One of the primary reasons we use metal hose is because it is corrosion resistant. In aggressive applications, other hoses—rubber or PTFE for example—simply cannot handle the media without deteriorating.

Many end users focus on alloy selection when it comes to fighting corrosion. This makes sense as even among the available alloy grades resistance to media varies. The flexible metal hose industry relies on 321 and 316L for the bulk of its needs, but there are many alloys that offer superior resistance to corrosion.

For instance, The Chlorine Institute recommends Monel® 400 and Hastelloy® C-276 for metallic hoses used in chlorine transfer for their increased compatibility with chlorine. Even within the chlorine industry, Hastelloy® C-276 is generally considered a better option than Monel® 400.

The need to select the right alloy for your application is an important one, but it is not the only one. The wall thickness of the hose is a critical feature to be aware of. Media inside and sometimes in the environment outside the hose will, at some level, penetrate the hose. It may be that the penetration is only to an insignificant degree, or it could be that is enough to impair the integrity of the hose.

Where corrosion is present, a thicker wall hose will provide a longer life than a thinner wall hose made of the same alloy and used in the same application.

Expected Rates of Corrosion

Corrosion rates, or the speeds at which metal deteriorate, help illustrate the role wall thickness plays in determining corrosion resistance.

Millimeters per year (mm/y) and Mils penetration per year (MPY)—a unit of measurement equal to one thousandth of an inch—are used to measure these rates.

For instance, 316 SS is compatible with sulfuric acid. At 99% concentration, sulfuric acid moving through a 316 hose has a 2.2 MPY expected rate of corrosion. This may not be an issue with a Schedule 40 pipe, but on a 0.010” wall hose, that’s 20% of the wall thickness. That is an issue!

Penflex Heavier Wall Hoses

Penflex leads the industry in wall thicknesses. For our 1” P4, the wall thickness is 0.015” but other manufacturers offer 1” hoses with wall thicknesses of 0.010.” In the above situation, a Penflex P4 hose would be in service for more than 2 years before approaching the wall thickness of a new hose made with 0.010” material. That’s worth thinking about!

Wall thickness matters. If you don’t consider that, then you could be using a thinner hose in a potentially dangerous scenario.

For any questions about wall thickness and corrosion resistance, please contact us.

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The Cost of Corrosion

Since the mid-1900s governments around the world have sought to determine the economic impact of corrosion. The most recent figures, released by NACE International in its 2016 IMPACT study, put the global cost of corrosion at an estimated US $2.5 trillion, equivalent to 3.4 percent of global GDP (2013).¹

The costs are significant and incurred across nearly all industries. And while the challenges facing these and other industries are considerable, the report went further to state that 15-35 percent of the cost of damage, or between US $375 – $875 billion, could be saved annually through corrosion prevention best practices.

It’s becoming better and better understood that Microbially Influenced Corrosion (MIC) plays a role in contributing to the corrosion wreaking havoc on systems and processes globally with the potential to increase rates of corrosion by two or three orders of magnitude.²

With a proper understanding of factors leading to MIC and designing, manufacturing and maintaining piping systems in response, its impact can be checked.

Microbially Influenced Corrosion

Some microbes found in water and soils can metabolize nutrients using oxygen or various other chemical compounds like sulfur or iron to produce corrosive agents. Others can change the electrochemical conditions at the metal surface without producing corrosive agents themselves.

These changes can lead to direct localized corrosion, an increase in general crevice corrosion, or possible corrosion inhibition. In any of these scenarios, MIC—also called microbiologically induced corrosion, bacterial corrosion, or biocorrosion—should be considered as a possible contributing factor.

Sulfate reducing bacteria (SRB), iron and manganese bacteria, and sulfur oxidizing bacteria are the three types of microbes commonly associated with MIC. SRB is responsible for most instances of accelerated corrosion damage to ships and offshore steel structures while iron and manganese oxidizing bacteria are most often associated with the corrosion of stainless steels.³

Given water is requirement for microbe growth, MIC is a problem in industries using seawater, surface water, municipal reclaim water, grey water and well water. Water treatment, power generation, oil and gas, marine, and pulp and paper are industries especially prone to MIC.

