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 °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.

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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.

Permeation

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.

Full Vacuum

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.

Abrasion

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.

Flexibility

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.

Torsion

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.

High Velocity

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.

Testing

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.

Oxygen Cleaning

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.

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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.

FAQs

Are you acquainted with flexible metal hoses? Perhaps the information below can resolve your confusions and you will find choosing metal flexible hose will be an effective and economical way to your piping problems in the long run.

Why select flexible metal hose?

Engine vibration makes flexible hoses mandatory elements of pipe systems. Flexible hoses are engineered to absorb vibration (horizontal, vertical or angular), offset misalignment of pipelines and dampen noise. These problems above all can shorten the life span of the entire systems.

There are generally options of flexible hoses - EPDM hose and corrugated metal hose. Both can act as a medium that reduce internal stress of pipelines, absorb vibration and correct misalignment. However, stainless steel flexible hose has some exceptional properties. First, it has much longer life span than the EPDM hose. The EPDM hose usually runs 10 to 15 years, while stainless steel hose will not fail all through the life of mechanical system. Second, metal flexible hose can operate at a wider range of extreme temperatures. Third, the stainless braid makes the metal hose withstand far higher pressures than EPDM hose.

What do you know about pressures?

When selecting a flexible metal hose, you may be confused about various pressures, including working pressure, test pressure and burst pressure. What do they mean? Do they have some relationships?
Maximum Working Pressure should be calculated on a continuous basis at ambient temperature of 70°F (21°C). Generally, it accounts for a quarter of nominal burst pressure.

Maximum Test Pressure refers to the maximum pressure before the corrugation of the hose start to deform. at ambient temperature of 70°F. It can be determined by multiplying the maximum working pressure by 150%.
Nominal burst pressure means that if the pressure is exceeding, the cord or braid will rupture.

T321, T304 and T316L, which one?

321, 304 and 316L are all common materials for flexible stainless steel hose. All of them belong to austenitic chromium - nickel alloys. However, there are small differences in their properties.

321 is a stabilized grade of stainless, for it is alloyed with Titanium which has a stronger affinity for carbon than chromium does at extremely high temperatures. Type 321 is the premier choice for exhaust headers.

304 is the most common stainless steel. It is similar to 321 except that 321 has an addition of Titanium. 321 is often replaced by 304 in a lot applications, such as header construction. Compared with 321, however, 304 don't have high temperature fatigue resistance.

In contrast with 321 or 304, 316L has addition content – molybdenum, which provides protection from various forms of corrosion. 316L is strongly recommended for chloride environments because it has a higher resistance to pitting and crevice corrosion.
In a word, consider the practical application before determining the exact material.

When does the hose need a liner?

Liners are designed when the velocity of liquid or gas exceeds the maximum velocity without liner. Interlock liners can decrease the turbulence posed by high speed and reduce the resonant vibration that may occur.

If the actual velocity is higher than the maximum velocity below, a liner is recommended:

Conversion formulas

What does safety factor refer to?

Safety factor is the ratio between the maximum working pressure with he nominal burst pressure. Generally speaking, the ratio is 25%. That is to say, the safety factor is 4:1.

What is the difference between live length and overall length?

Live length refers to the flexible or active length of a hose assembly. However, overall length means the length of the active length together with the length of fittings and braid sleeves. In a word, overall length is longer than live length.

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