Note: To print this bulletin on hose assembly failure, please click here.

Metal hose transfers media that is corrosive or hazardous— and often both. Thus hose failure presents significant health risks to those working in and around the piping system and to the surrounding environment. Depending on the application, hose failure can also damage equipment in which the hose is used, resulting in thousands, even millions, of dollars lost.

Preventing Hose Assembly Failure

As a safety precaution and to protect a company’s machinery, Penflex recommends regular inspection of hose assemblies. Beyond a more formal inspection, all those working anywhere around hose assemblies need to know the most common indicators of hose failure.

If any of the following indicators are observed, replacement hoses should be considered before failure occurs.

1. Loose, broken, bulged, frayed or worn braid.
2. Deformation of the hose, which may include twisting, kinking, denting or flat spots.
3. Slipped, cracked, or dented couplings. If couplings show excessive corrosion, this is also a red flag.
4. Traces of the media that’s inside the hose coming out or being on or around the assembly. This is an obvious indication of a leak.
5. Loose or damaged hose guards or covers.
6. Indications hose or braid corrosion.
7. Loose fitting attachments.
8. Hose assembly that are rubbing against one another or making contact with adjacent machinery or piping.
9. Unreadable or missing identification or tags if this information is required.

Some of these indicators can be fixed before replacement hoses are needed. In the case of loose or damaged hose guards or covers, look to replace the guards or covers first. However, a careful inspection of the hose and braid beneath the damaged protective layer should be carried out before replacements are installed.

Loose fitting attachments may be tightened and new tags may be created if information can be procured from manufacturer. If hoses are rubbing up against one another, we recommend the use of hose buns to create space between hoses or hoses and the ground or nearby machinery.

What to Look for During Inspections

Janet Ellison, Penflex Director of Quality and Engineering discusses these common indicators of hose failure, and underscores the importance of routine inspections, in the video below.

To download a copy of the field inspection checklist, click here.

If you have any questions about hose assembly inspection procedures, please contact us.

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Note: To print this bulletin on leak testing for cryogenic hoses, please click here. 

When it comes to leak testing, Penflex uses one or both of the following methods.

1. A submersion bubble leak test—also known as an air-underwater test—begins by placing dry air inside the hose. Then testers hold the hose under water for at least one minute while looking for bubbles.
2. A hydrostatic test uses water at a higher pressure inside the hose to verify the structural integrity of the end weld connections.

In the case of hoses designed to transfer gas made from molecules smaller than air, we need a testing method capable of detecting smaller leaks. Hoses which have an application in the cryogenic industry fit into this category.

Capable of detecting leaks as small as 10E-9 cubic centimeters per second (std cc/sec)—whereby it would take a year and a half for one bubble to escape under water—a mass spectrometer test using helium gas is a common requirement for these hoses. Mass spectrometer testing is the most accurate way to identify leakage.

In comparison, and in accordance with international standard ISO 10380 for corrugated hose, the leak rate that air-underwater and hydrostatic testing will capture is .001 std cc/sec.

Helium Mass Spectrometer Testing

First developed as part of the Manhattan Project during World War II, mass spectrometer testing relies on a sealed vacuum chamber filled with helium. If helium leaks out of the chamber, the machine will detect the leakage and assess the size of the leak.

When testing a hose, we first need to vent and calibrate the system. We then load the hose assembly into the chamber, connecting it with the inlet of the mass spectrometer machine. Once we have a good seal, we begin the testing process.

Through a series of pumps, the machine pulls a vacuum inside the hose. Once the displayed leak rate reduces to the required testing level, using the hood or the outside-in method, we spray the helium inside the chamber for a couple of seconds until helium saturates the outer dimension of the hose.

Mass spectrometer machine pulling vacuum on chamber containing hose assembly.

Helium is used as the tracer gas because its atoms are among the smallest. They can thus penetrate small leaks quickly. We record the fluctuation in leak rate to determine whether any potential leakage is present. As a policy, we sustain the hose in a helium rich environment for five minutes.

With the recorded values, we draw up a certificate showing that the hose assembly passes the mass spectrometer test.

Ronit Patil, Penflex sales engineer, discusses the process for testing hoses using a mass spectrometer in the video below.

If you have any questions about helium mass spectrometer testing or about how Penflex hose assemblies are used in cryogenic applications, please contact us.

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Often braid must be compressed and secured on the hose prior to welding as the braid must always be reasonably tight on the finished assembly.

Loose braid can lead to two primary problems.