Common MIC Scenarios in Oil & Gas 

Incidences of biocorrosion resulting in, among other things, oil reservoir souring and pipeline and process equipment corrosion have been reported in oilfield operations where water is present.⁴ MIC may be responsible for as much as 40% of internal corrosion within the oil and gas industry.

Sulfate reducing bacteria present in crude oil or injection water often attaches to the internal surfaces of pipelines and injection lines. SBRs covert sulfate into highly corrosive hydrogen sulfide and even the low concentrations of water in crude oil or condensed water in gas pipelines can be sufficient to allow them to multiply. The presence of biofilms and localized pitting corrosion could be indications of MIC.

It’s imperative that oil and gas piping systems are designed using the optimal alloy—the austenitic stainless steels are suitable options; carbon steel should likely be avoided—and geometry to minimize MIC’s impact.

For instance, flow velocity is an important consideration given the increased likelihood that microbes will adhere to the piping surface with slow media flow. Engineers need to keep this in mind when determining the inner diameter of pipes and hoses.

Microbially Influenced Corrosion in Power Generation

The significant amount of water required to generate electricity typically gets recirculated many times over and may be left stagnant for periods of time. Trapped gases, minerals and impurities make the water increasingly corrosive while biofilms can more easily develop under these conditions.

Power plants are thus susceptible to MIC. Often, it’s stainless and carbon steel tanks and piping that see this kind of corrosion. When it comes to metal hose and expansion joints, steps to maintain a material’s corrosion resistance—like purging welds and keeping surfaces clean and unscratched—will help to delay the onset of Microbially Influenced Corrosion.

Of course, the design and manufacture of piping components can only go so far to limit the impact of MIC. Regular mechanical cleaning and employing chemical treatments with biocides to prevent bacterial population growth would help curb its impact on piping once in service.

High Likelihood in Water Treatment

Water treatment sees some of the highest levels of MIC, and challenges laid out previously in this bulletin are often compounded by the age of many sewage systems and water treatment facilities. Protective coatings, whether physical barrier coatings or sacrificial coatings, are increasingly being used to mitigate the effects or delay the advance of MIC.

Given the wide variety of culprits and the varying ways in which they can accelerate corrosion, MIC is difficult to predict and its impact a challenge to estimate. As the subject gets more attention, that will likely change but it’s worth remembering that many factors are in play—from design and manufacture through to operation and maintenance—when it comes to reducing the impact of and minimizing costs associated with MIC.

We hope to have given some context and insight into this topic with our bulletin and encourage you to contact us with any questions.

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<sup>1.“Economic Impact,” NACE International, http://impact.nace.org/economic-impact.aspx.
2. “ATI Alloy AL-6XN,” ATI Allegheny Ludlum, last modified 2010, https://www.atimetals.com/Products/Documents/datasheets/stainless-specialty-steel/superaustenitic/al-6xn_tds_en_V2.pdf.
3. “ATI Alloy AL-6XN,” ATI Allegheny Ludlum, last modified 2010, https://www.atimetals.com/Products/Documents/datasheets/stainless-specialty-steel/superaustenitic/al-6xn_tds_en_V2.pdf.
4. Videla, Héctor A.  and Liz Karen Herrera Quintero, “Biocorrosion in oil recovery systems. Prevention and protection. An update,” Technical Journal of the Faculty of Engineering, http://www.tjfeonline.com/admin/archive/919.09.20141411133274.pdf.</sup>

Note: To print this bulletin about oxygen service cleaning, please click here. 

A bit like asking someone to quote an expansion joint and sharing only the required ID, a request for oxygen service cleaning without additional information is similarly vague.

Defined as a system conveying gaseous O₂ in concentrations of at least 23 percent, oxygen service applications are as diverse as they are numerous. Without knowing precisely how the hose will be used, any recommendation around special cleaning requirements would more accurately be defined as a guess.

Necessity of Cleanness

Regardless of whether the hose is intended for industrial or medical oxygen service, cleanness is critical to the safety of those working in and around the piping system. From greases, oils, dust and fibers to metal chips, burrs, weld slag and oxidation, contaminants in the assembly bore present potential ignition sources and possible combustion fuel.

