The term metal hose refers to a hose where all components of the flexible portion of an assembly are stainless steel or any kind of corrosion-resistant metal. Similar to how we broke down hose in basic terms, a metal hose also consists of a tube and reinforcement.
The tube, in which 321 stainless steel is a standard material, carries the media; meanwhile, the wire braid—in this case, 304 stainless steel is more common—gives the tube strength to carry the pressure of the application. For better corrosion resistance, 316 stainless steel is the next step up; in more aggressive applications, we’ll also see materials such as Monel, Inconel, and Hastelloy.
Ends for a metal hose assembly can be carbon or stainless steel, or any other metal that can be welded, brazed, or soldered to stainless steel. We talked briefly about end connections in Hose Pros 101, and those ends apply to metal hose assemblies as well. Some common end connections are NPT, pipe flanges, and pipe groove (Victaulic) connections. Like we do with industrial and hydraulic assemblies (as metal hose is just a more specified category within these categories), we also see JIC and cam and groove connections quite frequently. While these are just some of the most common connections, we can attach any metal fitting.
Temperature–Very High and Really Low
Unless specified, you may be wondering when you need to consider a metal hose versus a rubber or plastic hose for your assembly. One specification that stands out as an indicator for needing a metal hose is temperature. On one side, we have very hot applications, up to 1300°F. On this end of the spectrum, we have two considerations: outside environmental conditions, such as an assembly near a furnace in a steel mill; or, internal process conditions, like combustion gases or hot air for test facilities. Metal hose performs great at elevated temperatures, but be aware, stainless steel’s strength declines gradually as temperatures rise.
On the other hand, you may need to consider a metal hose for very cold (cryogenic) temperatures, -325°F and lower. Stainless steel improves mechanical properties as temperatures drop, even to cryogenic levels. The colder the temperature, the better they get. When thinking about your end connections, remember that carbon steel has an effective temperature range of 0-800°F; outside this range, its properties diminish quickly.
Another property of metal hose is low permeation—zero leak rate. All rubber and plastic hoses will permeate molecules through the tubing over time. Functionally speaking, the metal hose has a zero leak rate.
Vacuum is another condition that could sway your decision between rubber and metal hose. Because of the low permeation, metal hoses can also be uniquely qualified for vacuum-rated applications. If you need to guarantee the highest level of vacuum rating, a metal hose is a great choice! Because of its corrugated design, it also handles full vacuum from a structural perspective.
Because metal hose can be made from the most chemically resistant metals, this can be another consideration point when deciding to choose metal hose. Metal hoses can withstand the most corrosive of environments. Keep in mind: One aspect of chemical resistance is thickness; a thick component has more material to corrode away and will inevitably last longer.
Finally, it’s important to remember that metal hose is not particularly good with abrasion resistance, contrary to what you might think. The braid wires found in metal hose construction are only a few thousandths of an inch thick; if they’re put in an application where abrasion exists, they can quickly abrade and weaken until they eventually fail. In a scenario such as this, another layer of protection is required, such as a plastic spiral wrap or a strip-wound metal guard welded over the top of the assembly.
Metal hoses can also see this type of degradation from the high frequency of vibration in the system itself. The braid can cause abrasion on the crests of the corrugations. You can address an issue such as this by adding a sacrificial bronze braid layer in between the tube and stainless-steel braid; this softer bronze braid acts to protect both the tube and braid from degrading.
Hydroforming vs. Mechanical Forming
A metal hose starts as a flat strip, and it’s rolled and welded into a tube. This tube is then run through a corrugator to form the corrugation’s omega shape. There are two ways in which this is achieved:
- Hydroforming uses pressurized water to force the tube radially outward. This water pressure is uniform and, theoretically, reduces stress concentrations by applying an equal force on the material.
- Mechanical forming uses metal dies that stretch the metal onto the omega shape; this causes increased stress where the dies push and pull to stretch and bend the metal.
Metal hoses can be manufactured with improved bend radius and flexibility. A compressed hose with a high number of corrugations per inch allows for greater flexibility and reduced force to bend.
