Note: To print this bulletin on Positive Material Identification, please click here. 

The right materials are important to any design. In selecting the appropriate alloy for a metal hose assembly, engineers consider the temperature, pressure, movement—be it flexing, vibration or fatigue—and rates of corrosion anticipated within the given application.

It’s a careful calculation with many variables and selecting a less-than-ideal material could lead to faster rates of corrosion, decreased efficacy as a transfer medium, and premature hose failure.

While Penflex does specify the chemical and physical properties for our stainless steel, Hastelloy, Monel, Inconel and Bronze hoses with Material Test Results (MTR) from our raw material suppliers, some end users want additional assurance in the form of a final verification.

This final verification is Positive Material Identification (PMI) testing conducted before the hose assemblies are shipped to their final destination. PMI testing analyzes the composition of a component by reading quantities of its constituent parts, often delivered in percentages. Penflex uses an alloy analyzer that leverages X-ray fluorescence technology for its PMI testing.

Penflex Director of Quality and Engineering Janet Ellison demonstrates how PMI testing works and precisely why it can be an additional procedure worth undertaking in the video below.

The composition of stainless steel alloys defer and one of the key differences between 304 and 316 is 316 contains more molybdenum (Mo) for increased corrosion resistance. Assuming the application requiring these flanges involved corrosive media, this catch was an important one that would not have been made without PMI testing.

For a sample PMI test report, please click here.

For any other questions about alloy analysis, please contact us.

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A cycle life comparison between annular and helical designs.

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Annular and Helical

Broadly speaking, there are two types of corrugated metal hose. This categorization is based on the orientation of the corrugations and is the result of different forming processes.

Annular Hoses

Annular hoses have convolutions that line up parallel to one another at right angles to the longitudinal axis of the hose. The corrugations of an annular hose may look like a series of complete circles or rings. These hoses can be formed by a number of metal forming processes, including split and rotating die mechanical forming processes, hydraulic forming, and hydroforming.

Helical Hoses

Helical hoses have one continuous convolution resembling a screw thread. They are generally formed using a single rotating die that is in continuous contact with the outer diameter of the tube as the tube progresses longitudinally.

Annular hoses are more commonly used than their helical counterparts, with some estimates suggesting annular hose comprises 95 percent of the corrugated metal hose market.

Helical hoses aren’t as common as they once were and, while there are a few advantages cited, we remain skeptical of these claims. It has often been said that when a helical hose needs to be drained, it can be hung from one end and the liquid will follow the helix and drain from the hose. While it is very difficult to “drain” all traces of a liquid from an annular hose, we also believe that liquids adhere to the walls of helical hoses the same as they do to the walls of annular hoses, and that it takes more than “draining” to remove all traces of liquid.

In some cases, when fully compressed, helical hoses can achieve a higher working pressure than a standard pitch hose. This may make them the right choice in industries like bottle filling, but for the most part, users prefer annular hoses for their respective applications for a number of reasons.

For instance, the parallel convolutions of annular hoses make them easier to cut and easier to weld. And, as our recent U-Bend Fatigue Test demonstrates, annular hose has far better flexing characteristics than helical hose.

U-Bend Fatigue Test

Recently we performed a test to compare annular and helical hose. Specifically, the U-Bend Fatigue Test compared the cycle life of our P3 Series annular and our 400 Series helical hose. We used three six-foot assemblies of ¾-inch single-braided hose from each series.

While the pressure and bend radius were specific to a customer’s request, the test format was carried out in accordance with ISO standards 10380:2012(E). Each assembly was mounted onto the U-Bend Tester to form a vertical loop and then subjected to repeated sinusoidal flexing at a rate of 20 cycles per minute in a direction parallel with the axis of the hose.

The test pressure was 300 PSI, the dynamic bend radius was 6.0 inches, and water was the media. Failure would take place if there was any leak—marked by a drop in pressure—or if there was localized reduction of the hose radius more than 50% during the test.    

