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How to Handle Interlocked Hoses

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Corrugated hoses have replaced their interlocked predecessors in all but a few special applications and, as a result, most conversations about metal hose are about the corrugated kind.

There’s talk of pressure ratings, discussions around chemical compatibility and consideration given to temperature derating factors. Examining hose failures has users assessing leaks, looking at cracks and debating the cause of braid damage.

None of these topics relate to interlocked hose.

With a different construction, interlocked hoses present users with a unique set of considerations. If they are more familiar with corrugated hoses, these could be helpful to point out.

Greater Design Capacity…Though with Limitations

Interlocked hose machines can be adjusted in many ways to make slightly different sizes and constructions. This ability to deliver a wider range of products also makes it difficult to control some characteristics from one run to another. 

Sometimes a user might think the interlocked hose is too stiff, and other times too floppy. Or that it may too easily compress and extend on one occasion but prove too difficult on another. 

The manufacturing process is not the only contributing factor here. Consider the design of an interlocked hose. Movement is determined by how much the interlocked folds can move before hitting the nearest hose wall. 

Interlocked Hose Profile

Without anything to “set” the slip space in place, compression and extension of the hose can happen during shipping, handling, installation and operation in a way that may not be consistent throughout the entire length of the hose. 

This space, while necessary for movement, also means that interlocked hoses are not 100% leak tight. This is true even with the inclusion of special packing. This limitation created the need for a pressure tight solution, which eventually led to the development of corrugated hoses. 

How to Handle Interlocked Hoses: Common Oversights 

Sometimes users do attempt to put an interlocked hose in an application where a corrugated hose is better suited. Maybe the flow media is a liquid or there are high pressure requirements. When the hose leaks, the user may cite a failure, but the reason for failure would be an error in hose selection rather than a shortcoming of the hose. 

The special packing that is sometimes included to reduce air loss or manage low pressure requirements in an interlocked hose is of a synthetic material. It can melt out of the hose if exposed to too high of temperatures, and this does present a challenge when welding on the end fittings. 

Since metal can handle temperatures so much higher than the packing, sometimes users unknowingly subject an interlocked hose with packing to temperatures above design limits. 

Also, with regard to the packing, if the hose sees a lot of compression and extension, it may try to “sneak out” of its position in the curve of the hose. In these cases the seal is lost and the user could expect to see some seepage. 

Need for Lubrication

As an interlocked hose flexes, metal moves against metal. This contact can lead to material loss and shorter hose life which, fortunately, can be defended against with lubricants. 

Lubricants reduce wear, thereby extending service life, and to remove them through, for instance, ultrasonic cleaning would be unwise. Without lubrication, an interlocked hose would be difficult to flex and produce an uneven bend, and the metal on metal movement would surely lead to premature failure. 

The Big Don’t and a Unique Failure Mode

As with corrugated hose, torquing is a big “don’t,” though the result of torquing an interlocked hose is certainly unique. Twisting the hose will damage the interlocked connection, sometimes to the point of “unhooking” the folds. This can also happen if the hose is bent very far in excess of its minimum bend radius. In either scenario, once this happens, the hose will continue to fall apart. 

One way to gauge whether an interlocked hose has experienced torque, assuming it’s not immediately visible, is to paint a “laying line” or “flow arrow” on the outside of the hose. If the line begins to swirl around the hose, the user will have evidence of twisting.  

Use with Corrugated Metal Hose Assemblies

Thanks to the uniformity of the finished product and its pressure carrying capabilities, corrugated hose is the preferred option in most applications. 

However, beyond the niche use cases, interlocked hoses continue to play an important role in the metal hose industry. They are often used as liners in or as protective armor on corrugated hose assemblies. 

While liners manage flow velocity and protect the hose from the deleterious effects of flow-induced vibrations, armor acts as a bend restrictor and abrasion guard. While some users opt for short lengths of interlocked armor near the end fittings, if there is potential for over-bending, there’s a chance that the sharp edge of the short length of armor will end up digging into the hose. This is why other users opt for armor that runs the full length of the assembly. 

The development of new products gives us a fresh set of solutions and challenges, but in the case of interlocked hoses with their continued use, some of the solutions and challenges associated with previous product iterations remain relevant. 

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Is a Pressure Tight Interlocked Hose Really Possible?

Excerpt from Penflex's 1959 Metal Hose Catalog“The rugged twenty-four inch steel interlocked tubing installed on diesel engine exhaust bears little resemblance to a gold necklace for your lady. Strange as it may seem, it is a direct descendent.” 

Excerpt from Penflex’s 1959 Flexible Metal Hose catalog

When the inventors of interlocked hose first thought to fold the edges of thin metal strip together in a spiral-like fashion, thereby creating a flexible sheath, it wasn’t a means of fluid conveyance they sought.

A Jeweler’s Invention Shaped A Modern Industry

It was the mid-1800s and Heinrich Witzenmann and Louis Kuppenheim wanted to create something elegant, and purely ornamental. And the competition was stiff. 

Pforzheim, on the outskirts of the Black Forest in southwest Germany, had been dubbed “Goldstadt,” or “Gold City,” given the proliferation of jewelers and watchmakers that had come to call it home. If the pair’s creation was to stand out, it needed to be truly unique.  

And their necklace was, though in a way the two had probably never envisioned. It was more than ten years after developing the design that the jewelers recognized its potential in industrial applications. A new branch of their business dedicated to the development and production of interlocked hose was subsequently opened.