• Under pressure, the hose expands axially, or gets longer, even under low pressure. If the braid is not tight, the hose will grow in length until it takes up all of the slack in the braid. The assembly overall length will increase, and this can cause over bending and kinking of the assembly in service.
• If the braid collars are crimped or a rolled type braid collar clamped in place while the braid is loose, it is possible that the braid is slightly angled or is cocked on the hose. This results in the braid wires along some carriers being shorter than those of other carriers. The shorter wires will be overly tensioned when the assembly is put under pressure. The longer wires have slight slack in them and will not assume any of the pressure-induced tensile load until the shorter wires stretch and all of the wires are of equal length. The assembly is likely to squirm or deform before the braid wires equalize in length.

In Engineering Bulletin #123, we looked at the procedures for tightening loose braid with filament tape, screw clamps and plastics cable ties. In this bulletin, we’ll look at procedure for tightening large bore (8 – 12″ ID) assemblies using the vacuum pump tightening method.

1. Measure the hose length.
2. Calculate 10% of the hose length. (Hose length x .1.) This figure represents the maximum length that the hose should be shortened during the braid tightening procedure. Compressing further may plastically deform the corrugations.
3. Slide the hose into the braid.
4. Place a screw clamp around the braid approximately 2″ behind the hose end and tighten firmly.
5. Place a vacuum plug on one end of the hose. Have an assistant hold the plug in place or tape it in position with filament tape. Make sure that the plug is centered on the hose and the rubber seal is pressing on the corrugation sidewall, not on a sharp cut edge.
6. Connect the vacuum pump to the second vacuum plug.
7. Place the second plug in position on the hose end. (Rather than use two vacuum plugs, at Penflex we typically add a blind flange to a finished end of the assembly and a vacuum plug to the other side to minimize handling of the hose.)
8. Open the vacuum valve.
9. Turn on the vacuum pump.
10. Close the vacuum valve and closely monitor the hose length as it shrinks in length under the braid. Control the vacuum and the hose shrinking in length by partially opening or closing the vacuum valve.
11. When the hose shrinks down to the calculated compression length, close vacuum valve.
12. Place a second screw clamp around the braid approximately 2″ behind the hose end and tighten firmly.
13. Partially open the vacuum valve very slowly. As air bleeds into the hose and the vacuum decreases, the hose will try to extend in length. As the braid is held to the hose by the clamps, the lengthening hose will pull down and tighten the braid.
14. When the braid is tight on the hose, loosen the second hose end clamp slightly, allowing the hose slide under the braid and return to its original length.
15. The few inches of braid between the second end clamp and the end of the relaxed hose can be tightened with one or two added screw clamps or cable ties.

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Note: To print this bulletin about mechanical forming, please click here.

Why corrugate?

Corrugated hose is produced from corrosion resistant alloys and is designed to accommodate the transfer of corrosive media over time in a non-fixed configuration. It will withstand the flow of liquids and gases at temperatures beyond the capacity of hoses made from materials such as rubber, composite or PTFE. Corrugated metal hose also provides the option to operate at higher pressures than these other materials of construction.

Transforming a straight tube into a corrugated hose requires a forming process that creates a series of peaks and valleys that in turn permit the hose to flex. The end result is a hose that can bend, absorb vibrations and respond to thermal expansion without breaking.

There are three basic corrugation processes: mechanical forming, hydraulic forming and hydroforming. All three processes use machines that have many similarities and a few basic differences to create corrugations.

Each process has slight advantages and disadvantages that effect the final product. All are time tested and proven, and no one process can claim superiority over another. In this bulletin we explain the differences between each, so you can determine whether the differences are material to your application and requirements.

Regardless of the forming process used, all corrugated metal hose has variations in the wall thickness between the original straight tube and the final hose with its peaks and valleys. This conclusion is easily confirmed by measuring the wall thickness variations for each forming process.

Penflex mechanical forming processes

At Penflex, we use a split die mechanical forming process for our 700, 800, 900 and 1400 Series in sizes ¼” to 2” while all sizes of the P3 and P4 Series use a rotating die mechanical forming process.

Advantages of split die mechanical forming

As a tube is fed into the corrugator, a ring rolls around it, creating slight indentations at regular intervals. The ring has a smooth radius to minimize stress concentrations. The intervals will become valleys between the corrugations.

Next, a pair of c-shaped tools called “jaws” clamp onto the tube in the interval area. Another pair of jaws clamps onto the tube at the adjacent interval. The first pair of jaws remains stationary while the second pair moves towards the first, causing the tube to bulge out in a curved shape which forms the corrugations. Thus, the crest is formed with material that has been pushed together axially rather than clamped and expanded with internal pressure.