Keep in mind that a metal hose does not play well with torsion or twisting; this is something that can be avoided by proper installation. Using two wrenches to tighten a swivel nut will keep the hose free from torque. Proper routing is important too. Bending should occur in a single plane, as multi-planar bending creates torsion and decreases the service life of your hose.
The high velocity of the process fluid can be another consideration when looking into metal hoses. When referring to metal hoses, velocity is considered high at 75 ft/s or more. When your velocity goes above 75 ft/s, the media becomes turbulent, and the hose begins to whistle and vibrate. A flow liner of stripwound hose will smooth out the surface the media flows against and reduces turbulence.
At times, customers may require certain testing requirements. In the absence of a customer spec, every metal hose assembly built by a company using NAHAD standards receives an air-under-water test for one minute to look for leaks at the weld.
NAHAD suggestions include:
High-pressure pneumatic testing has been shown to result in fewer false-positive outcomes, meaning anything higher than 75 psi. Some pinhole leaks are so small they don’t register a bubble in one minute at pressures under 75 psi; however, increase the pressure, and bubbles form quicker. You can talk with your manufacturer to decide if this increased level of testing is right for you.
Welding & Inspection Certifications
This is another consideration driven by the customer. You must determine if your organization requires certification, as many do not. The most common certification for a metal hose is ASME Section IX, which tells you, as a buyer, that the weld procedure yields an effective weld, and that the welder is qualified to produce that weld. We won’t sugarcoat it: weld certifications are a labyrinth of understanding and interpretation. If you need details, give us a call and we’ll show you how deep the rabbit hole goes.
Weld inspections can also be performed by a certified inspector. Again, this is a customer-driven requirement. Inspections are normally non-destructive and simply visual. Increased levels use aids like liquid die penetrant to inspect the surface of the weld. Other non-destructive tests, like X-ray and ultrasonic, can detect discontinuities in the internal weld. There are other methods that must be discussed at the quoting and design stage but remember: visual inspection is a minimum requirement for all welds.
Metal hose is a great choice when it comes to oxygen applications, especially industrial oxygen; however, certain care must be taken to ensure the hose is clean and free of hydrocarbons prior to use. Oxygen itself is not flammable, but in the presence of a fuel—typically hydrocarbons—it allows those fuels to ignite at lower temperatures and burn hotter. Metal hose is generally safe because the manufacturing process should not introduce fuel onto the surface of the hose.
When it comes to internal cleanliness, cleaning the inside of the hose and then capping the ends after it’s been fabricated can ensure the hose is safe to use. Cleaning methods and degree of cleanliness should be discussed during the quoting and design stage.
Minimum Length Requirements
Each size and style of hose has a minimum recommended live length, meaning the minimum length of the flexible portion of the hose must be able to flex and hold pressure, generally behaving like a hose. At lengths shorter than the minimum recommended live length, the hose tends to act more like a pipe, with little flexibility; these cannot be guaranteed to perform as expected. Most fabricators will make these short assemblies and simply state the length is shorter than recommended, which could void the warranty.
Metal Hose Manufacturers
Opinions vary on who makes the best metal hose. From a market dominance perspective, Hose Master is the most prominent. They have manufacturing facilities all over the U.S. and fabricating distributors in every city, so their product is readily available. They are constantly innovating their product and manufacturing processes, so their hose is always state-of-the-art. Penflex, US Hose, and others are also well represented in the U.S. and make a quality product.
As with many more specific applications, a metal hose has many different considerations. We’ve attempted to layout as much as possible, but each application differs from the next. If you find yourself needing assistance, call The Hose Pros at TCH for further guidance.
Our focus and obsession is the distribution and fabrication of hose for industry and hose related products. Founded nearly 40 years ago, we are proudly owned and operated by the same family. Our manufacturing partners are some of the biggest and best names: Eaton, Parker, Dixon Valve, Hose Master, Brennen, Hannay Reels, and many others.
In short, we are a customer-centric hose company filled with happy professionals who can help you meet all your hose related needs. If you have any questions, please reach out to us by filling out the form below!