Note: To print this bulletin on metal hose tolerances, please click here.

There’s a certain satisfaction when things line up just right. When something runs like clockwork, or fits like a glove. Without discrete units of measurement, it’s easy to assume a perfect fit.

Dive into manufacturing and it’s a world of numbers. Tape measures, calipers, micrometers. In the metal hose industry, working with sheet metal, strip material is measured in thousandths of an inch. Assembly drawings mark overall length to a similar degree. No longer just a “feel,” fit now has defined measurements.

Once we know precisely what something is, we also know precisely what it is not. Stainless steel strip that is .018” thick is incredibly thin, but not as thin as strip that is .010” thick. To many the difference may not be noticeable. To others–and to our machines–it’s all too apparent.

But just how strictly must requested dimensions be matched?

Tolerance: Range of Acceptability

While our thickest strip (.035”) will not run in machines designed to make hose using our thinnest strip (.006”), and vice versa, each machine does have a (smaller) range of acceptable thicknesses. We refer to these ranges as tolerances.

For our thinnest strip, tolerance is +/- .00030” meaning the machine can run strip so long as it falls within this defined range on either side of .006”. From measuring the thickness of strip to the length of an assembly, measurements are taken throughout the manufacturing process to ensure dimensions fall within predetermined tolerances.

Why Tolerances are Important in Fluid Conveyance

Since metal hose assemblies are connected into an existing piping system or attached to a piece of equipment, parameters are needed to ensure the component fits with what is already there.

Industry association NAHAD has published guidelines on assembly overall length tolerances. This is considered the standard.

*Source: Corrugated Metal Hose Assembly Specification Guidelines, by NAHAD.

In the drawing below, this 1½” assembly has an overall length (OAL) of 24”. In line with the NAHAD guidelines, the tolerance is +/- ⅝.” The hose will fit into its chosen application so long as its OAL ranges from 23⅜” – 24⅝”.

Setting Realistic Expectations

Since the allowances of our manufacturing processes are so small, we understand the need for close tolerances. However, it is important to set realistic expectations for metal hose.

The unique flexing characteristics of metal hose limit how tight tolerance can be. For one, when the hose is prepped for an assembly, it is cut in the valley of a corrugation. Compared to a straight tube where a 1-foot piece will match another 1-foot piece, a 1-foot section of corrugated hose may not exactly match another 1-foot section because of this requirement for a valley cut.

An exact 1-foot measurement may stop and start along the sidewalls, crests or valleys of corrugations at each end. There’s no guarantee that the corrugation valleys will align exactly with the designed measurement. This can sometimes be corrected by adjusting the length of the fitting, but some fittings, such as flanges, are not modifiable.

For another, a hose’s ability to flex means it likely will not return to the exact same length once bent. Storing, testing and shipping can all change the length of a hose after it is made. This can be a point of frustration to the perfectionist, but it’s important to note too that hoses typically elongate once pressurized.

The geometric properties that allow metal hoses to absorb movement also mean an assembly’s length does not remain exactly the same throughout its life. It is now easier to see why tolerances cannot be zero.

Designing to Account for Tolerances

As metal hose typically has larger tolerances compared to machined parts, it is not always suited for critically tight applications. When designing a system for the first time, it is ideal to have some redundant length so that the assembly is not always on the limit of recommended use. This is applicable to loops, where the added length will create a larger bend radius and help increase the cycle life of the assembly. For straight lengths, hoses should be kept on the shorter side to limit the effect of loose braid in fabrication or be built in such a way that there is some axial give to allow the hose to elongate.

When replacing hoses in low tolerance, straight configurations, hoses should be built slightly shorter. A shorter hose can be tightened into place, whereas if a longer hose is compressed to fit a tight space, failure is likely. Of course, this type of configuration should be avoided if possible and alternatives such as expansion joints should be considered.

Systems that use metal hose must be designed to account for the inherent tolerance associated with metal hose assemblies. Tools on our website can help find the minimum length for certain applications. However, engineers are always standing by to assist.

Note: To print this bulletin on metal hose tolerances, please click here.

Note: To print this bulletin on fitting attachment welds for metal hose assemblies, please click here.

How Many Welds are There?

The typical metal hose assembly has four welds: two cap passes and two fitting attachment welds. (There are actually FIVE welds in a typical assembly if we include the longitudinal weld seam. This bulletin will, however, focus only on manual welds.)