A Geometric Design

To create interlocked hose, the edges of strip material are folded into one another. As the material runs into the machine, one edge is bent up and inward to create a curl running the length of the strip. As it continues its path, winding helically around a sizing mandrel, the other edge is folded into the curl.

This creates the interlocked convolutions that enable the hose to move. Movement is determined by the amount of space between the two folds and, as seen in the cross section above, this space creates an exit path for media. 

Interlocked hoses are not leak tight and, as a result, cannot be used in applications with pressure requirements. 

Inclusion of Packing Materials

In an attempt to deliver some pressure carrying capacity, manufacturers began to add packing material into the interlocked convolutions. 

However, gains were measured. For instance, Penflex’s interlocked hose–regardless of size–when packed with silicone is rated to just 20 PSI. 

These days, when used alone, interlocked hoses convey small solid particles like grain or plastic pellets for injection molding machines. The packing is not tight enough to seal against leaks from a liquid. 

M-100 Hose Conveying MolassesFurther gains came with the introduction of Penflex’s M-100 Pressure Hose, an interlocked hose with a specially formed groove to accommodate the packing material. Two-inch M-100 hose is rated to 190 PSI. 

While the packing does serve as a continuous gasket to make the hose pressure tight, we would limit its use to air and non-searching fluids at moderate pressures and temperatures. 

Historically, suggested applications for M-100 included steam hoses, cleaning boiler tubes, tar and asphalt hoses, vegetable oil hoses, diesel exhaust, expansion joints, rivet passing and conveying molasses. The hose’s heavy wall construction enables it to withstand significant external pressure, and M-100 has been used successfully in underground and underwater applications as well.  

With either design, temperature is a consideration given the packing material cannot withstand the same high temperatures that metal can. This may not be a concern given operating conditions but consider the heat of welding. Materials adjacent to the end fitting weld experience temperatures in excess of 800°F. The packing can “burn out” and leave a leak path in its wake. 

The limitations on pressure and temperature left room for further innovation. 

Advent of Corrugated Hose Technology

Judging by records from the US Patent and Trademark Office, corrugated hoses were making their way to market by the 1930s and 1940s. Initially the hoses were created in a similar way to their interlocked predecessors. The main difference was that rather than folding the edges together, they were crest welded. 

A pressure tight seal had been achieved!

Corrugated Hose Profile

Soon a more efficient method was developed whereby the strip was welded into a tube before being run through a corrugator to create the corrugations. Today’s corrugated hoses are made this way–though some machines now combine tube making and corrugation creation in a single, continuous process. 

Most feature an annular hose profile, and can achieve pressure ratings far in excess of their interlocked counterparts. Penflex’s 2-inch P4 hose with one braid layer is rated to 532 PSI. With two braid layers, it is rated to 850 PSI. 

While interlocked hoses are still used, both alone and as accessories on corrugated hose assemblies, the fact that they are not 100% leak proof is one of the main reasons they have largely been displaced by corrugated hoses. 

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Using Traveling Loops to Accommodate Axial Movement in Piping System

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Hoses perform a valuable function in piping systems by absorbing movement. Rigid as they are, hard pipe and equipment can crack under the stress of movements, while hoses, being flexible, can bend without breaking.

Picture expansion joints in bridges. Or flexible foundations under buildings in earthquake prone areas. These structural elements protect immovable objects from unstoppable forces. Hoses do the same thing in piping systems.

Why Can’t Hoses Move Axially?

Hose Squirm due to Axial MovementWhile braided hoses are pliable and seemingly capable of moving in many directions, they are not actually designed to accommodate all kinds of movement.

To be a pressure carrier, the hose must be a braided hose. It is the braid that prevents the hose from growing back into a tube when pressurized, and thus its strength largely determines a hose’s working pressure.

To function properly, the braid must be in tension. Compression along the longitudinal axis would bring the braid out of tension and, for this reason, hoses cannot accommodate axial movement.

Beyond a reduction in pressure carrying capacity, once a braid comes out of tension, there is a tendency for the hose to wiggle. It can exploit weaknesses in braid coverage and squirm out, leading to a sometimes dramatic-looking failure.

Though hoses themselves are not designed to move axially, they can still accommodate axial movement of the piping system.

Accommodating Axial Movement of Piping System

To accommodate axial movement within a piping system, hang hoses in traveling loop configurations. There are three broad categories of configurations. Horizontal and vertical installations are options within each one.

In a Variable Radius Traveling Loop, the end of the hose moves in and out in a horizontal configuration and up and down in a vertical configuration. Regardless of orientation, the radius changes throughout each cycle.

Variable Radius Traveling Loop Configuration

In a Constant Radius Traveling Loop, the end of the hose moves up and down in a horizontal configuration and in and out in a vertical configuration. Regardless of orientation, the radius remains constant throughout each cycle. While this installation requires more space than a variable traveling loop installation, it can accommodate more movement.

Constant Radius Traveling Loop Configuration

Traveling Loops with Movement in Two Directions combine the movements of Variable and Constant Radius Traveling Loop configurations. So long as the two movements do not prompt axial compression, the two movements can happen simultaneously.

Traveling Loop Configuration with Movement in Two Directions

When There Isn’t Enough Space

Traveling loops are an ideal configuration for hoses because the length of the installation limits stress on individual corrugations. While this ensures hoses reach maximum service life, it also makes traveling loops unsuitable in applications where there is not a lot of space.

