The braid on a metal hose must be kept in tension if it is to play its role as pressure carrier well. Without it, under pressure, a hose will grow back into a tube.

Compress a hose axially, and the braid will fall out of tension. Therefore, we do not use braided hoses to absorb axial movement. (A hose can absorb axial movement in a piping system if it is hung in a loop. In this configuration, the hose does not move axially though.) Fail to capture all braid wires in a cap pass, and those wires left out are no longer, or rather, never were, in tension.

Pressure ratings depend on all braid wires remaining in tension during operation. Theoretically, every wire carries an equal portion of the pressure and a few loose wires can reduce a braid’s pressure capacity below the intended working pressure.

The Cap Weld

Cap pass welds connect a hose and braid. They precede any fitting attachment welds and are judged based on several criteria, whether all braid wires are captured chief among them.

Beyond a reduction in pressure ratings, once wires pull out from the cap pass, the area of the hose just behind it becomes more susceptible. If any cycling or bending is taking place, fatigue will set in sooner, resulting in premature cracks and, eventually, failure.

The image below offers an example of braid wires that were not captured in the cap pass. In this instance, given the number of wires and their uniform geometry, it is likely significant sections of the braid slipped below the top of the ferrule during the welding process, never even coming into contact with the puddle spanning hose to ferrule.

If only a few wires were missed, localized stress at the end fitting caused by mechanical bending, vibration or other application forces could have contributed to pulled out wires.

Approaches to Welding the Cap Pass

Aside from skilled welding technique, the best way to ensure all braid wires are captured is to pull the braid 1/16” above the ferrule before welding.

Keeping the braid flush with the hose may make for easier welding, but the likelihood of failing to capture all braid wires offsets any potential gains in time saved by taking this approach.

Capturing every braid wire is not the only consideration for a good cap weld. A common approach, known as the burn down method, accomplishes this goal but fails to take into account two important details.

First, this weld cannot be purged and therefore ignores the metallurgical effects of welding. (For more information about argon purging, and its role in delivering high quality welds, have a look at this bulletin).

Second, the uneven geometry reduces the attachment weld flexibility and special care must be taken to ensure proper joint fit-up. This is important to ensure joint strength and quality.

Penflex’s method eliminates these issues and accomplishes the goal of capturing the braid wires while allowing for purging and creating an easy geometry for the attachment weld.

The Cap Pass Gold Standard

Here’s a look at a well-executed cap pass. In the first image, the ferrule has been removed, and looking at the braid from the outside, we can see there are no loose wires. In the second image, a close trim cross-section of the wire in the cap pass shows that 100% of the wires were captured.

Penflex Welder Training

Cap passes of this caliber 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 cap pass welds is one of the skills achieved through the training.

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

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Oftentimes engineers use metal bellows to damp vibrations in a piping system. While they protect surrounding pipes and equipment from damage, bellows themselves are not impervious to damage.

Work hardening occurs with each cycle and, as a result, the bellows become increasingly brittle over time. The more brittle an expansion joint becomes, the higher the likelihood of stress cracking—seen within the valley or at the crest of a convolution—and subsequent failure.

In rare cases, resonance can cause near immediate failure. Given the challenges associated with vibration, it’s a good idea to have a conversation with a Penflex sales engineer when designing components for these kinds of applications.

Applications Where Vibration is a Concern

The most common high vibration scenarios include exhaust and pump system applications.

High flow velocity can also lead to damaging vibration, though engineers take a different approach when designing bellows for applications where this is a concern.

Designing Metal Bellows with High Vibration in Mind

A flexible, 5-ply design with a low spring rate is the “gold standard” for high-vibration applications.

Flexibility is a key characteristic as flexible bellows are slower to work harden. In delaying the onset of embrittlement, we can also delay the advent of stress cracking and thus prolong service life.

Stresses are distributed across the bellows in a multi-ply expansion joint. This also slows work hardening. And while many variables contribute to flexibility, adding plies helps to make an expansion joint more flexible as well. Finally, where pressure is a concern, the multi-ply design delivers a robust wall thickness to accommodate higher working pressures.

Low spring rates are desirable as they will keep the forces exerted on pumps by expansion joints low.

When high flow velocity is a concern, Penflex uses the EJMA guidelines for liners. Based on different flow velocities and diameters, the guidelines offer recommendations for smoothing media flow within an expansion joint.