Remember, fire requires oxygen, heat and fuel, and in these applications, oxygen is non-negotiable. Beyond being a very effective oxidizer, oxygen lowers the ignition temperature of other materials. This coupled with the many opportunities for generating heat within a piping system means the stage for oxygen explosion could easily be set.

It’s the fuel—or the contaminants—that we have to control in order to minimize this risk.

Considerations from Design to Delivery

While this bulletin focuses on cleaning for oxygen service, cleanness should be top of mind throughout design, fabrication, packing and shipping.

For instance, higher pressure and higher velocity applications are better served with copper or nickel alloys rather than with stainless steel. Hose should be trimmed at the half corrugation to eliminate a crevasse between the full corrugation lip and fitting which could trap contaminants. Other considerations include carrying out braid trimming processes in a way that prevents braid segments from entering the hose and purging welds to prevent oxidation.

An Overview of Oxygen Cleaning Procedures

Cleaning specifications, inspection and cleaning methods, and recommended solvents are provided in literature and standards published by the Compressed Gas Association

(CGA) and the American Society for Testing Materials (ASTM), along with several oxygen producers, government entities, and individual users.

A Selection of Oxygen Cleaning Specifications

Most standards are similar for industrial oxygen service, but there is greater differentiation and precision required for medical oxygen service.

Given the range of standards, it’s important to establish which specification the cleaning must conform to with a customer prior to accepting an order.

CGA G-4.1 Cleaning Equipment for Oxygen Service

The cleaning standard most referred to in the metal hose industry is “CGA G-4.l Cleaning Equipment for Oxygen Service.” It provides a set of minimum requirements for industrial oxygen service.

CGA G-4.l lays out the solutions, equipment, procedures and post-cleaning contaminant levels for the following cleaning methods: steam or hot-water cleaning, caustic cleaning, acid cleaning, solvent washing (including ultrasonic cleaning), vapor degreasing, and mechanical cleaning. The kinds of contaminants and hose materials determine which method to use.

Hoses are typically submerged in the cleaning solution or positioned at a downward angle to allow the solution to be swapped along the inside and then flushed out. Drying the hose is the next critical step and may be achieved through baking or blowing the moisture out, though the simple configuration, or shape, of the corrugations complicates this process.

Inspections follow to determine whether post-cleaning contaminant levels meet CGA G-4.l.

“An acceptable contamination level for oxygen service equipment is approximately 47.5 mg per ft² (500 mg/m²) but could be more or less depending on the specific application (state of fluid, temperature and pressure). If the purchaser’s requirement includes a particle and fiber count, a representative square foot section of surface shall show no particle no larger than 1000 microns (.0384″) and no more than 20 particles per ft² (215 particles/m²) between 500 and 1000 microns. Isolated fibers of lint shall be no longer than 2000 microns and there shall be no accumulation of lint fibers.”

Acceptable contaminant levels after cleaning differ between the various regulatory and industrial cleaning specifications, but even the most stringent cleaning procedures and methods will leave some level of contaminants in the hose.

Final Thoughts

In conclusion, more often than not, cleanness requirements are developed between the buyer and the seller to meet a particular need rather than an objective standard.

For a product such as a metal hose assembly, the care and cleanness used in fabrication can include a wide range of practices and techniques to reduce contamination, even if the presence of contaminants can never be eliminated completely. Each additional process will add to the cost of the final product as delivered and at some point, the buyer will need to determine the value it brings.

Our Cleaning Corrugated Hose for Oxygen Service Guide is available upon request. For more information, please contact us.

Additional insights are provided by Janet Ellison, Penflex Director of Quality and Engineering, in the video below.

Any questions, please contact us. 

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Not all metal hoses are created equal. Distinctions in quality can be subtle and often go unnoticed. Without the ability to recognize superior technique, users will consistently evaluate product based on a limited set of criteria.

If the goal is to maximize the life of a hose in service, this is a short-sighted approach.

Purging is a proven process that enhances weld quality by decreasing—or even preventing—oxidation. This, in turn, maintains the corrosion resistance of parent materials, ultimately reducing leak rates in finished product. Visual cues, signaling whether a weld was purged or not, will enable users to better assess metal hose.