In these scenarios, some users may opt for U-Loops and V-Loops. The returns and elbows in these assemblies save space. And while these may look like traveling loops to the untrained eye, they actually absorb movement differently. U-Loops and V-Loops use two hoses, each moving in a lateral offset motion, to accommodate the axial movements of a piping system. This can stress the end fitting connection welds more acutely though this may not be an issue in an infrequently cycling application.

Where space is especially limited, or if there is no space beneath the piping, an expansion joint may be the best design for the application.

Unplanned Axial Movement 

While this bulletin focuses on how hoses can be configured in loops to accommodate axial movement in a piping system, where the design is carefully considered to avoid the hose experiencing this kind of movement, there are other scenarios where axial compression happens inadvertently.

Improper installation of hoses hung vertically can lead to axial compression of the hose. In the drawing below, you can see that without pipe support, the hose slouched and the braid relaxed. To prevent this, system owners use pipe hangers.

Unplanned Axial Compression from Lack of Pipe Support


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Accommodating Out of Plane Movement in a Piping System

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Flexible metal hoses are not designed to accommodate movement in more than one plane. 

Let’s clarify: a SINGLE flexible metal hose is not designed to accommodate movement in more than one plane.

To move in more than one plane would require the hose to twist. We call this rotation of the hose along its longitudinal axis torsion, and it is guaranteed to reduce service life. 

Flex Hoses are not designed to accommodate Out of Plane Movement

Wrong Shows Out of Plane Movement in a Vertically Installed Traveling Loop

This has to do with the way hoses are designed. Engineers plan for the stresses media flow and bending in a single plane exert on the hose. Exceeding these design limits leads to metal fatigue. The ultimate result is cracking. 

When a hose experiences torsion, design limits are exceeded and cracks can develop along the corrugation crests. We call this failure mode stress cracking. 

How Bad is it to Twist a Hose? 

Comparing annular and helical hoses can help illustrate the impact of twisting. With annular hoses, the corrugations are parallel to one another. With helical hoses, the corrugations line up at a slight pitch, like the spine on a spiral notebook.  

When pressurized, hoses seek to resume their former tube shape. With annular hoses, forces will exert outwards parallel to the longitudinal axis of the hose. With helical hoses, given the corrugations “swirl” around the hose, forces will exert both sideways at an angle to the longitudinal axis as well as outwards in line with it. This means helical hoses, when pressurized, have a natural tendency to twist, and in effect torque themselves. 

This is not a classic example of torque, but it is worth noting because this natural tendency to twist contributes to shorter service life when compared with annular hoses in the same application. A cycle test conducted in Penflex’s lab found annular hoses lasted almost 90% longer than helical hoses in one dynamic cycling application. 

Among other advantages, this is the kind of information that makes it easy to understand why annular hoses have largely come to replace the helical hoses that came before them. But it also stresses just how significant the impact of moving a hose out of plane can be. 

Matter of Opinion vs. Matter of Fact

We recommend avoiding out of plane movements as a hedge against torsion, but anyone who has been inside of a plant can attest to the fact that hoses may move all over the place. 

Given the infinite options for hoses and operating conditions, there is never going to be a one-size-fits-every-application rule when it comes to out of plane movement. 

Maybe hoses bending in more than one plane do last long enough. Maybe they don’t. Without consistent tracking and historical data, it is difficult to know that a hose has failed prematurely–only that it has failed.    

Possible torque of Metal Hose in Plant

In scenarios where there appears to be no adverse impact on service life, it is likely the piping engineer was conservative in design, giving the assembly a longer length to ensure less stress on individual corrugations with each movement. 

Perhaps it is not a high pressure application and the full pressure carrying capacity of the hose is not being realized, meaning there is “leftover” capacity to accommodate slight out of plane movements.  

So while hoses can move out of plane, the ideal design is one that avoids or at least limits it, especially when working with minimum live lengths and in high pressure applications. 

When to Use Another Hose

In some applications, movement in a piping system is such that a single hose will just not be able to address the situation. In these scenarios, our first line of inquiry is to find out whether the system can be re-designed to remove movement in the additional planes.

If this is not an option, we would need to consider a more complex arrangement where multiple hoses could be installed to accommodate the movement in multiple planes.

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Metal Market Brief: December 2022

With these regular updates, we seek to paint a clearer picture of the myriad factors impacting supply and demand of stainless steel and special alloys which–ultimately–affect the price and availability of our products.

Steelmaking Surcharges

Steelmakers use a base-plus-alloy surcharge pricing structure to accommodate the wide variability in input costs. While base prices remain relatively stable, often locked in by long-term contracts, surcharges are adjusted monthly to reflect the volatility in raw material and, increasingly, energy prices.

In our previous Market Brief, we discussed the March spike in nickel prices and its impact on the cost of stainless steel. Producers sought to recover costs above “normal” through elevated surcharges. For instance, North American Stainless issued a $2.1484/lb. surcharge on its 316/316L product in April, a 15.4% month-on-month increase.1 Surcharges peaked in May before falling back in line with levels seen before the spike.

Beyond raw material inputs–which include chromium, molybdenum, manganese, and iron ore in addition to nickel–energy has become part of the pricing matrix.

