Penflex provides a unique breadth and depth of durable metal hose products, though cost-competitive, lighter-weight options are also available. The P3 and P4 Series Stainless Steel Hoses are two of our most popular products and a comparison between them hints at our range of offerings.

Both hoses use a rotating die process to form corrugations gradually from the inside out. This ensures a highly uniform hose, the insides of which remain free of surface effects that can result from forming corrugations from the inside out.

P3 and P4 are available in 321 or 316L with one or two layers of 316L or 304L braid. The hoses come in standard pitch, but we also offer compressed pitch versions for applications requiring a more flexible hose.

As laid out above, there are many similarities between these Series, but there are also some key differences. The main distinctions to be made revolve around wall thickness, hose geometry and size.

P3 Hose: A Thinner Wall for Increased Flexibility

The metal strip used to make P3 is thinner than that used to make P4. This makes P3 a lighter hose and, given the lower metal component, a more economical hose.

It’s also the reason why P3 is our most flexible hose, and one of the most flexible hoses on the market. A thinner piece of metal is inherently easier to bend than a thicker piece of metal. Thus, a thinner wall hose will be more flexible than a heavier wall hose, other factors remaining the same.

When it comes to the geometry of Penflex’s P3 hose, we designed the Series with a lower corrugation height to allow higher working pressures. While this can reduce the hose’s flexibility, we compensated for this by increasing the number of convolutions per foot. A higher corrugation count will ensure a more flexible hose.

We can compress P3 even further, in which case we end up with a hose that is extremely flexible, kind of like a Slinky ™.

Full product details for P3 Series Standard and Compressed can be found via the links below:

• P3 Series Standard
• P3 Series Compressed

P4 Hose: A Heavier Wall for Increased Corrosion Resistance

Made from heavier strip, P4 is a thicker wall hose with better corrosion resistance. It’s a more durable hose, though in applications where corrosion resistance or lifespan are not top priorities, that may be an unnecessary attribute.

Increased corrosion resistance through the use of thicker strip comes at the expense of flexibility. Penflex P3 and P4 Series demonstrate this inverse relationship well.

However, in designing P4, we were able to overcome some of this. We increased the number of corrugations per foot and raised the height of the corrugation to bring some flexibility back.

As noted earlier, lower convolution height allows for higher working pressures, so to compensate for lower flexibility, we “lost” some of the working pressure potential. Thus, while P4 often has higher working pressures than P3, that is not always the case. Both Series offer competitive working pressures and to pick between the two based solely on such criteria would mean overlooking the real distinctions between P3 and P4.

The hoses differ as well in the sizes available. We currently run P4 from ¼” to 4” while P3 is only available up to 2”.

Full product details for P4 Series Standard and Compressed can be found via the links below:

• P4 Standard
• P4 Compressed

A Note on Hose Certifications 

All sizes of P3 and sizes up to 2” P4 are in accordance with sections of ISO 10380 and all sizes of both Series have weld seams that are PED certified.

For more information about our hose certifications, please click here.

Penflex also produces a range of heavy wall hoses, exceeding the capabilities of the P4. Any questions regarding the difference between Penflex P3 and P4 Series Hoses, or regarding our other heavy wall hoses, please contact us.

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Note: To print this bulletin on common expansion joint accessories, please click here.

Accessories are added to an expansion joint installation to meet specific customer, application or code requirements. Some are used to control movement, others to protect the bellows in the event of pressure thrust, and still others to defend against corrosion caused by media inside or from the environment outside the bellows.

Here is an overview of the most common expansion joint accessories. 

Limit Rods

The primary function of limit rods, sometimes called limit bars, is to restrict the range of movement—whether that be axial, lateral or angular—during normal operations, thereby preventing premature failure. Nut stops placed along the rods determine just how much extension and compression an expansion joint will accommodate.

The Expansion Joint Manufacturers Association (EJMA) standards state that “in the event of a main anchor failure, limit rods are designed to prevent bellows over-extension or over-compression while restraining the full pressure loading and dynamic forces generated by the anchor failure.”

The number of limit rods is calculated based on the pressure thrust that needs to be contained. EJMA recommends a minimum of three rods.

Tie Rods

Tie rods are similar to limit rods, though they cannot accommodate axial movement. Limit rods can. The sole function of tie rods is to continuously restrain the full bellows pressure thrust in normal operation while allowing only for lateral deflection.

If only using two tie rods—and placing those two rods 90 degrees from the direction of rotation—you could accommodate angular rotation.  This could be a cost-effective alternative to a hinged expansion joint.

Control Rods

Control rods are commonly confused with limit and tie rods, but it’s important to understand precisely what is specified as the accessories serve different functions.

These accessories are only used in universal expansion joints to distribute movement between the two bellows. Unlike limit and tie rods, they are not designed to carry the pressure thrust in any circumstance. 

Shrouds

 Also called covers or guards, shrouds encase the bellows, protecting it from external damage and external flow. Sometimes, a shroud will be needed just during installation to, say, protect the expansion joint from getting hit with a hammer. Other times, the shroud will be needed throughout operation.

Liners

Liners, also called sleeves, are metal devices installed on the inside of a bellows to prevent flow-induced vibration which leads to stress cracking and failure. All Penflex bellows and expansion joints are designed in accordance with the allowable values specified in the latest EJMA standards. These let us know how thick liners need to be to prevent flow-induced vibration.

Liners also prevent the build-up of material on the inside of the bellows. This would be a consideration when working with some media, like concrete for example, that hardens below a certain temperature. Shielding the inside of the bellows also protects them from caustic media.