Understanding Weld Purging

As heat combines with air during the welding process, bands of temper colors form alongside the penetration of the bead on the front and back sides of the weld. On stainless steel, these strips of color are indications of oxidation, often in the form of chromium carbide precipitation. This chemical reaction decreases corrosion resistance and diminishes the mechanical properties of parent materials. Shorter hose life is the ultimate result.

Chromium and carbon, which have an affinity for one another, can bond to form chromium carbides at elevated temperatures. This presents a challenge as chromium is a key alloying element in austenitic stainless steels and the one that imparts these metals with their superior corrosion resistance. When chromium migrates to bond with carbon, its distribution becomes uneven, thereby weakening its ability to prevent corrosion.

The chart below reveals the correlation between oxygen content and the amount of oxidization that has taken place along the weld seam in the wake of welding.

Given resistance to corrosion is one of the primary reasons users opt for stainless steel, maintaining this characteristic throughout the manufacturing process is essential.

When TIG welds are purged, inert gas is used to “purge” air from inside the hose and at the front side of the weld where it is feed through the welding torch. By preventing atmospheric contamination, temper colors do not form, and corrosion resistance in the weld material remains similar to that of the parent material.

Additionally, as welders can better control the melt when purging, there will be a smaller heat affected zone (HAZ) around the weld. As the HAZ is a target for chemical attack, minimizing this area will ensure longer hose life.

Purged vs. Unpurged Welds

Purged welds have a cleaner, more uniform appearance than unpurged welds. The bands of colors that form during the welding process range from gold to grey, with lighter shades of gold representing minimal chromium carbide precipitation.

Here is an example of some very good, clean welds from our operation. These are flexible pipe assemblies and you can see both the attachment fillet welds where the flexible component attaches to the pipe and the FJP pipe butt welds were very well done by Penflex welders.

At the other end of the spectrum, grey coloring is often accompanied by a rough surface, termed “gross oxidation” or “sugaring.” Differences in the quality of purged and unpurged welds can also be clearly seen when looking at the inside of the weld.

Per the image below on left, sugaring is a telltale sign of an unpurged weld. The uneven surface, caused by burn through, will trap bacteria, corrode and possibly crack prematurely. In sanitary applications, this is unacceptable. In contrast, the purged weld to the right shows no signs of burn through and exhibits a fully penetrated and consistent arc.

Argon Purge Gas  

While there are several options for purge gases, argon is preferred to other commonly used gases because it is inert, noble and unreactive. Nitrogen, on the other hand, is a reactive gas that, in combination with heat, can alter the base properties of the filler material.

Some welders use nitrogen when cost is a concern, but as the ensuing chemical reactions could be detrimental to the application, Penflex recommends using pure argon, especially when manually welding.

The American Welding Society promotes the use of argon purge gas as well. The only standard color chart it publishes is D18.2 and it is done with argon on stainless steel. Penflex uses this chart as a reference of acceptability on all our seam welds and assembly welds.

Weld purging tools and technique

Certain steps in the purging process cannot be overlooked and the right equipment is required if welds are to be purged properly.

During setup, argon needs to be “flooded” into the hose slowly to avoid any mixing with oxygen and flow rates vary based on hose size. A purge tool matching the dimensions of the hose is needed to minimize the amount of argon that can escape and fresh oxygen that may penetrate the seal. Using the correct size reduces the amount of time needed to purge and ensures successful back purging.

Penflex welders use stainless steel purge tools designed for our hoses and built in-house with our resident experts and machinery. The purge tools range in size from ¼” – 12” and are also available for sale to fabricating customers. (For more information, contact us.)

Once unwanted gas is displaced from the weld area, purging should continue for no less than 30 seconds after welding is complete and metal is no longer red. At this point, the metal will have cooled and will no longer be subject to oxidation.

The bright, clean finish of an argon purged weld indicates corrosion resistance on par with that of the parent material. Weld purging is a best practice and Penflex is committed to seeing it become the industry standard through education on its inherent benefits and with our purge tools.

Check out this video below on argon purging with Penflex CWI, CWE and NDT Examiner Dave Gregor.

Any questions, please contact us.

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What is Fretting Corrosion?

Fretting corrosion refers to the damage of uneven material surfaces in contact with one another. These uneven surfaces, also known as asperities, appear to be smooth but a microscope would reveal their sharp grooves and rough pits.