The Impact of Energy Costs

Rising energy costs are a concern for European mills especially. The continent relied heavily on Russian natural gas to the tune of 40% of its annual supply.2 Dwindling and subsequent shutoff of flow from Nord Stream 1 came when supply shortages in the face of increasing demand were already being felt as economies re-opened post-pandemic on the heels of an unseasonably cold winter in 2021. Prices soon climbed into uncertain territory.

The chart below shows the cost of natural gas via forward month Dutch TTF Natural Gas Futures over the past five years. It shows prices hovering between 10-25 EUR/Mwh until 2021. Prices peaked in August 2022 at 339 EUR/Mwh and at time of writing this brief on December 12, 2022, stand at 134.5 EUR/Mwh, a relaxation but hardly a reversion to the mean.3

Dutch TTF Natural Gas Futures - 12 December 2022


The departure from previously reliable low-cost energy has encouraged steelmakers to incorporate energy into their surcharge pricing models. Average energy surcharges issued by British stainless steel long product manufacturers fell for the first time last month since July.4

However, with the situation far from certain, firms are taking additional measures to reduce the impact of energy costs on their bottom line. Worried its annual energy bill of £110,000 could reach £400,000 at current consumption levels, Leeds-based Pland Stainless distributed more “thermally efficient” clothing to its staff as it turned the heat down a few degrees.5

US Enjoys Relatively Cheap Energy

Henry Hub prices are recognized as the benchmark for US natural gas prices. For the week of December 2, 2022, spot prices were 6.06 USD/MMBtu.6 This equates to 19.58 EUR/Mwh.

Henry Hub Natural Gas Spot Price (Dollars per Million Btu)

Energy prices may be included in US surcharges, but do not have the same impact on pricing as they do in Europe. For example, Cleveland Cliffs issued a November surcharge for 316/316L precision strip less than or equal to .015” thick of $1.6565/lb. Natural gas accounted for just $.0157/lb., or less than 1%, of the total.7

Albeit on a different scale, energy prices have fluctuated more than normal in the US as well. The U.S. Energy Information Administration recently released their expectations for higher wholesale energy prices this winter in every region of the country. Figures they shared included an anticipated 31% increase in the Southwest and a 60% increase in the mid-Atlantic and Central regions.8

A Checkered Report: Stainless Steel Demand in 2022

Brussels-based research and development association worldstainless expects a 0.6% reduction in stainless steel consumption globally in 2022.9 To be sure Europe is battling higher-than-usual energy prices and dealing with the war in Ukraine while the Fed’s continued pursuit of higher interest rates makes many in the US weary, but the most significant driver of decreased demand is China’s slowing economy.

Chinese consumption is expected to decline 2.3% in 2022, the first year-on-year fall since 2008.10 Production has slowed along with consumption in large part due to the country’s zero-Covid policy.

In addition to being the world’s largest consumer of both steel and stainless steel, China is also its largest producer. Six of the world’s ten largest steel makers are Chinese entities. The country was responsible for 52.9% of all steel and 56.0% of all stainless steel production in 2021.11 12 This is to say that what happens in China influences global statistics and perspectives on the industry as a whole to a significant degree.

Case in point, reduced demand is not being seen in other parts of the world. Gains of 3.1% in the Americas and 5.1% across Europe and Africa are expected in 2022.13 Increased demand in these regions combined with fewer Chinese exports has maintained pressure on prices.

Indian Steel Returns to the World Market

A further restriction in supply came during the months May through November when the Indian government employed a series of export tariffs on steel products in an attempt to protect the domestic market from inflationary pressures. The results were, instead, damaging to Indian producers.

Dependent on export sales, steelmakers such as Tata Steel, JSW Steel, Jindal Steel and Power, and Steel Authority of India reported poor third quarter results. Though net sales rose 7.9%, operating profit declined 69%, and net profit plunged 96% from a year earlier.”14

List of Products Subject to Indian Export Tariff in 2022

In response to pressure from steelmakers, the tariffs were lifted in November. If producers outside India saw some respite from competitive Indian exports while the tariffs were in place, they can expect the return of competition with this news.

Pressure on Pricing Continues

While the market has seen a softening of late, the inflationary environment in which we find ourselves in 2022 is keeping prices sustained at higher levels. The figure below from the St. Louis Fed charts stainless steel prices over the past five years.15

FRED Economic Data - Producer Price Index - Metals and Metal Products, Steel Wire, Stainless Steel - Jan 2018 - Oct. 2022


1 (2022, March 1 and 2022, April 1). North American Stainless for Products Alloy Surcharge for Orders Promised for Delivery. North American Stainless.

2 McHugh, D. (2022, September 6). Europe is facing an energy crisis as Russia cuts gas. PBS News Hour.

3 (2022, December 12). EU Natural Gas. Trading Economics.

4 (9 November 2022). The rise and fall of European gas prices. MEPS International Ltd.

5 (16 November 2022). Leeds stainless steel firm staff get thermals as heating is reduced.

6 (12 December 2022). Henry Hub Natural Gas Spot Prices. U.S. Energy Information Administration.

7 (1 November 2022). Stainless Steel Raw Material Surcharges for Orders Promised for Shipments October 30, 2022 through December 3, 2022. Cleveland Cliffs.

8 (6 December 2022). Short-Term Energy Outlook. U.S. Energy Information Administration.

9 (20 October 2022). Global stainless steel consumption changes. worldstainless.

10 (20 October 2022). Global stainless steel consumption changes. worldstainless.

11 (25 January 2022). December 2021 crude steel production and 2021 global crude steel production totals. worldsteel association.