Techniques for executing a successful cap pass were discussed in a previous bulletin; here we’ll take a look at the most common types of fitting attachment welds, what it takes to perform them, and common design considerations.

Direct Fitting Attachment

The most common fitting attachment method is the direct fitting attachment method, or the direct connect method.

The direct fitting attachment method is a two-step process which begins with a cap pass. The hose is cut in the valley, what we would call a standard cut. Braid is then pulled 1/16” of an inch above the edge of the hose and the ferrule is aligned with the edge of the hose before the welder completes a cap pass to join these components.

Once hose, braid and ferrule are joined via cap pass, the welder places the end fitting atop the cap pass. The end fitting must be level on the hose and centered before being tacked into place.

The welder then uses the proper filler rod and amperage and welds the fitting to the hose using an argon purge. There must be fusion between the first and second welds, though you do not want to burn through the fitting wall or remelt the cap pass.

A good fitting attachment weld will show a nice even weave pattern with coloring that indicates proper gas coverage. Welds should be shiny, rather than dull and gray.

Brightly colored with an even wave indicate a well-executed fitting attachment weld.

Discoloration in excess of acceptable limits would indicate purging was not done correctly and too much oxygen got into the weld. A “V” wave pattern would indicate too much heat, while a “V” wave pattern with a horizontal line through the middle would indicate both too fast of a travel speed and excessive amps.

Discoloration (left) and a “V” pattern in weld bead (right) are examples of poorly
executed fitting attachment welds.

Assuming properly executed welds, the direct fitting attachment method has been verified by burst testing to achieve the full catalog pressure rating.

Smooth Transition Fitting Attachment

Increasingly, in applications with extremely low tolerance for contamination, the smooth transition fitting attachment method is used. Here, the fitting is reverse beveled on the inside, the hose is often cut at the crest, and a slight gap is left between hose and fitting to allow for complete fusion of components with disparate thicknesses.

This additional preparation ensures a crevice and burr free connection. Crevices, as can be found behind the lip of a standard cut hose or which can result from lack of complete fusion, offer space for residue build-up that could eventually contaminate flow media. Altering the weld geometry with the crest cut and beveled end fitting safeguards the assembly from any debris from the hose cutting process or from operation that could get trapped inside the hose.

Reducing potential for contamination makes smooth transition attachment appealing to users in specific marketing, especially those in the cryogenic and pharmaceutical spaces. Chlorine Transfer Hoses, if they are to be in compliance with The Chlorine Institute’s Pamphlet 6, also must have smooth transition welds.

Here’s a look at how we would prepare a JIC fitting for a smooth transition attachment. Note, if the outside wall of the fitting is already tapered (for example 10° on the weld end of the female JIC) the taper on the inside need only be 10° – 20° to achieve the desired net angle of 20° – 30°.

Common End Fittings for Hose Assemblies

Beyond the pipe threads shown in the picture of nice welds above, other common end fittings include hex male NPTs, female JICs, and various kinds of fixed or floating flanges, the latter paired with stub ends.

Important Design Consideration

The outside circumference of the end fitting must overlap with the edge of the cap pass if the weld is to reach its full pressure bearing capabilities. As tube sizes are measured by outer diameter (OD) and hose sizes are measured by inner diameter (ID), the potential for a mismatch is greatest when attaching tube ends. As an example, there is little overlap between a 1” hose and a 1” tube end.

Some adjustment is needed, especially if the hose will be used in a high-pressure application.  Solutions include opting for a smaller hose so there is more overlap–for instance a ¾” hose with 1” tube ends–or, if the same dimensions are needed, flaring the end of the tube to match the hose in order to create that needed overlap. This latter option would, however, also compromise pressure capacity and is not a sound practice for high pressure applications. Lastly, there are special tube size reducers, but they are expensive and also have lower pressure capacity.

Dissimilar Materials: When is it Okay?

Typically if a hose is 316L, we’ll use 316L end fittings. Using similar materials ensures the characteristics of the parent materials remain intact, and while most welders are qualified to welding procedures for joining similar materials, welding dissimilar materials requires separate weld procedures.

There is a common exception to the rule however! When working with floating flanges or raised face slip-on flanges, you’ll also need a stub end. Since the flange does not come into contact with flow media, only the stub end, the flange can be a different material and there is no issue.