Results

At approximately 200 cycles the helical hose samples all demonstrated a twist sideways in sync with the bias of the hose when in its relaxed condition. This bias comes from the impact of a single die that rotates around the tube to form the hose. Under pressure a helical hose will torque as it elongates and place pressure on the cap and fitting welds at the ends of the hose. Annular hose does not exhibit these characteristics.

As expected, we got longer dynamic cycling out of annular hoses as flexing happens between the convolutions rather than through them.

For the helical hoses, the cause of failure appeared to be a stress fracture. (See image below on left.) For the annular hoses, it was the rubbing between hose and braid that slowly notched a hole into the surface. (See image below on right.)

Impact of Oiling Hoses on Cycle Life

While the cycle test outlined above was conducted without oiling the hoses—a standard requirement for a test such as 10380:2012(E)—hoses may be oiled in service to prolong their service life. Without lubrication, braid friction may lead to premature hose failure.

Burst Testing at a Million Cycles

In a third test, we oiled a new set of three six-foot  ¾” single-braided P3 assemblies and took them all to 1,000,000 cycles with the same pressure and bend radius as used in the previous test. The hoses were then removed from the U-Bend Tester and burst tested.

The goal was to see if cycling would affect burst pressure. Turns out, the oiled annular hoses were in almost the same condition as before cycling.

Achieving similar burst pressures between the original state and after one million cycles speaks to the quality of Penflex P3 Series and structural integrity of our welds.

To read the complete report, click here. For any questions about the U-Bend Fatigue Test, please contact us.

¹ Nominal Burst represents maximum pressure at 70°F.
² For more details on Penflex P3 Series Stainless Steel Hose, click here.

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Note: To print this bulletin on turnaround hoses, please click here.

Turnarounds are more than a routine maintenance event. The pause in production and significant costs for labor, equipment and materials make them a very expensive whole-business event, one that impacts a refinery’s workforce, its customers, and even its shareholders.

Every project has its own characteristics which makes each one a challenge.

While a poorly executed turnaround can be a major setback for a refinery, one that is well-executed can mean competitive advantage, increased commercial performance, improved operations and safety, and heightened morale. The stakes are high.

Turnaround Hoses

Understanding the mission-critical role metal hose plays in these events, Penflex designs and manufacturers assemblies to provide safe, durable and corrosion resistant options for refineries and turnaround operators.

Our hoses are often used as hydrocarbon drain lines, but other common applications include steam injection lines, transfer lines from storage tanks and vessels, and temporary bypass piping. In short, they are used anytime highly corrosive media needs to be transferred.

With contractors onsite and employees performing tasks outside their usual scope, safety is a top priority during turnarounds. And while the market is moving away from rubber hoses given the likelihood of catastrophic failure decreases when using metal, metal offers the additional benefits of corrosion resistance, the ability to work at higher temperatures, and overall durability.

Key Benefits

To ensure a longer life in service, Penflex turnaround hoses incorporate these characteristics.

• Heavy wall. Our line of heavy wall hoses offers greater corrosion resistance than do standard duty metal hoses, especially in areas where aggressive chemicals are being used.
• Double braid. In addition to increasing the working pressure of a hose, the second braid layer protects the first. It’s a proactive approach given how hoses are handled during turnarounds.
• Purging of welds. Argon purging of welds ensures clean, contamination-free welds. With numerous chemicals involved in turnaround operations, this approach delays corrosion.

Other key characteristics include:

• Lightweight. Our hoses are easy to move throughout turnaround operations and workers are less likely to hurt their backs in the process.
• Pressure rated Type A stub ends. While many of our competitors use non-pressure rated Type C stub ends, we use pressure rated Type A stub ends.
• Tested to 1.5x working pressure. Penflex uses conservative working and burst pressures for our metal hoses.

With a focus on safety, durability and corrosion resistance, Penflex offers a complete range of metal hoses with options for wall thickness, flexibility and pressure ratings to provide the ideal solution for a refinery’s unique turnaround needs.