Fretting is caused by oscillatory slip and rapid repeated motion that causes contact between the two surfaces. While the surfaces are often not designed to touch, even the slightest amount of contact—for instance, an amplitude of motion as little as 3×10^-9m—could eventually lead to fretting corrosion.

This contact can be caused by vibration under a load or pressure fluctuations in tight-fitting parts. Within the hose industry, we often see fretting corrosion on the hose as a result of constant contact with the braid.

A Word About the Term “Corrosion”

Strictly speaking, corrosion is meant to refer to damage caused by chemical attack. Fretting corrosion is therefore, at its most basic, mechanical damage.

However, as the two surfaces rub against one another, fresh surfaces become exposed. Oxidation occurs and oxide debris soon forms. In stainless steel, iron oxide can develop, which lends a reddish color to the area affected. The image below shows fretting on a material.

A Word on “Braid Wear”

Depending on the material or application used, fretting can have abrasive wear, adhesive wear, or both. Abrasive wear occurs when a surface slides across another surface, the former having a rougher surface than the latter. This causes material loss on the softer surface. Adhesive wear occurs during direct frictional contact whereby both surfaces begin to lose material fragments. This type of wear can increase roughness and create protrusions. Since the fragments cannot escape contact during fretting, they further contribute to wear.

When it comes to metal hose, we might also refer to fretting corrosion as “braid wear” as the relative movement between the braid and hose is an example of abrasive wear. The tensile strength of the braid material is higher than that of the hose material—about 100 ksi versus 75 ksi—and thus the hose is “sawed-into” by the stronger wire which acts like a little knife. The image below shows an example of braid wear.

Factors Contributing to Fretting Corrosion

Like many other types corrosion and wear, the degree of severity on the material(s) depends on many factors including contact load, amplitude, temperature, relative humidity, and inertness of the material(s).

Since fretting corrosion is often mistaken by false brinelling, is it important to make the distinction between both types of wear. The main difference is that false brinelling occurs under lubricated conditions—which is often used to preventing fretting—and fretting corrosion occurs under dry conditions.

How to Reduce Fretting Corrosion

High dynamic cycling applications offer greater likelihood for fretting corrosion, especially when contact is between similar metals. For instance, a stainless steel hose with a stainless steel braid. In situations like these, adding lubrication can reduce the severity of fretting corrosion.

Another option is to use dissimilar material, or if the hose and braid material will be the same, introduce a dissimilar material between the other two surfaces. We might do this by adding a layer of “loose” bronze braid between the stainless steel hose and stainless steel braid which works in the same way as lubrication.

Beyond movement, the outside environment plays a significant role in the rate of fretting corrosion and must be taken into consideration when designing an assembly. For example, in humid conditions, the wear becomes less severe due to the moisture acting as a layer of protective lubricating film.

Ultimately though, it’s regular maintenance and routine material inspections that will defend against corrosion of all types—including fretting—and increase the life span of a hose in service.

For any questions on this bulletin, please reach out and contact us. 

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Note: To print this sour gas transfer bulletin, please click here.

Natural Gas Market

As demand for natural gas increases, effective extraction and efficient processing of sour gas is top of mind for many in the industry. The acidic gases contained within are toxic, flammable and highly corrosive.

The main driver of increasing demand, which jumped 4.6 percent in 2018 according to the International Energy Agency, is the switch from coal to gas. As countries like the United States and China look to reduce air pollution from coal to gas switching, other regions like the Middle East expect power generation to be a key driver of industry growth in the coming years.

The industrial sector also presents a ready market for natural gas where it is used as energy for processes and feedstock for chemicals. Continued growth in global demand is expected.

Sweet and Sour Natural Gas

Like crude, there are different kinds of natural gas, namely sweet and sour. Sour gas is natural gas that contains significant amounts of acidic gases, namely hydrogen sulfide (H₂S). It may also contain other acidic gases like carbon dioxide (CO₂)—and can thus be termed an ‘acid gas’—but it’s important to recognize that an acid gas is not always a sour gas. Without the presence of H₂S, an acid gas is not technically a sour gas. Sweet gas, which does not contain concentrations of acidic gases, is preferable.