Accessories

While four is the number of welds on a typical hose assembly, there are accessories added to hoses that increase this number. Accessories like an inside interlocked liner–used in high-flow applications–or an outside interlocked armor–used as an abrasion guard or bend restrictor–increase the number of welds on an assembly.

Penflex Welder Training

High caliber end fitting attachment welds are consistently achievable with proper training. Penflex offers a one-week ASME Section IX-certified program for welders with mid-level experience. The training is designed to improve technique, and perfecting fitting attachment welds is one of the skills achieved through the training.

For more information about our Welder Training Program, please click here.

Note: To print this bulletin on fitting attachment welds for metal hose assemblies, please click here.

Note: To print this bulletin on how to prepare NPT ends for leak testing, please click here.

How to Prepare NPT Ends for Leak Testing

Penflex hose assemblies undergo an air under water test followed by a hydrostatic test to ensure there are no leaks and to confirm the structural integrity of the welds.

To perform these tests, special fittings, or “closures,” are needed to create a temporary seal. It is through these fittings that the test media, whether gas or liquid, will flow.

The minimum working pressure of these fittings must meet or exceed the assembly test pressure. For instance, the test pressure in an air under water test for hose assemblies ¼” – 4” in diameter is 75 PSI. The test fittings used must have a minimum working pressure of 75 PSI.

Closures for hydrostatic testing must be stronger given the higher pressures used in these tests which, by default, is 1.5 times working pressure for an assembly with one braid layer. For instance, 4” P4 hose has a working pressure of 300 PSI. Therefore, the hydrostatic test pressure will be 450 PSI and the test fittings used must meet this pressure requirement.

Naturally any test fittings used for a hydrostatic test will work for an air under water test, but the reverse is not necessarily true.

Risk of Damage to End Fittings

Threaded pipe nipples, as well as unthreaded pipe, are susceptible to damage from wrenches during the application of the test fittings.

This is no inevitable result though! A good set-up and careful use of force will ensure no damage occurs.

Attaching the Test Fittings

The process below is designed to minimize contact with the braided section of the hose.

1. Ensure the threads are clean. Use a brush to remove any dirt or debris that could affect the ability to achieve a leak-tight seal.
2. For smaller diameter hoses, a vise grip will keep the hose stable when screwing on the test fittings. Situate the hose in a piece of rubber insulation to protect the outside of the toe nipple before tightening the vise. The vise should never press against the braided hose.
3. Apply a layer of thread sealing to the top half of the threads followed by Teflon pipe tape, wrapped in a clockwise direction. The combination of sealing and tape delivers a leak tight seal without requiring excessive tightening that could deform assembly fitting threads.
4. Screw on the test fitting using the right size wrench, being careful not to overtighten. There is no need to bury the threads all the way into the fitting.
5. Repeat on the other end of the assembly.

Pipe Thread Alternative

A hex nut is an alternative end fitting that is less susceptible to damage.

Note: To print this bulletin on how to prepare NPT ends for leak testing, please click here.

Note: To print this bulletin on radiographic testing, please click here.

You’ve fallen and banged up your arm quite badly. There are scrapes and bruises and you’re in a lot of pain. You worry something might really be wrong and so you go to the emergency room for an x-ray. The radiograph confirms what the naked eye cannot: a broken bone.

X-ray technology can be used to see the inside of a weld as well as the inside of your arm. When electromagnetic waves pass through a material, the image rendered will reveal any imperfections. For instance, the digital reading below from an RT inspection on a butt weld shows slight darkness within the weld. This is an indication of incomplete fusion.

Welds that pass RT inspection will show complete fusion and be absent of any porosity.

The Most Sensitive NDT Methods

Any discussion of non-destructive testing (NDT) for metal hoses would be remiss if it failed to acknowledge the limited scope of methods used.

There are numerous NDT methods, some of which contain multiple techniques for reaching similar conclusions though with varying levels of granularity given the use of different equipment and the adherence to different processes.

Leak testing (LT) is the most common NDT method for metal hoses. All Penflex hoses undergo LT inspection of seam and orbital welds. If being made into assemblies, the hoses are then subjected to air under water and hydrostatic tests to assess end fitting connections. For certain applications, we can use a helium mass spectrometer to identify even smaller leaks. While these techniques allow us to identify current leaks, they offer no indication of whether future leaks may occur.