Inconel® 625 LCF

Another consideration for expansion joints in high-vibration application is the material of construction. Inconel® 625 LCF was specifically designed for the metal bellows industry. LCF stands for “low cycle fatigue.”

It is an excellent choice for high-vibration applications due to its better thermal fatigue resistance and better cycle fatigue properties when compared to other, similar alloys.

Vibration Analysis

Penflex sales engineers can confirm an expansion joint will not operate within the resonant range, assuming system frequencies are known.

Work hardening and subsequent stress cracking is a common cause of expansion joint failure, but one that can be avoided through thoughtful bellows design. For more information, please contact us.

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Vibration is common in many piping systems. However, once it becomes a defining characteristic of the application it is safe to assume that vibration poses some very real risks to system integrity.

Deciding whether an application is “high vibration” is mostly a matter of opinion. Ascertaining amplitude without the use of sophisticated measuring equipment is difficult. Even if vibration could be quantified, design codes offer little in the way of defined limits.

Indications of Vibration Induced Failure

Circumferential cracks, typically on the crest but also in the valley of the corrugation, are symptomatic of vibration fatigue.

So too is braid wear. Braid wear is indicative of significant movement of the braid relative to the hose, like what you might expect to see in high vibration applications. The tensile strength of the braid wires is higher than that of the hose, so the hose loses material first.

In essence, the braid wires “saw” into the hose, creating divots as seen below.

It’s important to note these indicators are not exclusive to vibration fatigue. Cracks can be a sign of torsion while braid wear can also result from mishandling. However, if a hose has failed due to vibration, it is likely that cracks or braid wear will be visible upon inspection.

Causes of Excessive Vibration in Piping Systems

Mechanical vibrations from pumps or moving equipment attached to the hose can induce movements that create premature ware and early failures.

Another kind of vibration is “flow induced,” generally caused by high flow velocity inside the hose. The rule of thumb for maximum recommended flow velocity in a straight run of braided hose is 150 ft/sec for gas and 75 ft/sec for liquids.

Considerations for High Vibration Applications

There are no hard-and-fast rules when it comes to vibration. As a result, one can only attack the problem through trial and error.

When vibration failures occur, the typical response is to change the mass or stiffness of the assembly. Sometimes a combination of the two will yield a better result. Adding components such as external bend restrictors, additional braids and internal flow liners or increasing wall thickness can all be inexpensive fixes to overcome premature vibration failures.

Flexibility is another important consideration in high vibration applications, and there are some requirements as to minimum live length of a hose assembly in such scenarios. Those requirements can be found in this table here.

For assistance with flexible piping components in high vibration applications, contact us at sales@penflex.com.

In the video below, Penflex Director of Quality and Engineering Janet Ellison discusses how to approach installing hoses in high vibration applications.

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Robust lance hoses are needed to deliver oxygen to a Basic Oxygen Furnace (BOF). The heat of the surrounding environment, the frequent handling—and mishandling—of the hoses, and the need to maintain strict cleanliness present a set of challenges for engineers and maintenance personnel.

There are various options when it comes to lance hose design. One of the biggest decisions to make revolves around what material to use for the inner hose, whether to opt for metal or rubber.

In either design, layers of braid or insulation and metal armor complete the hose assembly.

Working Temperatures

Metal has a higher maximum working temperature than rubber. Rubber hoses require insulation to reach working temperatures of 1000 °F while the 300 Series austenitic stainless steels can handle temperatures up to 1500°F. Exotic alloys such as Inconel 625 can accommodate even higher temperatures.

It’s often the ambient heat that engineers must keep in mind when designing components for steel mills, but this can vary greatly depending upon proximity to furnaces and other pieces of equipment.

Hose Flexibility

As lance hoses move in and out of the furnace, flexibility is a desired characteristic. Rubber hoses are often more flexible than metal hoses in smaller diameters, but that difference decreases or becomes negligible as hose size increased.

Lances are typically six, eight or ten inches in diameter, fitting into this second category where neither metal nor rubber have a definitive edge in terms of flexibility.

Lightweight Lance Hoses

Despite having a heavy wall construction, metal hose assemblies weigh significantly less than rubber hose assemblies. The increased weight of the latter, which is about 1.5 – 2 times more, can be difficult to handle and put a lot of stress on the piping system.

