By Jeffrey A. O’Hearn, Product Manager, Eaton Corp.
Teflon hose is seldom specified for routine hydraulic or pneumatic systems, but dominates in aerospace and tough industrial environments. Here’s how to choose the right hose and avoid several common mistakes while doing so.
Hoses made with a core of Teflon® (PTFE) deliver out-standing performance under severe conditions. That’s why they are found in places like the space shuttle, aircraft landing gear and braking systems, and the self-contained breathing apparatus firefighters depend on to keep them safe.
Teflon hoses may be a good choice for applications where the following characteristics are required or specified:
• Heat and cold resistance
• Corrosion resistance
• Extremely low permeability
• Light weight
• Flex/fatigue resistance
• Ease of cleaning/sanitizing
• UV/ozone resistance
Just as there are differences between materials used in hoses, so are there differences between hoses using the same base material, like Teflon. Here are some of the basic characteristics to consider in choosing a hose for a specific application.
Smooth or convoluted bore? The primary differences are size and bend radius. Smooth bores are generally available only in bore sizes of 1 in. or less. For that size, a smooth bore hose has a minimum bend radius of 12 in., whereas a convoluted bore hose has a 3-in. minimum bend radius.
Conductive or non-conductive? Fuel-line hoses carrying gasoline or other low-viscosity hydrocarbons at high flow rates tend to build up static charges that can arc through the Teflon to the braid, creating a pinhole in the Teflon. Specifying conductive Teflon will allow the static charge to bleed off harmlessly to the fitting.
Wall thickness? Thicker walls are better for applications where the hose is flexed severely, as they are more resistant to buckling. Thick wall hoses are also less permeable to gasses. Thin wall hoses tend to cost less, because they contain less material.
Braid material? Type 304 stainless steel is the baseline braid material for most Teflon hoses. Type 316 stainless is recommended for marine hose applications. Monel is available for hoses exposed to severe marine corrosion environments, and bronze is available for marine applications and also where hoses may be rubbed together or against other equipment. In such cases, the excellent lubricity of bronze may deliver longer life than stainless steel.
Braid material also plays into pressure rating of a hose. Some materials and hoses can handle pressures to 5,000 psi. Monel and bronze-braided hoses typically have lower pressure ratings.
Fittings? Teflon hoses are suitable for use with crimp, swage, or reusable fittings. Use the type you prefer, as there are no significant performance differences.
Interior or exterior? Hoses exposed to severe environmental conditions can be fitted with several forms of external protection, such as extruded thermoplastic and silicone sleeves, fire-resistant sleeves, and various types of metallic protective enclosures. Hoses used in vacuum can be fitted with internal coils or sleeves to prevent collapse.
Three common mistakes
To avoid application errors in your designs, beware of these common misunderstandings or “myths” about hoses made of Teflon.
1. Teflon hoses are a high-cost item: While it is true that the Teflon PTFE paste used to make high grade hose is a premium material, the hose itself is often a more economical choice than traditional products when total lifecycle cost is considered. There are a number of factors to consider when comparing hose cost and performance.
A hose made with Teflon will outlast an ordinary rubber or thermoplastic hose under virtually any operating condition. The advantage is greatest under the most severe conditions but it is almost always present, and usually significant. Whether this factor alone is enough to justify the higher initial cost of a hose made with Teflon will vary from application to application, but in most cases the answer is “no.”
That begins to change, however, when costs other than initial purchase price are considered.
• How much does the scheduled downtime for a hose change cost in terms of lost production, direct labor, hose and fitting inventories, and impact on other dependent processes?
• How much does unscheduled downtime due to unpredictable hose failure cost?
• How much does contamination of products or process fluids by hose materials cost annually?
• How much does it cost to replace expensive boiler additives lost to effusion through rubber or thermoplastic steam hose cost?
• How much could be saved if rubber or thermoplastic hose jackets were as corrosion resistant as the stainless steel braid used with most hose made of Teflon?
A good example of the benefits of hoses made with Teflon can be found in the tire industry. The presses used in tire manufacture use steam to cure the rubber, water to cool the die, and compressed air to blow the water out of the die prior to introducing steam for the next cycle. A single hose is used to supply steam, water, and air to the press. The OEM rubber hoses originally supplied with the presses had to be changed every two weeks on average to avoid in-process failures. Replacing them with hoses made with Teflon and equipped with spring guards to minimize flexing fatigue has extended the hose-change interval to six months.
Hoses made with Teflon can also reduce warranty costs for products like commercial/industrial coffee machines that tend to receive little or no maintenance from end users. They also add a factor of safety when used to drain hot products like fryer grease in fast food restaurants. Other advantages of hoses made with Teflon can have significant cost impacts in more specialized application areas. For example, carbon will not stick to the Teflon core of hoses used as compressor discharge lines. That means the compressor damage caused by pieces of built-up carbon becoming detached from the discharge hose walls and circulating through the system is completely eliminated. How much does it cost to replace an industrial compressor?
The bottom line here is simple. Hoses, like everything else, ought to be evaluated on their total lifecycle cost, not just their initial purchase price. Viewed in this way, hoses made with Teflon very often turn out to be the lowest-cost option.
2. Teflon hoses tend to kink and crush easily: The truth is that all hoses tend to kink and crush easily, especially when they are improperly applied. Hoses made with Teflon are no better or worse than other materials when recommended bend radii and loadings are adhered to.
In applications where kinking or crushing is a possibility, it is a simple matter to add internal or external spring guards to reinforce the hose at critical points. Selecting a hose with thicker walls and a convoluted bore will also minimize kinking and resist crushing.
Note that Teflon hose with a convoluted bore has a recommended bend radius that is three to four times tighter than a PTFE hose of the same size with a smooth bore. Convoluted bores also exhibit greater crush resistance in vacuum applications.
3. All hoses made with Teflon are the same: Teflon is a trade name that is applied to a whole family of fluoropolymer-based materials, not all of which have the same properties. Two different Teflon materials are typically used in the hose industry, a melt-extrudable form (such as PFA, FEP, or ETFE) and a paste-extrudable form (PTFE).
Melt-extrudable materials necessarily have a lower melting point than paste-extrudable materials. The also exhibit lower flex fatigue resistance. These lower physical properties are all reflected in the products using melt-extrudable Teflon materials.
The premier PTFE resin used in hose construction is Teflon 62. Because Teflon 62 is not a thermoplastic, it cannot be continuously extruded, which has traditionally limited the length of hose that can be produced. Now the latest generation of paste extrusion equipment can produce long lengths of PTFE hose. For a 1-in. ID hose, a 400 ft continuous length is now possible.
Footnote: Teflon® is a registered trademark of DuPont used under license by Eaton.
Filed Under: Aerospace + defense, Fluid power, Hydraulic equipment + components, Pneumatic equipment + components