12 Stainless steel meltshop production. worldstainless.

13 (20 October 2022). Global stainless steel consumption changes. worldstainless.

14 Jauhari, U. (17 November 2022). Pricing Challenges Likely to Persist for Steelmakers in Q3. mint.

15 (12 December 2022). Producer Price Index by Commodity: Metals and Metal Products: Steel Wire, Stainless Steel. U.S. Bureau of Labor Statistics. Retrieved from FRED, Federal Reserve Bank of St. Louis.

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Alloy Selection in Ammonia Service

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Feeding the World’s Population

Between 1900 and 2000, the world’s population grew from 1.6 billion to 6 billion. Today, it registers at 7.9 billion.[1] Something happened in the 20th century that allowed this great explosion of population to take place.

While a mix of inputs are required for plant growth, nitrogen is considered the most important given how much is required. As such, crop yields are limited by the amount of nitrogen available and farmers in centuries past relied upon manure to augment the work of nitrogen-fixing bacteria in the soil to deliver this critical element.

In the early 1900s, a pair of German chemists discovered how to synthesize ammonia (NH3)–a nitrogen-hydrogen compound–and thereby boost the amount of nitrogen available to plants. Inorganic nitrogen fertilizers, often injected into the soil as liquid ammonia, allowed for great increases in crop yield, the kind of increases that could spur a population explosion.

Given the role it plays in feeding the world’s population, ammonia is one of the most widely produced chemicals. One hundred and eighty million metric tons are produced annually.[2] With the world’s largest population, it may come as no surprise that China is the top producer. India, Russia and the United States follow.[3]

Ammonia is also used in commercial refrigeration systems and in household cleaners, and its use as a potential hydrogen fuel source is a popular topic of conversation as well

Ammonia as a Health Hazard

Direct exposure to ammonia in high concentrations is hazardous to human health and numerous government agencies and industry associations have developed various specifications, procedures and training sessions to limit leaks. These efforts, along with correct handling and preventive maintenance, have kept incidences of leaks and human injury to relatively low levels.

Interestingly, Fertilizer Grade Ammonium Nitrate is listed as a chemical of interest in the U.S. Department of Homeland Security’s Chemical Facility Anti-Terrorism Standards.[4] While this signals the potential for its use as a chemical weapon, for the purposes of this bulletin, it underscores the importance of safety in the design and operation of ammonia piping and transfer systems.

Metal Hose in Ammonia Service

Common applications for hoses in these systems include connections between fixed loading and unloading systems, and in nurse tank trailer, rail and truck transport. Metal is often the preferred material of construction given its chemical compatibility. Metal hoses also offer a more robust design given braid layers protect the inner core from abrasion and welding is the end fitting attachment method.

The 300 Series stainless steels are suitable options for most ammonia service applications, including those involving anhydrous ammonia, a liquid solution that is used both as fertilizer and commercial refrigerant. Anhydrous ammonia corrodes copper and zinc alloys and can also attack rubber and certain plastics.

A gas at room temperature, anhydrous ammonia is cooled to its liquid state before being transported under pressure to its destination. When working with anhydrous ammonia gas at elevated temperatures, the 300 Series stainless steels are not recommended. Contact the factory for details about other options.

When working with ammonium bromide, ammonium sulfate or ammonium chloride in concentrations above 10%, 316L is recommended above 304 and 321 which are only partially resistant to these media.

For a more complete listing of alloy compatibility with ammonia have a look at our corrosion resistance chart.

Hoses for ammonia service are often used for loading and unloading of nurse tank trailers.

Concerns Around Explosiveness

While ammonia is non-flammable, it can ignite in the presence of certain compounds, namely halogens, with explosive force. Chlorine and various chlorides are halogens, so great care must be taken to remove contaminants during production and to avoid their entry into the system during shipment, storage and installation.

Key precautions must be taken during manufacturing, and include removing any chips or debris from the inside of the hose after cutting and purging welds with argon gas. Welds and welders should be certified under ASME Section IX, the industry standard for quality welding.

Considerations on Stress Corrosion Cracking

In addition to being a potential ignition source, contaminants also exacerbate corrosion in a hose. With its strong affinity for water, it is important to prevent an influx of moisture into an ammonia piping system.

Chloride contamination from the ingress of water can reduce the service life given material sensitivity to these compounds. Stainless steel and chlorides are a pairing highly susceptible to stress corrosion cracking (SCC). This form of corrosion occurs at the intersection of a susceptible material, working or residual stress experienced above the SCC threshold, and a corrosive environment. Cracks may lead to leaks if not identified soon enough.

Regular inspection of hoses in ammonia service is important for identifying cracks, as well as damaged braid, deformation of the hose, cracked fittings, or traces of media on or around piping components that could indicate imminent failure.

For further questions, please contact us.


[1] United States Census Bureau. U.S. and World Population Clock. Retrieved August 11, 2022 from

[2] Alexander Tullo. 8 March 2021. Chemical & Engineering News. Is ammonia the fuel of the future?. Retrieved August 11, 2022 from

[3] Johnny Wood. 29 October 2021. Forbes. Scaling Ammonia Production For The World’s Food Supply. Retrieved August 11, 2022 from

[4] Cybersecurity & Infrastructure Security Agency. Chemical Facility Anti-Terrorism Standards (CFATS). Retrieved August 11, 2022 from

Metal Market Brief: August 2022

With these regular updates, we seek to paint a clearer picture of the myriad factors impacting supply and demand within the metal market, for stainless steel and special alloys especially, which–ultimately–affect the price and availability of our products. 