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

Thin wall hoses at close corrugations are—on account of their geometry—highly flexible hoses and typically do show better results in cycle tests. Thus, one of the reasons flexibility is a key point of focus for many metal hose users is it is often equated with service life.

But to treat flexibility and service life synonymously misses the big picture.

In an earlier bulletin, we discussed the numerous aspects of hose geometry, braid construction and material composition that impact flexibility and touched on the tendency to oversimplify and point to a single factor.

Similarly, to say that a flexible hose will have a longer life in service is too easy.

In this bulletin, as we talk about flexibility, we are referring to how many cycles a hose can go at a certain pressure.

Once in service, various operating conditions—like bend radius and internal pressure—can impact the flexibility of a metal hose assembly.

Additional variable: Bend radius and flexbility

Bend radius is the radius of the bend measured at hose centerline. The smaller the bend radius, the more deflection and stress there will be at each corrugation, accelerating hose failure. Thus, the larger the bend radius, the longer the life of the hose will be.

There’s little downside to increasing the bend radius, while decreasing it too much can overwhelm a hose’s flexibility, and result in deformation. For this reason, a larger bend radius is recommended when repeated bending stress is anticipated.

In applications subject to flexing, whether it be static or dynamic flexing, it’s important to calculate the Minimum Bend Radius and the Minimum Dynamic Bend Radius.

• Minimum Bend Radius. The smallest radius to which a hose can be bent without being deformed.
• Minimum Dynamic Bend Radius. The smallest allowable radius of a hose being used in an assembly in dynamic movement service.

To understand how long a hose must be to withstand a certain bend radius, use the Min Live Length Calculator within our Technical Tools section or reference the Vibration and Minimum Hose Length Engineering Bulletin to see the relationship between hoses with nominal OD from ¼” to 12” and the minimum live length for vibration required.

Additional variable: Internal pressure and flexbility

Furthermore, the pressure at which media is moved through the hose as well as any external pressure the hose may be subjected to can impact flexibility, or rather alter the flexibility required for a particular application.

The flow velocity in corrugated metal hose should never exceed 150 ft/sec for gas or 75 ft/sec for liquids, but when a hose is installed in a bent condition, the flow values should be reduced proportionally to the degree of the bend.

Where the flow velocity exceeds these rates, an interlocked metal hose liner or larger hose ID is recommended.

Specific pressure ratings for each type of Penflex metal hose are found in our catalog.

Janet talks us through a few considerations around variability in a hose’s flexibility in the video below.

For any questions regarding metal hose flexibility, please contact us.

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Please note: To print this bulletin on hose flexibility, click here.

Assemblies transferring corrosive media at various temperatures, from very low cryogenic temperatures to temperatures as high as 1500°F, often under pressure and usually subject to some form of motion—be it flexing, vibration or fatigue—require metal hose. There’s simply no other material suitable for such applications.

A metal hose’s ability to bend in response to static or dynamic movement without deforming is one of its key characteristics. It’s also the most difficult to understand given the number of inputs that ultimately determine how a hose bends.

The context in which flexibility is discussed can be cause for further confusion. We could be talking about how easy a hose is to bend, how acutely it may be bent, or even how many cycles it can go at a certain pressure.

It’s no surprise then that explanations of hose flexibility are prone to oversimplification. In this bulletin, as we discuss flexibility, we are referring to how large or small a hose can be bent as well as how easy it is to bend that hose.

The overlooked role of hose geometry in flexibility

There is a common misperception that the corrugation forming process (i.e. mechanical, hydroforming, etc.) determines how flexible a hose is. But that’s too easy. Rather, it is the geometry of the hose that dictates just how flexible it will be.

While the corrugation forming process does impact wall thickness—and variations in wall thickness are a result of all forming processes—it is important to highlight that the forming process itself does not determine hose flexibility.