For this, we may perform penetrant inspection (PT) on the hose. PT highlights surface defects like pits or cracks that could develop into leaks once the hose is in operation.

And, while Radiographic Testing (RT) is most often used on full penetration butt welds on pipe, tank and plate joints, if requested, we can support Radiographic Testing (RT) of hoses–whether that be to inspect the longitudinal seam or the end fitting connection welds. RT is included within a subset of NDT inspection called volumetric testing, which entails looking at the whole mass from top to bottom. These are the most sensitive NDT methods.

Governed by ASME

RT is typically a request in scenarios where failure presents serious risks to safety. These could be applications in aerospace or nuclear power plants, for example. Radiographic testing gives piping system owners and equipment manufacturers greater assurance that components will perform as expected in the intended application. In other applications, such assurances may be unnecessary.

While RT does provide an additional means of measuring quality, it is also a requirement for compliance with ASME B31.3, the organization’s standard for process piping systems. If a plant is ASME B31.3 compliant, its pipe and hoses must be compliant with the code as well. Radiographic testing is among the requirements for compliance.

All NDT examination methods are outlined in Section V of the ASME Boiler and Pressure Vessel Code. Radiographic testing requirements, methods and techniques are detailed in Article 2 of the 1000-page-plus document.

RT in Combination with Other NDT Methods

ASME B31.3 often requires RT inspection in combination with another NDT method, typically visual inspection (VT). If a component fails VT, it would not then be subjected to RT. Beyond avoiding unnecessary testing, combining methods provides a more comprehensive view of the finished assembly in all aspects of surface conditions and weld quality.

The requirements for testing depend on fluid service. For instance, Category D Fluid Service covers non-flammable and non-toxic media flowing through the hose at temperatures ranging from -20°F to – 366°F at a maximum pressure of 150 PSI. Hoses used in this fluid service must undergo air under water and hydrostatic tests, as well as VT inspection, and 5% of each welder’s work is subjected to radiographic testing.

Then consider the requirements for Category M Fluid Service which governs highly toxic media flow. Air under water and hydrostatic tests are followed by VT and finally RT on 20% of all butt and miter joints.

Challenges of Radiographic Testing

The most obvious challenges to RT inspection include the equipment requirements, administration of inspection by qualified personnel, and the necessity of carrying out testing far from personnel or in a protected space where radiation can be contained.

The inspector needs a good set-up to take the right views and must be well versed in what they are looking at. While the code focuses on RT inspection of butt welds, radiographic testing can be carried out on any type of weld. This may present a challenge for metal hosers as some labs may never have seen the cap weld to fillet weld connections that are so ubiquitous in our industry.

For a successful inspection, the tester must understand the “load path” from end fitting to hose into braid.

Radiographic Testing Requirements for a Weld Shop

Beyond satisfying user requirements, RT inspection is used to demonstrate a welder’s capability. As a shop that does work per ASME B31.3, Penflex must test at least 5% of all butt welds performed by each welder on a routine basis whether the work itself has a B31.3 requirement or not.

A welder who consistently passes RT inspection is a very skilled welder indeed.

This butt weld connecting a 304 stub end to a concentric reducer is an example of a high quality weld. Something like this would be used as part of the 5% requirement.

Note: To print this bulletin on radiographic testing, please click here.

Note: To print this bulletin on ASME B31.3 and B31.1 compliance, please click here.

ASME Compliance for Metal Hoses

The terms ASME B31.3 and B31.1 get tossed around a lot in our industry, but what does compliance with these oft-cited standards really mean?

ASME Standards for Piping Systems

The American Society of Mechanical Engineers (ASME) has established numerous standards for the design, manufacture and testing of various “mechanical devices” over its long history. Its aim has been and remains today to ensure safety and reliability.

Among these mechanical devices are piping systems, and one of the first standards issued by the organization was, in fact, related to pipe. “Standard for the Diameter and Overall Dimensions of Pipe and Its Threaded Ends” was issued in 1887 and sought to address issues around pipe standardization emerging with the advent of mass manufacturing.

Since then, all piping standards issued by ASME come under the umbrella of the B31 Series. B31.3 and B31.1 are standards within the B31 Series. There are others.

ASME B31.3 for Process Piping

ASME B31.3 contains the requirements for piping systems found in the industries where metal hose is most often used: petroleum refining, chemical manufacturing, pulp and paper, semiconductors, cryogenics, etc.