In this edition, we review recent economic indicators that support concerns around an impending recession while also looking at one sector that’s bucking the trend of slowing growth: aerospace. 

A requirement in the production of stainless steel and special alloys–as well as aircrafts by extension–is nickel. We then outline the relationship between these two industries to better explain the anxieties around future supply. 

Central Banks Aim to Curb Inflation

The news is full of headlines about inflation and predictions about the U.S. Federal Reserve’s policy response. After its meeting in late July, the Fed announced it will lift interest rates by another 75 basis points in an effort to curb inflation. During the press conference Chairman Jerome Powell noted that future rate hikes of a similar magnitude are likely.

Central banks around the world are taking similar action. The European Central Bank raised its three interest rates in July, ending an era of negative rates. It was the first increase in 11 years. The Bank of Canada raised its rates earlier in the month as well, increasing the overnight rate a full percentage point to 2.5%. 

July Manufacturing PMIs

Rate hikes to tame inflation support sentiments that a recession is looming. Orders have slowed and inventories are growing, according to The Institute for Supply Management. Findings from its July Manufacturing ISM® Report on Business® showed continued expansion in the U.S. manufacturing sector, but at a slower rate than in previous months. The Manufacturing PMI fell to 52.8% from 53.0% in June.1 

A slowdown is being felt on the other side of the world as well. Driven by criticism to curb its carbon emissions and catalyzed by continuing pandemic-related shutdowns and the ensuing softening of demand, production in China has slowed. The country’s official manufacturing purchasing managers’ index (PMI) similarly fell in July, to 49.0% from 50.2% in June.2 The 50-point mark divides growth from contraction. 

Aerospace Bucks Slowing Growth Trend

While negative sentiment may reign this week, there are signs of accelerating growth in some sectors. Aerospace supply chains are returning, adding to the demand for stainless steel and, more so, for certain nickel alloys. 

Aircraft manufacturers are projecting a full recovery by 2024 but orders are coming in now. Most recently, in July, news that Delta placed an order with Boeing for one hundred 737 Max 10 planes, with options for 30 more made headlines. Deliveries to the Atlanta-based Delta are not expected to be complete until 2025. 

Nickel alloys, such as Hastelloy C-276, are used in hot section components like exhaust systems, bleed air systems, heat shields, fasteners, honeycomb seals, and hydraulic lines. Stainless steel Types 304, 316 and 321 are commonly used for fuel tanks.

Nickel Alloy Suppliers See Increased Profitability 

The spike in demand amid an inflationary environment has allowed nickel alloy producers an opportunity to increase profitability, helping to offset losses suffered during the pandemic. 

One Haynes International executive reported off the back of stellar third quarter earnings that aerospace orders have rebounded to 95% of pre-pandemic levels. The company, a producer of high-performance alloys, saw top line revenue come in at $130.2 million, a jump of 47.7% over the same period last year.3 Gross margin, operating income and net income all saw similarly impressive results. 

Demand from aerospace is unlikely to meaningfully impact stainless steel supply, but the same cannot be said for the exotics. Production capacity constraints continue to plague supply chains and re-roll mills are encouraging customers to buy early and often. 

Nickel: A Key Alloying Element

One of the biggest drivers of stainless steel and nickel alloy prices is the cost of nickel. It is a key alloying element in the 300 series stainless steels, giving them their austenitic structure, and thereby making them easier to work with and suitable for a diverse range of applications. Thanks to nickel, these alloys have good formability and can be easily shaped into many products, from a corrugated hose to an orthopedic implant. 

Nickel also improves weldability and toughness. While other metals (including non-austenitic stainless steels) become brittle and fracture at low temperatures, 300 series steels do not and as a result, are used in cryogenic applications. Nickel also improves strength at elevated temperatures. 

For the austenitic stainless steels, nickel content ranges from 8% – 14% of total composition, with Type 304 seeing nickel content range from 8% – 10.5% while Types 316/316L alloys see nickel content range from 10% – 14%.4 There is no substitute for nickel in the production of these stainless and other alloy steels. 

Annual Nickel Production and Reserves

Stainless steel prices and, ultimately, its long term availability are closely tied to the nickel market given the production requirements. 

While nickel is a relatively common element, annual production is quite small compared to other extractable resources. Two-point-seven million tons of nickel were produced in 2021 while–as a point of comparison–68 million tons of aluminum were produced.5 

Production trends often echo the situation with reserves, or the amount that can ultimately be extracted through mining. Known reserves of bauxite ore, the primary source of aluminum, are estimated to be between 55 billion and 75 billion tons and far outweigh known reserves of nickel, which are estimated at 300 million tons.6

To note, it is widely accepted that extensive nickel deposits lay beneath the ocean floor but mining technology has not yet evolved to a point where extraction is efficient–or even possible. These potential reserves are not included in the tally above. 

Competition for Nickel Production

The stainless steel industry is the largest consumer of nickel each year, accounting for 69% of annual production. Another 7% is used to make non-ferrous alloys, and a further 3% to make alloy steels.7

But there is a new game in town whose growing presence is putting pressure on the stainless steel industry and its consumers–electric vehicles. Certain electric vehicle (EV) batteries, including the most widely used lithium-ion batteries, require nickel. Currently EV battery manufacturers consume 11% of world nickel production annually, but as the push for e-mobility and carbon neutrality escalates, these manufacturers will be demanding bigger and bigger slices of the pie. 