When we say hose geometry, we are referring to these characteristics:

• Inside diameter (ID). ID is the distance between opposite points inside the hose.
• Outside diameter (OD). OD is the distance between opposite points outside the hose.
• Wall thickness. Base metal thickness plays a role in the eventual thickness of hose wall.
• Corrugation width. This is the width of an individual corrugation.
• Corrugation count. Also called the pitch, this is the distance from one corrugation to the next or the number of corrugations per foot.

In the video below, Penflex Director of Quality and Engineering Janet Ellison discusses the characteristics described above.

Additional characteristics to consider in hose assemblies

When considering braided hose assemblies, braid design is an additional variable. When we say braid design, we are referring to its construction which includes the following properties.

• Number of bands of wires or number of carriers
• Number of wires per band
• Diameter of the wires
• Angle of the braid from longitudinal axis

Material selection: The part mechanical properties play

Changes in the mechanical properties of the strip or wire used to make braided hose affect the force needed to bend a hose, and thus impact its flexibility. When we talk about these mechanical properties, we are referring to:

• Tensile strength. Ability to resist tension, or the forces that elongate a hose.
• Temper. Delivered though heat treating, temper refers to the toughness of a hose, or its ability to absorb energy without fracturing.
• Elongation. Amount of strain a hose can experience before failure.

Increases in tensile strength, temper and elongation increase the force required to bend a hose, thereby reducing flexibility.

For example, a high nickel, high tensile material such as Inconel 625 ™ requires more force to bend than stainless steel. On the other hand, Monel 400 ™ has a lower tensile strength and requires less force to bend than 321 or 316L Stainless Steel.

When considering flexibility from this perspective alone, we could say that Monel 400 ™ hoses are likely to be the most flexible, followed by 321 or 316L Stainless Steel hose and finally by Inconel 625 ™ hoses.

Hose flexibility: A complex calculation

All of the attributes listed above play a role in the flexibility of a hose or braided hose assembly, and each one is taken into consideration individually as well as collectively when designing the right solution for a particular application.

For instance, if all else remains the same, increasing wall thickness will decrease flexibility. Alternatively, decreasing wall thickness will increase flexibility.

However, increasing wall thickness can be offset by increasing the corrugation count, or increasing the OD—or both—to retain flexibility. Of course, all these characteristics are limited by the ranges within which changes can occur.

If a wider corrugation is desired to reduce the metal content and thus the cost of a hose, a thinner wall hose can be used to retain flexibility and further reduce weight per foot. These changes, however, reduce burst pressures and the hose will fail earlier than a heavier hose when subjected to the predicted penetration of the media.

The important point to keep in mind is that design of a braided hose is always a matter of optimization, and the effects increasing or decreasing one of the above characteristics can be offset by other changes to the design to achieve the goal set for a particular hose.

To learn more about the impact of operation conditions like bend radius and internal pressure on flexibility, take a look at Engineering Bulletin #140 on the topic.

For any questions regarding metal hose flexibility, please contact us.

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Please note: To print this bulletin on hydrogen embrittlement, click here. 

A form of stress corrosion cracking, hydrogen embrittlement is the loss of a metal’s ductility and subsequent inability to maintain its load bearing capacity due to the absorption of hydrogen.

As a result, metal will crack or fracture under stresses less than yield strength. The difference between anticipated and actual stresses that lead to failure is determined by various factors including the amount of hydrogen absorbed.

Several forces must be in play to create a situation where hydrogen embrittlement is present. First, a susceptible material. Second, exposure to hydrogen. Lastly, stress on the component. Failure due to hydrogen embrittlement cannot happen without the presence of all three.

Hydrogen embrittlement process

Even at room temperature, steel can absorb hydrogen atoms. Once absorbed, the atoms recombine to form hydrogen molecules. Over time, these molecules diffuse throughout the metal and form bubbles at grain boundaries. The bubbles exert pressure which weakens the metal, eventually reducing ductility and tensile strength.