It covers everything related to the piping system including those critical flexible components–metal hoses. The sections related to metal hose discuss material composition, design parameters, welding procedures and minimum testing requirements.

Some rules cite compliance with another ASME standard. For instance, standards for flanges are laid out in ASME B16.5. If a hose assembly with flanges is to comply with B31.3, its flanges need to comply with B16.5.

Similarly, other rules cite compliance with standards administered by different organizations. For instance, metal hoses must be specified to BS 6501, Part 1, a standard from the British Standards Institute that compliments ISO 10380, a standard from the International Organization for Standardization.

Categories of Fluid Service

Not all hoses are treated equally within ASME B31.3. The code divides pipes and hoses into six service categories based on flow media and operating conditions.

•  Category D Fluid Service
•  Category M Fluid Service
•  Normal Fluid Service
•  High-Pressure Fluid Service
•  Elevated Temperature Fluid Service
•  High Purity Fluid Service

The parameters for categorization determine testing and design requirements. For instance, pressure ratings of 2500 psi or greater would require compliance with High Pressure Fluid Service, and based on the testing requirements are going to be more applicable to hard pipe than to thinner-wall hose.

Those requirements are found in Chapter IX of ASME B31.3 and include the following:

• Charpy impact testing
• Liquid penetrant examination (LT)
• 100% Radiography inspection (RT)
• Hydrostatic or pneumatic/gas testing at 1.25 times working pressure

These requirements are more stringent than the requirements for hoses used in Category D Fluid Service, where media is non-flammable, non-toxic, and will not exceed 150 psi at service temperatures between -20°F and 366°F.

•  Visual examination
•  5% RT inspection
• Air under water test at 1.5 times working pressure

A hose may be ASME B31.3 compliant if it meets the requirements for Category D Fluid Service, but that same hose will not necessarily also be compliant with ASME B31.3, Chapter IX for High Pressure Fluid Service.

If a hose will not be used in service as defined by Category D, Category M, High-Pressure, Elevated Temperature, or High Purity, it would be considered for use in Normal Fluid Service.

While this is the most common use case for metal hoses, it’s worth pointing out just how comprehensive the code is (fluid service categories are just one example) to highlight that compliance with the code usually means compliance with certain sections or appendices rather than with the entire code.

ASME B31.1 for Power Piping

ASME B31.1 contains the requirements for piping systems in electric power generating stations, industrial and institutional plants, geothermal heating systems, and central and district heating and cooling systems. The flow media is most often water or steam.

With a more defined application, ASME B31.1 does not have the same need for separate service categories. The codes are similar to a certain degree though, broadly speaking, because the impact of a shutdown at a power plant can affect many thousands of people immediately, the requirements are more stringent to ensure a greater level of reliability. As an example, while ASME B31.3 references BS 6501 for the metal hose requirements, B31.1 has specific sections concerning them.

On the other hand, some requirements for hoses are the same. Both standards reference Section IX of ASME’s Boiler and Pressure Vessel Code (BPVC).

ASME Section IX

“Section IX: Welding, Brazing and Fusing Qualifications” of ASME’s BPVC sets the standard for quality welding. Welds must be performed in accordance with a documented Weld Procedure Specification (WPS) by a welder certified to code.

ASME Section IX is similarly comprehensive and not all sections relate to welding metal hose. Due to the quality materials in play, TIG welding is the preferred end fitting attachment method. Only the parts of Section IX relating to gas tungsten arc welding (GTAW), another term for TIG, are relevant.

How Relevant are ASME Codes in the Metal Hose Industry?

These codes are voluntary. Their use ranges across industries and across companies within those industries. Many users don’t require ASME compliant hoses. Many do. Even if there is no requirement for ASME compliance, many components, like flanges, will comply with B16.5 automatically.

There is also overlap between sound design and manufacturing processes and the ASME codes. For instance, Penflex is capable of fabricating hose assemblies to conform to either B31.3 or B31.1, but we would not do so unless compliance is required per the customer.

That said, there are many aspects of our production that inherently align with the code, like our standard air under water leak test. The fact that all Penflex welders are ASME Section IX certified would be another example. End fitting attachment welds are going to be done by ASME Section IX qualified welders, even if there is no requirement for this by the end user.