Estimates for the future of electric vehicles are enthusiastic. The International Energy Agency expects EV sales to reach 20% of all car sales in 2030, increasing stock 11-fold from today’s levels to 200 million vehicles.8 This projection depends on the production of 180 million EV batteries over the next eight years. 

Unless production levels increase, nickel could be a limiting factor to the industry’s growth. In such a scenario, stainless steel and nickel alloy consumers could also expect to pay more. 

U.S. Geological Survey Adds Nickel to Critical Minerals List

The rise of EVs, resurgence of resource nationalism, price volatility and concerns over access are pushing some of the world’s largest economies to shore up supply chains of critical resources.

Albeit a bureaucratic gesture, the USGS added nickel to its List of Critical Minerals earlier this year, a designation given to minerals essential to the economic or national security of the United States which have a supply chain vulnerable to disruption.9  

Of the 50 minerals on the list, 23 are used in steelmaking processes underscoring just how important the industry is to the nation’s security. Eight of the minerals are used in the production of rechargeable batteries, an indication of the commitment to and focus on advancing technologies that will diversify our energy mix and reduce carbon emissions.  

Along with cobalt, lanthanum, manganese, and praseodymium, nickel is one of the few minerals required to support both industries. Its addition to the list just this year suggests concerns that supply may not be able to keep pace with increasing demand.


1 Institute for Supply Management®. (2022, August 1) Manufacturing PMI® at 52.8% July 2022 Manufacturing ISM® Report On Business®. Retrieved August 3, 2022 from

2 National Bureau of Statistics of China (2022, August 2) Purchasing Managers Index for July 2022. Retrieved August 3, 2022 from

3 Haynes International. Haynes International, Inc. Reports Strong Third Quarter Fiscal 2022 Financial Results. Retrieved August 3, 2022 from–3rd-q-.pdf

4 Penflex. (2021, June 29) Differences Between the 300 Series Stainless Steels. Retrieved July 28, 2022 from  

5 U.S. Geological Survey. Mineral Commodity Summaries 2022. Retrieved July 28, 2022 from

6 U.S. Geological Survey. Mineral Commodity Summaries 2022. Retrieved July 28, 2022 from

7 Nickel Institute. About Nickel. Retrieved July 28, 20222 from

8 IEA (2022), Global EV Outlook 2022. Retrieved August 3, 2022 from

9 U.S. Geological Survey. (2022, February 22) U.S. Geological Survey Releases 2022 List of Critical Minerals. Retrieved July 29, 2022 from

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How Much Weight Can a Hose Hung Vertically Support?

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Hoses are sometimes hung vertically. Oftentimes, it’s for a temporary application and, typically, it’s a large bore hose that’s being used. In these situations, a user may wonder how long the hose can be before the combination of its own weight and the weight of media flowing through it become too much.

The long and the short of it is it’s unlikely that there would be an issue.

Finding the Impact on Pressure Ratings

Braided hoses are designed to resist internal pressure as noted by their pressure ratings, and when a hose hangs vertically, some of the pressure carrying capacity does get “used up.”

What gets “used up” is determined by the weight of the hose, braid, end fittings, and flow media. Totaling these forces, converting the sum into units of pressure, and subtracting the result from catalog ratings will give you the updated pressure limits.

Example Using Penflex Single Braided 10” 700 Series

Let’s say we are using a 10” x 12’ hose to direct water from a container above into a pit below. The assembly has a slip-on flange at each end. We calculate the weight of the hose, braid and end fittings as follows.

Item Weight per Unit Total Weight
Hose 716-160 12.85 lb./ft 154.2 lbs.
Braid 1SB-160 6.1 lb./ft 73.3 lbs.
End Fittings SOF 43 lbs. 86 lbs.

To determine the weight of flow media, multiply the hose’s total volume by media density. Penflex’s 716-1SB-160 has a volume per foot of 1018.96 in3. In a 12’ run, the total volume will be 12,227.52 in3.

Item Weight per Unit Total Weight
Flow Media Water .0361 lbs./in3 441.41 lbs.
 Total Weight of Hose and Media 754.9 lbs.

To convert force to pressure, divide by the net effective area. This is the area of the hose using the radius which comes from the average of the inner and outer diameters. The ID of 716-160 is 9.82” and its OD is 11.18.” Using the formula below, we find the effective net area is 86.56 in2.

E = ((I +O)/4)2 x p
E = 27.56 x
E = 86.59 in2

Then, to finish the conversion, divide total weight by net effective area.

Pressure = 754.9 lbs./86.59 in2
Pressure = 8.72 PSI

716-1SB-160 has a MAWP of 230 PSI. Just 8.72 PSI will be “used up” when this hose hangs vertically. As mentioned earlier, unless the operating pressures are close to the MAWP, or hose lengths are quite long, a hose hung vertically will not see any meaningful reduction in pressure ratings.

For further questions, please contact us.

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Alloy Selection for Sulfur and Sulfuric Acid Applications

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Sulfur (S) is one of the most abundant elements on Earth, and references to its antimicrobial and anti-fungal properties date back to ancient times.

While there are mentions of its use in topical ointments and as a fumigant, sulfur’s yellow mass, the bright blue flame it emits when burned, along with that acrid smell, led to one more ominous association. It is to burning sulfur that the Bible refers with each mention of “brimstone” and eternal damnation was the theme of many a “fire-and-brimstone” sermon.