The spacing between intervals determines the amount of material available to form the corrugation. The longer the space between the intervals, the taller the corrugation will be. The wall thickness and shape of the corrugation together are referred to as the “profile” of the hose. One hose has a “higher profile” than another if the difference between its outer diameter (OD) and ID is greater than the difference between the OD and ID of the other hose. Corrugation height, pitch and profile are key factors in determining flexibility.

Key features of this process:

1. 1. Capable of forming heavy wall materials. Split die forming is a robust forming process and can sustain the loads and pressures needed to form thicker materials, or higher tensile materials such as Hastelloy and Inconel. Heavier walled hoses will have a longer life than thinner walled hoses when exposed to media that has predictable corrosion penetration rates and will also have higher working and burst pressures than thinner walled hoses.
   2. ID of hose remains untouched. With a split die process there is no internal interface between any tooling and the ID of the hose, which, of course is the surface where any stress risers or minute scratches can become a point of chemical attack.
   3. Can adapt to work with thinner walled tubes. Split die processes have the added advantage of being able to form corrugations within thinner walled hoses when needed. Overall, they offer a greater range of ability to form wall thicknesses than hydroforming does.

Advantages of progressive die forming

As the tube is fed into the corrugator, a ring rolls around the OD creating a shallow indent into the tube. Through a series of dies that rotate around the tube the indentations are deepened and more closely spaced. The dies are in constant contact with the tube and thus there are far less instances where there are stress risers and virtually no external scratches as can occur with external split dies. Indeed, the progressive dies often have the appearance of polishing the hose, giving it a bright shiny surface. No internal fluid that can become trapped in the corrugations is used and a further benefit of this process is it creates an extremely uniform OD and consistent corrugation shape.

Advantages of hydraulic forming

As the size of a metal tube increases, the wall thickness gets very thin in proportion to its diameter. Consequently, the metal is not as stiff as it is in smaller-sized hoses and could, in fact, be squeezed into the shape of an oval under pressure applied manually.

If the ring process used in mechanical forming were applied, the walls of the tube would buckle in. For this reason, larger hoses with diameters exceeding 2 ½” in Penflex’s 700, 800 and 1400 Series are formed instead by pushing the metal outward from within the tube. This is called hydraulic forming and can be accomplished in several ways.

At Penflex, the metal tube is slipped over a bumping rubber shaped like a donut. The bumping rubber fits just inside the tube. Next, a hydraulic plunger squeezes the donut, flattening it. As the donut widens outward, it pushes on the wall of the tube, while the corrugator advances the tube drawing the material up into a bulge. The peak of a corrugation is thus created.

When the pressure on the plunger is removed, the elastic nature of the bumping rubber allows it to go back to its original shape. The tube then advances a set distance and the process is repeated. In this way, we form corrugations one at a time.

After the peaks, or bumps, are put in the tube, a set of outer split dies grab the tube in the valleys between the corrugations.  The dies press towards each other to form the corrugations.  This step in the process is similar to that used in the mechanical forming corrugation method.

For a quick summary of the mechanical forming processes used at Penflex, watch the video below.

Hydraulic vs. hydroforming

With hydroforming, the force used to push the metal outward to form the corrugation is generated by water instead of a compressed rubber bumper. The water pressure from inside the tube pushes the metal into a die on the outside of the tube which gives the bump its shape. Hydroforming may be used to form the corrugations individually, or in a group of several humps all at once in a multi-station form. In both hydraulic and hydroforming, the tube is pressed outward and formed by split mechanical dies. Where the metal interfaces with the dies the results will be the same whether the movement is created by water or rubber.

Usually, the hydroforming method is used on thinner walled tube and the thicker wall hoses are formed using the “bumping rubber” hydraulic method. In addition, the hydroformed hose has to clamp all around the hose in two places to seal the internal section of tube to be formed. This clamped section often has a different OD than the rest of the hose and presents a pattern of one small corrugation spaced out by a series of larger corrugations. This difference in ODs could create a corrugation where the braid is not in tension with the hose under pressure.

Misperceptions around hydroforming process

The term hydroforming can be highly misleading as it connotes a soft, harmless forming process, which is not the case. In no way is an assertion that hydroforming is superior to other methods provable.

Hydroforming uses liquid, water or oil, under internal pressure to force the straight tube out against external split die mechanical dies. The dies can be a set of two dies, or in some cases the dies are set up with a series, with 5-6 split dies joined in sequence to make one set of corrugations per machine cycle. The dies compress inward, while collapsing together axially at the same time that the tube is pressurized. The external split dies, a ring split in half, can exhibit wear like any die, and the compression inward with worn dies can cause scratches and stress risers. This is inevitable and unavoidable whenever split dies are used that are not properly maintained.