If compliance is requested, a certification is typically issued by the quality department to confirm design, testing and welding was carried out in accordance with the respective section or sections of the pertinent code.

Along with material selection, code compliance is another decision dictated by the piping system owner. Their ability to communicate clearly the relevant sections of these, or any other, codes helps to identify suppliers capable of meeting their needs.

Note: To print this bulletin on ASME B31.3 and B31.1 compliance, please click here.

Note: To print this bulletin on what’s the difference between static and dynamic bend radius, please click here.

Static and dynamic bend radius are flex hose measurements that relate to two types of applications. Generally speaking, a hose can either be installed in a static application or a dynamic application.

Manufacturer catalogs publish minimum values for both bend radii. Good cycle life is no longer a realistic expectation in the different applications beyond these values.

Static Applications

Consider a hose that’s installed to correct misalignment between a piece of equipment and the piping system outlet. No cycling is expected. The hose will not move once installed. If it is bent during installation, that bend will remain in place for the duration of the hose’s service life.

Without movement, this is a static application.

Dynamic Applications

On the other hand, consider a hose that’s installed in a traveling loop to absorb axial movement in the piping system. Cycling is expected with each expansion or contraction of the surrounding pipe. The hose will experience repeated movements throughout the duration of its service life.

Given there is movement, this is a dynamic application.

If a hose is installed to absorb any movements, the application is a dynamic one. We referenced traveling loops above. Hoses installed to absorb angular or offset movements would also be expected to move dynamically. With a longer list of use cases, dynamic applications are more common than static applications.

Stress That Leads to Fatigue and Failure

Whenever a hose moves, whether in a static or dynamic application, corrugations in the sections experiencing movement–in other words, the sections that are bending–will experience relative movements.

Corrugations on the inside of the bend will compress while corrugations on the outside will extend. The tighter the bend, the greater the deflection.

Comparison of deflection in corrugations
between dynamic and static minimum bend radii for a 2” hose

Imagine a paper clip. Take that straight piece and bend it back and forth repeatedly. What happens? At a certain point, you won’t be able to put the piece back the way it was. Keep going, and eventually the piece breaks off.

This is a great example of how movement stresses material, and at some point how those stresses can exceed material strength. The same is true with metal hoses. With each bend of the hose and concurrent deflection of its corrugations, there is stress.

Why are Static Bend Radius Values Less Than Dynamic Bend Radius Values?

So while the static and dynamic bend radius values tell a user how far a hose may be bent in a certain kind of application before it will deform and no longer perform as expected, with an understanding of bending stress, we can understand why a hose can be bent tighter in a static application than it can in a dynamic application.

One of the stress catalysts has been removed: movement.

Additionally, without movement, there is no expectation that the hose will need to resume its original shape. For this reason, if the hose takes a permanent set in a static application, it’s likely not a problem.

A Bigger Bend Radius is Better

With an understanding of bending stress, it also becomes quite clear that a larger bend radius will reduce stress on individual corrugations when repeated cycling is expected.

Do not take the minimum bend radius values published in your manufacturer’s catalog for your ideal design parameters. Operating at minimums will not yield maximum results. Where the configuration and space allow, we encourage users to opt for a longer hose in order to receive a bigger bend radius.

How are Static and Dynamic Bend Radius Values Determined?

There are known tests for determining minimum bend radius values. For instance, Penflex hoses are tested to international standard ISO 10380.

To determine static bend radius, an unpressurized hose is bent 10 times around a mandrel of specified dimension based on hose size. The hose is then pressurized and subjected to a leak test. If it passes the leak test, then a value for minimum bend radius can be given based on the mandrel’s dimensions.

To determine dynamic bend radius, a pressurized hose is hung in a constant radius traveling loop with a bend radius specified by the standard and subjected to 10,000 cycles. A hose fails if it begins to leak or if the radius is reduced by more than 50% during the test. If neither scenario plays out, the hose passes the test and a value for minimum dynamic bend radius can be given based on the bend radius used for the test.

The standard writes that “the test shall be conducted with the hose at the relevant maximum allowable working pressure” meaning the pressure used to conduct the testing is at the discretion of the manufacturer. Pressures may be reduced to meet the specified bend radius values. This is to say that comparing bend radius values from different manufacturer catalogs is not always an apples to apples comparison.

Note: To print this bulletin on how to handle interlocked hoses, please click here.

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.