The element’s association with the fiery depths of Hell faded as the critical role it plays in plant and human health became better understood.

Sulfur and Sulfuric Acid Uses

Historically extracted from areas surrounding volcanoes and hot springs, sulfur is, today, most often produced as a byproduct of natural gas and petroleum refining. Sulfur-containing contaminants are removed and converted to sulfur in various forms, namely sulfuric acid. It is then used across a wide range of industries.

Agriculture and food and beverage markets rely on sulfur to support growth, stave off pests, bacteria and fungi, and prolong shelf life. A primary application is fertilizer production. Other applications include crop dusting, food processing and winemaking.

Typically introduced in the form of sulfur dioxide, sulfur works as a preservative when added to items such as dried fruit. Its chemical compounds are used in sugar refining to strip brown sugar of its color. And, while sulfites naturally occur during fermentation, some winemakers add more during this stage of production to further protect and preserve their vintages.

The paper making industry also relies on sulfur’s “bleaching” power. Other applications span cellophane and rayon manufacturing, water treatment, and renewable energy. More energy dense than lithium-ion batteries, lithium-sulfur batteries are promising to push the limits of rechargeable battery technology.

When sulfur burns and comes into contact with oxygen, as happens when sulfur-containing fuels such as coal or diesel are burned, the reaction produces sulfur dioxide (SO2), a commonly cited air pollutant. While legislation to curb emissions has initiated a shift away from coal-fired plants and created a market for low sulfur marine fuels, the need to extract sulfur from energy production processes remains, and thus a need for chemically compatible piping systems remains as well.

Tractor spraying fields with fertilizers made with sulfur

Alloy Selection

To produce, transfer, and administer sulfur in its various forms, flexible piping components are needed, and materials of construction may differ based on what is moving through the hose, and in what concentration, pressure, temperature, and environmental factors.

While 316 stainless is a suitable option for most sulfur applications—and the 300 series stainless with the widest corrosion resistance to sulfur and sulfur compounds—there are some notable exceptions. For alum, sulfur acid in 5% – 10% concentrations, and saturated sulfurous acid, 316 SS is “partially resistant” meaning we would not recommend it for continuous use.

When it comes to the following solutions, we would not recommend 316 SS in any use case.

  • Sodium hydrogen carbonate (aka sodium bisulfate), saturated
  • Sulfur chloride, dry
  • Sulfuric acid, 50%
  • Sulfuric acid concentrated, boiling

Sulfuric acid is most commonly found in concentrated solutions and it, along with the others listed above, require special alloys with higher percentages of nickel and chromium due to their aggressive oxidizing nature. Alloy options for such application would include Inconel™ 625 and Hastelloy™ C276.

For concentrated sulfuric acid, the relative order of corrosion-resistance, in descending order, would be:

  • Hastelloy™ C276
  • Incoloy™ 825
  • Inconel™ 625

Oftentimes solutions contain chlorides, necessitating further consideration so far as alloy selection goes. Beyond media composition, high flow velocity can speed corrosion rates, and come into play as well when selecting an alloy. In these cases especially, it’s clear that wall thickness also plays a role in corrosion resistance and must also figure into engineering design.

For a more complete listing of alloy compatibility with sulfur and sulfuric acid media, have a look at our corrosion resistance chart. For further questions, please contact us.

How Long Will a Metal Hose Last in Service?

Knowing how long a metal hose will last in service would make life easier. We could more accurately plan purchases for replacement parts and then schedule time to install those parts, all while reducing the likelihood of failures.

While this isn’t a pipe dream—some companies have successfully determined average service life for hoses in specific applications through careful observation and record-keeping—it is unrealistic to expect that an answer can be given without such attention to data collection and the monitoring of outcomes.

Any information that could be given at the manufacturer’s level would reflect how hoses behave in certain testing circumstances. We know that the same hose will last longer if pressure is reduced or bend radius is increased, and—conversely—that service life will be shorter if pressure increases or bend radius decreases.

It is impossible to test every possibility. And of course, we are only talking about two given variables that impact service life. Whether a hose is installed properly is another, and there are many more.

Retesting to Reaffirm Service Life

Some refineries and chemical plants look to their suppliers to retest hoses to ascertain whether they are “still good.” Such a process can give users a false sense of certainty, mistaking a hose that has been successfully retested as one that will last for another, often unspecified, stint in service.

This is misguided.

While pressure testing can be used to determine the continuing strength of a hose, it will not predict its remaining life span. We have seen retested hoses put back in service only to fail a week later.

Passing such a test does not negate the unknown impact of exposure to corrosive media, harsh environments, bending, twisting, intermittent flow, vibration, and improper handling. With this in mind, a retesting agency should not be responsible for how much longer a hose will last in service.

When to Take a Hose out of Service

Without data on how a hose operates under certain circumstances—which can only be collected by the end user—we cannot accurately predict how long a hose will last. The challenge then becomes when to take a hose out of service to avoid premature failures. And there’s no exact science to it.

We recommend regular inspections using a checklist to help maintenance personnel identify potential problems. If any of the indicators are observed, replacements should be considered. Keeping track of observations, and how long each hose lasts in every application, will, overtime, yield the kind of data that will allow users to predict service life for hoses in their facility.

Here’s Janet Ellison, our director of quality and engineering, to talk about the points highlighted in this bulletin.

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