In essence this hydroforming uses the same process as Penflex’s mechanical process, except that water forces the tube outward in hydroforming in contrast to the ring rotating around the hose forms the tube inward in a mechanical process. Hydroforming starts with a straight tube that is the ID of the hose and Penflex’s mechanical forming starts with a base tube that is close to the OD of the hose. In the same fashion the pressurized water does the same thing as a compressed rubber donut does in the hydraulic process. It makes no difference if the straight metal tube is formed using fluid or rubber, both forces press against the same kind of dies.

Conclusions

Hose flexibility is not a function of the forming process. A hose’s ability to bend is determined by a myriad of factors—inside diameter (ID), outside diameter (OD), corrugation height, pitch and profile, and wall thickness.

Variations in wall thickness are a result of all forming processes. All hose has variations from the original strip thickness at the peaks and valleys. Forming metal inherently rearranges its structure, thus to claim that there is no adverse impact to wall thickness is demonstrably not true.

Comparison of Wall Thickness in Mechanically Formed and Hydroformed Hoses
(Per findings of Metal Hose Consulting, an independent testing laboratory. Contact us for further details.)

Hydroforming claims to reduce concentrated residual stress. This claim is not unique to hydroforming as the design of the tooling and the ability to avoid stress concentrations in the hose geometry are much more important than the medium used to apply forming force. For instance, if the tooling of a mechanical forming machine maintains a smooth, tangent radii with no discontinuities concentrated residual stress will also be reduced. This is the case in the manufacture our P3 and P4 products which use a progressive die system on the outer diameter of a hose to create corrugations.

Hydroforming claims to minimize work hardening. By definition all forming work hardens the base metal present in the manufacture of corrugated hose regardless of the manufacturing process used. As all hoses are made using external dies all hoses will exhibit similar effects of work hardening. In fact, some hardening is a good thing as we want a hose to be flexible, and to return to a natural state without deformation. If the final hose was supplied in the same annealed condition as the original strip it would take a permanent bend when flexed, just like a paper clip when it is bent. With some work hardening the hose will want to return back to its original state. Work hardening also improves the pressure capability of a hose. If a hose was made with minimal hardening of the base material, which is fully annealed, then it would be soft enough to fail at a lower internal pressure than hose that has had work hardening. We have tested hydroformed hose against both P3 and P4 hoses and found that the actual burst tests are lower than P3 and P4.

Comparison of Burst Tests in Mechanically Formed and Hydroformed Hoses
(Per findings of Metal Hose Consulting, an independent testing laboratory. Contact us for further details.)

Hydroforming claims it is a clean process which uses only water to form the hose while most other products require lubrication. Hydroforming may use water to create the corrugations that create metal hose, but that water is forced through the tube using machinery, namely hydraulic pumps, that do require lubrication. It’s not just water that is used in this corrugation process. Additionally, water has to be removed from inside the hose. At Penflex, all of our processes use an emulsifiable mineral oil diluted with water where the concentration is less than 0.01%, which is a considered a “clean process.”

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California Proposition 65 Statement

A California law, Health & Safety Code § 24925.5 et seq., better known as Proposition 65, requires that warnings be provided for products that may expose individuals to trace amounts of lead and other substances, irrespective of their safety or compliance with other laws. Penflex Stainless Steel, Inconel, Monel, Hastelloy and Bronze Hose and Braid are considered “lead free” and safe to convey or dispense water for human consumption through drinking or cooking under federal law, but because they may contain trace amounts of lead (less than 0.05% by weight) we provide these products with the following warning under Proposition 65, which applies only in the State of California.

The following is the label we use and we suggest that any party using our product and repackaging it for shipment to California download the label and attach it to the product.

View California Proposition 65 Label

Using corrugated hose for vacuum applications, both in partial and full vacuum applications, is common. Under vacuum, a hose will compress axially, and may even squirm. In this situation, the braid loses its pressure-carrying function and does nothing except act as a cover to protect the hose. If the ends of a hose assembly are securely attached in known locations, squirm most likely won’t be a problem. The most important thing is to assure that there can be no lateral buckling. We always suggest that you incorporate the “cautionary statement” below when supplying a hose under external pressure since the majority of our testing and the normal use of metal hose is internally pressurized.

For the statement involving the use of corrugated hose for vacuum applications and for further information, click here.