Situations leading to hydrogen absorption

The likelihood of hydrogen absorption can increase with various manufacturing and operational processes involving heat. This is due to solubility of hydrogen.

When it comes to metal hose, welding presents an opportunity for hydrogen absorption. At Penflex, we use TIG welding on stainless steel. While this approach to welding doesn’t typically lead to hydrogen absorption, we still take precautions to prevent it. This includes material preparation and post-weld cleaning to remove any residual carbon content.

Once in use, hydrogen absorption can occur when a component is exposed to chemicals or if it has experienced some kind of corrosion. While we seldom encounter hydrogen embrittlement, the applications where premature cracks likely occurred as a result of it involved hydrogen gas.

Preventing hydrogen embrittlement

Thicker materials with higher carbon content are often more likely to experience this kind of the stress corrosion. If hydrogen absorption is expected to occur during service, lower carbon steels might be considered. Penflex offers L grade stainless steels—like 316L—in such circumstances.

Baking metals is a common means of removing hydrogen during the manufacturing process as is avoiding quick changes in temperature that might lead to condensation. It’s also important to keep hose assemblies off the ground and protected from exposure to chemicals which might lead to corrosion and premature failure.

If you have any questions, please contact us.

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While we use stainless steels for their resistance to corrosion and strength at high temperatures, we know that all stainless steels are not created equal.

Most Penflex hose and braid products are made with 304, 304L, 316, 316L and/or 321 stainless steel as these are the most common austenitic steels. These iron-chromium-nickel steels are among the most corrosion resistant of all stainless steels thanks in large part to their chromium content.

Chemical Composition Requirements (%)
ASTM A240

Carbon hardens iron and thus its primary role in these ferrous alloys is to increase material strength. In general, the strength and therefore the pressure rating of metal hose decreases as the temperature increases. Thus, as the operating temperature of a metal hose assembly increases, the maximum allowable working pressure (MAWP) of the assembly decreases.

When H Grade Stainless Steels are Specified

In Engineering Bulletin #112, we discussed how to calculate MAWP of an assembly at operating temperatures above 70°F [21°C], but special considerations must be taken into account when operating temperatures exceed 1000°F [540°C].

In these instances, ASTM A240 specifies the use of “H grade” alloys (i.e. 304H, 316H, 321H) for ASME applications. H grades have a higher carbon content than the standard and L grades of their corresponding alloys which ensures increased strength at elevated temperatures. They must also have a grain size of ASTM No. 7 or something even more coarse.

Alternative Alloys

In ASME applications where service temperature exceeds 1000°F—as our 321 and 316L hose and 304L braid alloys are not H grade—we move up to an alloy even better for high temperatures and that is Inconel 625.

The maximum service temperature of Inconel, a superalloy with nickel as its primary metal, far supersedes that of most austenitic stainless steels. For example, the maximum service temperature of Inconel 625 is 1800°F [982°C].

To compare the maximum service temperatures of austenitic steels and other common metals, take a look at Engineering Bulletin #106.

If you have any questions about the materials best suited for your applications, please contact us.

Alternatively, Janet Ellison, our Director of Quality and Engineering, offers a few additional insights in the video below.

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Sources include ASTM A240 Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General Applications.

Note: To print this bulletin on interlocked liners, please click here. 

High flow velocity can cause high frequency vibrations. Under such circumstances, corrugations may move all over—from both “inside and out” as they are all connected—leading to cracks in over-stressed zones. Neither the crests nor valleys of the corrugations are protected from this kind of stress.

Flow Velocity

To avoid circumferential cracks, we have maximum recommended velocities for gas and liquid flowing through the hose. The rates are determined not only by the type of media but also by the configuration of the assembly. The following table comes to us from NAHAD Metal Design Guide, Section 5 – Liner to Handle High Media Velocity.

Maximum Recommended Flow Velocity

These recommendations come to us from NAHAD, but it is worth noting that there are other international bodies with different limits. For instance, the British Standard 6501 lists the maximum flow velocity for gas in a straight run of hose at 60 m/s, or 197 ft/sec. On the other hand, ISO 10380 lists the maximum at 30 m/s, or 98 ft/sec for gas.