While definitions defer between states and countries, natural gas is typically considered sour if hydrogen sulfide content exceeds 5.7 milligrams of H₂S per cubic meter of natural gas, or about 4ppm.

H₂S is extremely corrosive and can be lethal to breathe. Often in combination with water, which leads to sulfide stress cracking, hydrogen sulfide damages drilling equipment and corrodes piping during extraction, transportation and processing of natural gas.

Highly poisonous, the toxicity of H₂S is comparable to carbon monoxide and several instances of death have been reported due to short-term, high-level exposure.

High concentrations of CO₂ decrease the amount of energy produced when burning natural gas and also present challenges when liquifying natural gas. CO₂ will freeze before the gas is liquified causing blockages in flow lines and other operational problems.

Thus, before sour natural gas can be consumed it must be “sweetened” to remove H₂S and CO₂. The removal of hydrogen sulfide is typically done through an amine gas treatment process while the removal of carbon dioxide can be done using solvent systems, absorption towers, membrane separators or various cryogenic processes.

Alloy Selection in Sour Gas Applications

Given the highly corrosive nature of H₂S, careful consideration is needed when designing metal hose assemblies for sour gas. At concentrations up to about 25 percent, H₂S is a reducing agent while at higher concentrations it becomes an oxidizing agent. H₂S concentrations in natural gas vary but can be as high as 90 percent.

316 SS is resistant to both reducing and oxidizing acids due to its molybdenum (Mo) content and can be used in sour gas applications. It even “fares” better than some exotic alloys like Monel 400 (NiCu 400).

However, if price is a secondary concern, then using AL-6XN which has twice as much Mo than 316 SS or one of the exotic alloys such as Inconel 625 or Hastelloy C-276—with significantly greater amounts of Mo (as much as four to eight times the amount compared with 316 SS)—is preferable. Hose assemblies made from these alloys will demonstrate greater corrosion resistance in sour gas applications.

To summarize, the above-mentioned alloys are listed below in descending order based on molybdenum content, one of the indicators of resistance to corrosion caused by H₂S.

• Hastelloy C-276
• Inconel 625
• AL-6XN
• 316 Stainless Steel
• Monel 400

Beyond Mo, the greater the nickel (Ni) and chromium (Cr) content of the alloy, the better suited a hose will be for such environments. Of those alloys listed above, Hastelloy C-276 and Inconel 625 are the only high nickel alloys with nearly 60% Ni composition.

As the elemental composition of media following through piping systems varies and there are other factors to consider—such as wall thickness, movement, temperature and pressure—it’s important to keep in mind that the information shared here is general in nature. Ultimately, piping system designers should be deciding which alloy is best for a particular application given their familiarity with these various inputs.

Any questions, please contact us.

Sources: “Gas 2019: Analysis and forecasts to 2024,” International Energy Agency, https://www.iea.org/gas2019/.

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Note: To print this bulletin on Positive Material Identification, please click here. 

The right materials are important to any design. In selecting the appropriate alloy for a metal hose assembly, engineers consider the temperature, pressure, movement—be it flexing, vibration or fatigue—and rates of corrosion anticipated within the given application.

It’s a careful calculation with many variables and selecting a less-than-ideal material could lead to faster rates of corrosion, decreased efficacy as a transfer medium, and premature hose failure.

While Penflex does specify the chemical and physical properties for our stainless steel, Hastelloy, Monel, Inconel and Bronze hoses with Material Test Results (MTR) from our raw material suppliers, some end users want additional assurance in the form of a final verification.

This final verification is Positive Material Identification (PMI) testing conducted before the hose assemblies are shipped to their final destination. PMI testing analyzes the composition of a component by reading quantities of its constituent parts, often delivered in percentages. Penflex uses an alloy analyzer that leverages X-ray fluorescence technology for its PMI testing.

Penflex Director of Quality and Engineering Janet Ellison demonstrates how PMI testing works and precisely why it can be an additional procedure worth undertaking in the video below.

The composition of stainless steel alloys defer and one of the key differences between 304 and 316 is 316 contains more molybdenum (Mo) for increased corrosion resistance. Assuming the application requiring these flanges involved corrosive media, this catch was an important one that would not have been made without PMI testing.

For a sample PMI test report, please click here.