P4 corrugated hose in sizes ¼” to 2″ is manufactured and tested in accordance with the ISO 10380 International Standard as Type 2-10 hose. ISO 10380 conformity is another example of Penflex’s commitment to provide exceptional products to our customers.

ISO 10380 is an International Standard which specifies the requirements for the design, manufacture and testing of corrugated metal hoses and hose assemblies for general purposes.

The Standard covers sizes from DN 4 to DN 300 (1/16″ to 12″), Working Pressures from PN 0.5 bar to PN 250 bar (7 psi to 3625 psi), specifies pressure derating factors for elevated temperatures, two methods of construction and three types of flexibility of hose assembly.

For Type 2-10 hoses, the bend radii for testing each hose size is defined by the Standard for both static (Pliable) and dynamic cycling (U-Bend) testing. The pressure in bars for which a hose shall meet all requirements, must be selected from one of the pressures listed in section 5.7.2 of the standard. When cycle tested in a U-bend, hoses shall have an average life of 10,000 cycles.

P4 Products: ISO 10380 Qualifications [Type 2-10 Hose]

If you have any questions or comments, please contact us.

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Note: To print this bulletin on calculating maximum allowable working pressure, please click here. 

As defined by the international ISO 10380-2012 standard, Maximum Allowable Working Pressure (MAWP) is the maximum pressure for which the hose assembly is designed. MAWP is specified by the manufacturer.

It is commonly accepted in our industry that the MAWP of an assembly should be no more than one-fourth of the burst pressure, or the pressure at which the assembly ruptures. This gives the assembly a safety factor of 4:1.

To establish Maximum Allowable Working Pressure, a straight hose assembly with a live length equal to ten (10) times its nominal diameter, but not less than twenty (20) inches, is subjected to gradually increasing hydrostatic pressure. The pressure at which the assembly ruptures is recorded as the burst pressure of the assembly.

To calculate a number for Working Pressure which we publish in our catalog, we would first apply the 4:1 safety factor mentioned above and then reduce the result by additional 20% to account for welding as the method of attachment.

The reason for this additional reduction is based on the fact that tensile strength of the braid wires located in the Heat Affected Zone (HAZ) might be affected by the heat and become lower.

An example

If the burst pressure of the hose was 1000 psi, the published MAWP calculated as the following:

1000 / 4 = 250 psi;

250 * 0.80 = 200 psi

For double-braided hoses, the MAWP published in our catalog is based on the addition of 60% to the number we published for single-braided hoses.

Therefore, if the single-braided hose assembly had a published WP of 1000 psi, then the MAWP of the double-braided hose would be calculated as:

1000 x 1.6 = 1600 psi

Note:

• MAWP published in our catalog is the maximum working pressure at 70°F. At elevated temperatures, the Temperature Adjustment Factor has to be applied. Please refer to our catalog for Adjustment Factors at different service temperatures.
• It is essential that the maximum operating pressure, including the surge pressure to which the hose is subjected in service, not exceed the specified Maximum Allowable Working Pressure.

Practical points to note:

• When making an assembly, it is important to make sure that braid is tight on the hose and all braid wires end flush with ends of hose.
• Testing has shown that the braid sleeves must be snug on the hose or the burst value of the hose will be significantly reduced.

For more information, please contact us.

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The Non-Destructive Testing Program (NDT) at Penflex is ASNT compliant and will offer these services to customers as well as perform customer requirements for NDT Inspections.

The program has three in house NDT Methods:

• Visual Inspection (VT)
• Liquid Penetrant Inspection (PT)
• Leak Test Inspection (LT)

SNT-TC-1A Level III oversight of the program’s Level II’s and testing techniques is part of that program. Two SNT-TC-1A Level II NDE’s are qualified in three methods of NDT Inspections as well as comply to ASME Section V Articles 6, 7, 9, and 10. Penflex has a LTA with an A2LA ISO 17025 Accredited Metallurgical testing lab and can provide those services as well.

In addition, Penflex has a Welding Program that includes ASME Section IX, PED 287-1, EN ISO 15614 Weld Procedure Qualifications with ISO 3834-2 Welding Coordination compliance in process.

The Welding Program includes full-time staff with in house CWI/CWE for continuous improvement of welding processes as well as ongoing development of Welding Matrixes formatted and maintained in an auditable position.

Developing and qualifying Welding Procedures in exotic alloys for customer’s requirements can now be done in house. Penflex’s CWI,CWE,NDE Level II Inspector has 35 years of experience in the Pressure Vessel, Piping, Structural, Fabrication and Welding industries.

Please contact us for further details on the Program or for pricing for Non-Destructive Testing services.