Together with shrouds, liners are the most commonly specified accessories we see.

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For information about Penflex Expansion Joints, please click here or to print this bulletin, please click here. We’ve also pulled together a 40-minute webinar that covers expansion joint basics. Take a look!

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Some applications lend themselves to either a metal hose or to an expansion joint, but more often than not, there is a preferred option. A few key application criteria will let us know when to use an expansion joint instead of a hose.

Axial Movement

Expansion joints are very good at absorbing axial movement, or the compression and extension of the bellows along a single axis. Whether the component is installed horizontally or vertically, it doesn’t matter.

This kind of movement is typically associated with thermal growth and contraction and found in piping and ducting systems across a wide range of industries.

Externally Pressurized Expansion Joints have a unique design whereby media comes into contact with the outside rather than the inside of the bellows. This external pressurization allows the components to absorb larger amounts of axial movement than more traditional expansion joint designs.

Metal hose is not designed to accommodate axial movement. When a hose is compressed axially, the braid falls out of tension and can no longer serve in its pressure carrying capacity. Once this happens, even at moderate working pressures, the hose is liable to squirm, braking through braid and resulting in a failure.

Vibration Damping

Expansion joints are sometimes a better option in applications with extreme vibration, like what you might find in exhaust systems or piping systems with pumps.

As they can accommodate movement in all directions—axially, laterally, or angularly—they can better absorb vibrations that might otherwise prompt a hose failure or cause stress on the piping system as a whole.

Exotic Material Requirements

Certain alloys, like 800H, Nickel 200, Titanium Grades 1 and 2 and Duplex 2205, may be desired for extremely hot or highly corrosive applications. They are, however, not readily available in a metal hose product and an expansion joint may be the only viable option in some scenarios.

Paper mills, for instance, often use titanium expansion joints to defend against the corrosion so inherent in their operations.

Space and Size Limitations

Expansion joints may be the right solution simply because they take up less space than a hose or because the diameter required lies outside the typical range of hose sizes.

It would be much easier to find a 30” expansion joint than a metal hose with a 30” ID. Penflex manufactures large bore hose up to 24” in diameter but expansion joints can be made up to 120” in diameter.

In some situations, the decision to use an expansion joint instead of a hose will be an easy one. Other times, the answer may not be so clear. Rather than deciding between an expansion joint and a metal hose, the answer may in fact be a combination of the two!

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For information about Penflex Expansion Joints, please click here. We’ve also pulled together a 40-minute webinar that covers expansion joint basics. To watch, visit our video library and scroll to the bottom for the training session with Sales Engineer Igor Smola.

To speak with one of our Sales Engineers about you may need for your applications, click here.

Liquid Penetrant Inspection (PT) is a nondestructive testing (NDT) method to identify surface defects so small they might be missed by the human eye.

When it comes to metal hose assemblies and expansion joints, PT allows us to find discontinuities that could undermine structural integrity or serve as points of chemical attack.

Since PT works on any non-porous material and can be done in several different ways, as well as being an easy process requiring minimal equipment, it is a widely used inspection method.

Step-by-Step: Liquid Penetrant Inspection

Regardless of which type of penetrant is used, the steps are similar.

1. 1. Pre-clean surface. The metal surface must be free of all contaminants that could prevent penetrant from seeping into defects.
   2. Apply penetrant and wait. Whether the penetrant is brushed or sprayed on—or the component is dipped into it—any cracks in the metal surface will “pull” penetrant inside during “dwell” time.
   3. Clean surface of excess penetrant. This needs to be done with care so as not to remove penetrant from the defect.
   4. Apply developer. A wet or dry developer is applied to the entire metal surface to draw penetrant from defects and bring it to the surface.
   5. Assess results. Penetrant magnifies any discontinuities—in essence being the “X” that marks the spot—making those surface defects easier to see. The contrast under blacklight using fluorescent penetrant or the contrast of red dye on white developer using a visual penetrant makes the discontinuities all the more apparent.

PT with Fluorescent Penetrant

Fluorescent penetrants contain dye that glows when exposed to UV blacklight—we use a ZYGLO fluorescent penetrant with Level 3 sensitivity. Fluorescent penetrant is a more sensitive system than one using visible penetrants.

In the image below, you can see defects on a flanged bearing exposed under ZYGLO testing. Any components that do not pass PT are scrapped and new components are made and tested to complete an order.

PT with Visible Penetrant

Visible penetrants contain red dye that is easily distinguished against the background of a white developer.

Using PT to Proactively Improve Processes

Beyond identifying surface discontinuities, the irregularities uncovered can serve as an indication of poor welding technique. For instance, a visible penetrant was used to uncover the cracks in a failed hose assembly later magnified under a microscope below.

We can see a lack of fusion, incomplete penetration of the weld joint, a concave weld profile, improper weld size, and off-center weld placement, all of which resulted in the weld becoming a pivot for vibration.

If further analysis is done off the back of liquid penetrant inspections, proactive steps can be taken to improve welding procedures.

Liquid Penetrant Inspection at Penflex

Penflex carries out PT inspections at our lab in Gilbertsville, PA. We are SNT-TC-1A qualified and comply with ASME V, Article 6, ASME B31.3 and ASME B31.1 PT requirements. We also meet AWS D1.1 NDT requirements. Our on-site CWI, CWE, and NDE Dave Gregor will tell you a bit more in the video below.

If you have any questions about this, or any other NDE testing we do, please contact us.

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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.

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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.

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

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