 1 1 /4 turns past finger tight. This can be done by hand using an ordinary wrench. And the fittings can be disassembled and reconnected many times.

Two-ferrule fittings operate in the following manner. As the fitting nut is tightened, the front ferrule makes a leak-free primary seal on the tube and fitting body. The rear ferrule hinges downward, swages, and grips the tube. The sealing and swaging action of the fittings creates a metal-to-metal seal that performs leak free over a wide range of fluid viscosity, pressure, shock, and vibration.

Several alternatives to two-ferrule fittings may work in some applications, but they have limitations. Important characteristics of seals and fittings are fluid compatibility, reliability, and ease of installation. Most fittings or seals meet some of the criteria, but few devices perform well in all aspects.

Metal-to-metal seals are advantageous because they withstand a variety of fluids. Two examples that traditionally have been used in hydraulics are 37° flare fittings and compression fittings. 37° flare fittings have a flared end that presses onto a mating part until fluid cannot get past the interface. This creates a metal-to-metal seal. Compression fittings squeeze a metal ring between a nut and a threaded fitting. But reliability is often a problem with these metal-to-metal seals. For instance, sudden shocks or long-term vibration can cause them to fail.

Manufacturers often suggest O-ring face seal fittings to address the shortcomings of traditional metal-to-metal seals. The seals basically provide a compressive elastomer inter-face between two metal parts.

One problem with O-ring face seals, however, is that they are relatively tedious to install and the O-rings are prone to falling out during installation. To solve this problem, manufacturers developed half-dovetail designs with a lip to hold in the O-ring. This lip can be difficult to machine and can make controlling squeeze on the elastomer more difficult.

Unfortunately, O-ring face seals still have shortcomings. For instance, the elastomer is subject to fluid-compatibility problems. Hydraulic fluids are changing to address environmental and safety concerns and to match the precision of electromechanical devices. But these fluids are not always compatible with elastomers in the seals. Although O-ring manufacturers may develop seals that are compatible with new fluids, the fluids change frequently and compatibility problems continue to crop up.

Compatibility is also a problem in nonhydraulic applications that use different fluids for multiple tasks. O-ring elastomers are not compatible with all fluids. In spray-painting equipment, for instance, the line needs to carry paint and then withstand cleaning solvent. Many O-ring elastomers are compatible with one fluid but not the other.

Chemical compatibility isn't always the only problem. For instance, some companies solve environmental problems by using water hydraulics instead of oil. Unfortunately, water has a lower viscosity than most oils used in fluid power, which presents more sealing challenges.

The fundamental relation-ship between viscosity and sealing is that as viscosity drops, fluids become more difficult to seal. Water has a relatively low viscosity and is more difficult to seal than fluid-power oils. Corrosion is another concern with water hydraulics. Although oils don't corrode metals, water hydraulics often require stainless-steel fittings.

The hydraulics industry must solve its leakage problems. This is critical to its survival. Two-ferrule fittings are a promising option in industrial applications because they withstand a variety of fluids without concern for compatibility. And they are a practical solution because the fittings are easier and less expensive to install than many other fittings.

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