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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. 

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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“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. 

Further 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!

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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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?

While 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.

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.

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.

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.

For further questions, please contact us.

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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. 

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.    

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.

For further questions, please contact us.

Note: To print this bulletin on ammonia service, please click here.

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.

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.

Footnotes

[1] United States Census Bureau. U.S. and World Population Clock. Retrieved August 11, 2022 from https://www.census.gov/popclock/

[2] Alexander Tullo. 8 March 2021. Chemical & Engineering News. Is ammonia the fuel of the future?. Retrieved August 11, 2022 from https://cen.acs.org/business/petrochemicals/ammonia-fuel-future/99/i8

[3] Johnny Wood. 29 October 2021. Forbes. Scaling Ammonia Production For The World’s Food Supply. Retrieved August 11, 2022 from https://www.forbes.com/sites/mitsubishiheavyindustries/2021/10/29/scaling-ammonia-production-for-the-worlds-food-supply/

[4] Cybersecurity & Infrastructure Security Agency. Chemical Facility Anti-Terrorism Standards (CFATS). Retrieved August 11, 2022 from https://www.cisa.gov/chemical-facility-anti-terrorism-standards.

Note: To print this bulletin on how much weight a hose hung vertically can support, please click here.

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.

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.

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 p
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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