Industrial settings with strenuous applications, such as thermal cycling and vibration, are recognised as some of the toughest environments for tube fittings. In all cases, such applications demand highly precise, leak-free technology to ensure protection from vibration and thermal cycling. For example, in power generation, nuclear and chemical plants, worker and atmospheric safety is business-critical.
Engineers face numerous options when choosing tube fittings for instrumentation, process and control systems and equipment. In most cases, buyers need a solution that will minimise risk, prevent leakage and provide robust value for money.
In instrumentation installations such as impulse pipework, typically a three-piece (single ferrule) or four-piece (double ferrule) design is preferred. While both types of fittings have their own merits, the four-piece fittings are slightly better known in the market. But in some cases, there are definite advantages for choosing a three-piece, single ferrule design.
With just three pieces required - a nut, body and single ferrule - this type of fitting is easy to install. The ferrule design also provides a strong anti-vibration hold on the tube.
Single ferrule fittings typically offer six core benefits:
Many engineers constructing fluid or gas handling systems aim to eliminate potential leak paths. This approach also applies to the design of tube fittings.
With single ferrule fittings such as CPI™, only two potential leak paths need sealing. But double ferrule fittings have three or more paths to seal, depending on the fitting design and composition.
With double ferrule designs, the front ferrule contacts the body seat over the entire surface. This causes the end load to be distributed over a wide area.
With Parker’s CPI™ design, there is less body seat contact; the end load is concentrated over a smaller area. Higher contact pressure enhances the ability of the single ferrule solution to seal low-density gases, which means it can out-perform its double ferrule counterpart in some applications.
The CPI™ fitting also has the benefit of the one-piece ferrule, utilising Parker’s unique Suparcase™ system. This offers better corrosion resistance and is less likely to be damaged in service, but it’s not available on the front ferrule in twin ferrule tube fitting designs.
Tubing vibration can affect the ferrule seal. However, with single ferrule designs, a light compression grip at the rear of the ferrule isolates seal points from system vibration. This creates a cushioning effect, providing excellent vibration resistance – again, a potential advantage over double ferrule designs.
During temperature cycling, metals naturally expand and contract; this causes dimensional changes in fitting connections. With single ferrule fittings, the ferrule features a ‘spring loaded’ effect; this creates a constant tension between the fitting body and nut.
The spring action compensates for cycling changes. By storing excessive force in the bowing action of the ferrule, it maintains effective sealing points. There is no corresponding spring action with double ferrule designs.
The CPI™ single ferrule tube fitting uses a Molybdenum Disulfide coated nut, which reduces the torque required to make up the assembly by as much as 40%. This coating also gives the added advantage of precise and consistent re-makes. As a result, the fitting can last longer in service - reducing potentially costly maintenance work.
Both single and twin ferrule designs can potentially seal all leak paths, as long as they are assembled precisely. With double ferrule fittings, there’s scope for incorrect alignment, lining fittings up backward, or leaving a ferrule out completely.
Ultimately, with fewer components, there is less that can go wrong; single ferrule solutions are quicker and easier to assemble, so the scope for error is reduced. With Parker’s CPI™, fittings are sold completely assembled and ready for immediate use.
Offering superior corrosion resistance, the CPI™ product series is manufactured to the highest quality standards and available in a range of sizes, materials, and configurations. There is documented heat code traceability on stainless steel fittings for nuclear and other critical applications.
Article contributed by Dave Edwards, fittings product manager, Instrumentation Products Division Europe.