For any other questions about alloy analysis, please contact us.

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A cycle life comparison between annular and helical designs.

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Annular and Helical

Broadly speaking, there are two types of corrugated metal hose. This categorization is based on the orientation of the corrugations and is the result of different forming processes.

Annular Hoses

Annular hoses have convolutions that line up parallel to one another at right angles to the longitudinal axis of the hose. The corrugations of an annular hose may look like a series of complete circles or rings. These hoses can be formed by a number of metal forming processes, including split and rotating die mechanical forming processes, hydraulic forming, and hydroforming.

Helical Hoses

Helical hoses have one continuous convolution resembling a screw thread. They are generally formed using a single rotating die that is in continuous contact with the outer diameter of the tube as the tube progresses longitudinally.

Annular hoses are more commonly used than their helical counterparts, with some estimates suggesting annular hose comprises 95 percent of the corrugated metal hose market.

Helical hoses aren’t as common as they once were and, while there are a few advantages cited, we remain skeptical of these claims. It has often been said that when a helical hose needs to be drained, it can be hung from one end and the liquid will follow the helix and drain from the hose. While it is very difficult to “drain” all traces of a liquid from an annular hose, we also believe that liquids adhere to the walls of helical hoses the same as they do to the walls of annular hoses, and that it takes more than “draining” to remove all traces of liquid.

In some cases, when fully compressed, helical hoses can achieve a higher working pressure than a standard pitch hose. This may make them the right choice in industries like bottle filling, but for the most part, users prefer annular hoses for their respective applications for a number of reasons.

For instance, the parallel convolutions of annular hoses make them easier to cut and easier to weld. And, as our recent U-Bend Fatigue Test demonstrates, annular hose has far better flexing characteristics than helical hose.

U-Bend Fatigue Test

Recently we performed a test to compare annular and helical hose. Specifically, the U-Bend Fatigue Test compared the cycle life of our P3 Series annular and our 400 Series helical hose. We used three six-foot assemblies of ¾-inch single-braided hose from each series.

While the pressure and bend radius were specific to a customer’s request, the test format was carried out in accordance with ISO standards 10380:2012(E). Each assembly was mounted onto the U-Bend Tester to form a vertical loop and then subjected to repeated sinusoidal flexing at a rate of 20 cycles per minute in a direction parallel with the axis of the hose.

The test pressure was 300 PSI, the dynamic bend radius was 6.0 inches, and water was the media. Failure would take place if there was any leak—marked by a drop in pressure—or if there was localized reduction of the hose radius more than 50% during the test.    

Results

At approximately 200 cycles the helical hose samples all demonstrated a twist sideways in sync with the bias of the hose when in its relaxed condition. This bias comes from the impact of a single die that rotates around the tube to form the hose. Under pressure a helical hose will torque as it elongates and place pressure on the cap and fitting welds at the ends of the hose. Annular hose does not exhibit these characteristics.

As expected, we got longer dynamic cycling out of annular hoses as flexing happens between the convolutions rather than through them.

For the helical hoses, the cause of failure appeared to be a stress fracture. (See image below on left.) For the annular hoses, it was the rubbing between hose and braid that slowly notched a hole into the surface. (See image below on right.)

Impact of Oiling Hoses on Cycle Life

While the cycle test outlined above was conducted without oiling the hoses—a standard requirement for a test such as 10380:2012(E)—hoses may be oiled in service to prolong their service life. Without lubrication, braid friction may lead to premature hose failure.

Burst Testing at a Million Cycles

In a third test, we oiled a new set of three six-foot  ¾” single-braided P3 assemblies and took them all to 1,000,000 cycles with the same pressure and bend radius as used in the previous test. The hoses were then removed from the U-Bend Tester and burst tested.

The goal was to see if cycling would affect burst pressure. Turns out, the oiled annular hoses were in almost the same condition as before cycling.

Achieving similar burst pressures between the original state and after one million cycles speaks to the quality of Penflex P3 Series and structural integrity of our welds.

To read the complete report, click here. For any questions about the U-Bend Fatigue Test, please contact us.

¹ Nominal Burst represents maximum pressure at 70°F.
² For more details on Penflex P3 Series Stainless Steel Hose, click here.

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