When flow velocity exceeds the maximum recommended rate, we suggest lowering the velocity by using a larger diameter hose with reducers at each end of the hose to connect it to the piping system.

When this is not practical, an interlocked hose can be placed inside the corrugated hose as a liner. The interlocked liner helps prevent turbulence from high velocity flow from creating vibration in the corrugations. High frequency vibration of the corrugations could lead to circumferential cracking and—thus—assembly failure.

Usually, the flow liners are welded to the end fittings or near the end cap welds. On long assemblies, consideration should be given to the state of the interlocked hose (whether it should be mid-state or closer to fully extended). Some designers may weld the liner only on the in-coming end and allow the liner to extend into the inside of a pipe end on the outlet side to avoid any impact that variable high velocity flow could have on the end connection weld.

Construction of Hose Assembly with Interlocked Liner

How High Can Velocity Go with a Liner?

There is no mention of maximum flow velocities for hoses with a liner, but there is a general consensus within the metal hose industry that staying within 2 – 2.5 times the unlined hose limits is prudent. Velocities in excess could likely result in damage to the liner.

There are several considerations when taking the approach to add interlocked liners which Penflex Director of Quality and Engineering Janet Ellison discusses in the video below.

If you have concerns about flow velocity in a hose assembly or want to know whether your assembly requires an interlocked liner, please contact us.

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Note: To print this bulletin on stress corrosion cracking, please click here.

Stress Corrosion Cracking (SCC) happens at the intersection of a susceptible material, working or residual stress experienced above the SCC threshold, and a corrosive environment. The cracks that develop under this unique set of circumstances would not have developed under the stress or within the given environment alone.

Bridges have collapsed, ceilings have fallen in and the list of fatal structural failures caused by SCC goes on. Leaks caused by SCC have also led to catastrophic failure. Typically, such failures are seen in pressure vessels, pipework, highly stressed components and in systems when an excursion from normal operating or environmental conditions occurs.

The problem with SCC is that it seems to happen ‘unexpectedly’ during a period of satisfactory service. As the environment is generally mildly corrosive to the service material, cracks can easily go undetected. Metals or alloys can look bright and shiny despite being filled with microscopic cracks.

Controlling for Stress Corrosion Cracking

To control SCC, engineers must first select a material that is not susceptible to the service environment. Then they must ensure that any temporary changes to the environment do not alter that material’s susceptibility. For example, cleaning is a change with potentially adverse affects. Residues are often left behind.

Material and environmental pairings where SCC is most likely to occur include:

• Brass and Ammonia
• High-Strength Steels and Hydrogen
• Stainless Steel and Chlorides

However, as mentioned earlier, it isn’t just the impact of environmental corrosion on a material but the combination of environmental corrosion and the application of a tensile stress above critical values that leads to SCC.

The stresses that contribute to SCC are produced either as a result of the use of the component in service or as a result of the residual stresses introduced during manufacturing. When the likelihood for SCC is high, engineers must carefully design piping systems to minimize stress concentrations. They may also suggest various heat treatments to reduce residual stresses.

Other means of controlling SCC include using corrosion inhibitors during cleaning operations, employing a closed system to control the environment, and coating the material to isolate it from the environment.

It’s important to remember that while SCC is a type of corrosion, not all types of corrosion are classified as SCC.

Penflex Applications Engineer/Technical Sales Igor Smola discusses Stress Corrosion Cracking in the video below.

For further reading on environmental corrosion, please see:

• Engineering Bulletin 119: Corrosion of Common Alloys in Dry Chlorine (Cl2)
• Engineering Bulletin 115: Environmental Corrosion of Stainless Steel
• Engineering Bulletin 105: Chloride/Chlorine Levels and Stainless Steel Alloy Selection

If you have any questions, please contact us.

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