Complete rubber hose assemblies used for oxygen lancing are generally far more expensive than a metal hose assembly and, given life span ends of being the same, one wonders whether the increased price justifies an arguable marginal increase in flexibility. And a heavier, harder to handle flexible hose at that.

For more information about Penflex’s metal oxygen lance hose assemblies, take a look at this handout.

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Penflex provides a unique breadth and depth of durable metal hose products, though cost-competitive, lighter-weight options are also available. The P3 and P4 Series Stainless Steel Hoses are two of our most popular products and a comparison between them hints at our range of offerings.

Both hoses use a rotating die process to form corrugations gradually from the inside out. This ensures a highly uniform hose, the insides of which remain free of surface effects that can result from forming corrugations from the inside out.

P3 and P4 are available in 321 or 316L with one or two layers of 316L or 304L braid. The hoses come in standard pitch, but we also offer compressed pitch versions for applications requiring a more flexible hose.

As laid out above, there are many similarities between these Series, but there are also some key differences. The main distinctions to be made revolve around wall thickness, hose geometry and size.

P3 Hose: A Thinner Wall for Increased Flexibility

The metal strip used to make P3 is thinner than that used to make P4. This makes P3 a lighter hose and, given the lower metal component, a more economical hose.

It’s also the reason why P3 is our most flexible hose, and one of the most flexible hoses on the market. A thinner piece of metal is inherently easier to bend than a thicker piece of metal. Thus, a thinner wall hose will be more flexible than a heavier wall hose, other factors remaining the same.

When it comes to the geometry of Penflex’s P3 hose, we designed the Series with a lower corrugation height to allow higher working pressures. While this can reduce the hose’s flexibility, we compensated for this by increasing the number of convolutions per foot. A higher corrugation count will ensure a more flexible hose.

We can compress P3 even further, in which case we end up with a hose that is extremely flexible, kind of like a Slinky ™.

Full product details for P3 Series Standard and Compressed can be found via the links below:

• P3 Series Standard
• P3 Series Compressed

P4 Hose: A Heavier Wall for Increased Corrosion Resistance

Made from heavier strip, P4 is a thicker wall hose with better corrosion resistance. It’s a more durable hose, though in applications where corrosion resistance or lifespan are not top priorities, that may be an unnecessary attribute.

Increased corrosion resistance through the use of thicker strip comes at the expense of flexibility. Penflex P3 and P4 Series demonstrate this inverse relationship well.

However, in designing P4, we were able to overcome some of this. We increased the number of corrugations per foot and raised the height of the corrugation to bring some flexibility back.

As noted earlier, lower convolution height allows for higher working pressures, so to compensate for lower flexibility, we “lost” some of the working pressure potential. Thus, while P4 often has higher working pressures than P3, that is not always the case. Both Series offer competitive working pressures and to pick between the two based solely on such criteria would mean overlooking the real distinctions between P3 and P4.

The hoses differ as well in the sizes available. We currently run P4 from ¼” to 4” while P3 is only available up to 2”.

Full product details for P4 Series Standard and Compressed can be found via the links below:

• P4 Standard
• P4 Compressed

A Note on Hose Certifications 

All sizes of P3 and sizes up to 2” P4 are in accordance with sections of ISO 10380 and all sizes of both Series have weld seams that are PED certified.

For more information about our hose certifications, please click here.

Penflex also produces a range of heavy wall hoses, exceeding the capabilities of the P4. Any questions regarding the difference between Penflex P3 and P4 Series Hoses, or regarding our other heavy wall hoses, please contact us.

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Accessories are added to an expansion joint installation to meet specific customer, application or code requirements. Some are used to control movement, others to protect the bellows in the event of pressure thrust, and still others to defend against corrosion caused by media inside or from the environment outside the bellows.

Here is an overview of the most common expansion joint accessories. 

Limit Rods

The primary function of limit rods, sometimes called limit bars, is to restrict the range of movement—whether that be axial, lateral or angular—during normal operations, thereby preventing premature failure. Nut stops placed along the rods determine just how much extension and compression an expansion joint will accommodate.

The Expansion Joint Manufacturers Association (EJMA) standards state that “in the event of a main anchor failure, limit rods are designed to prevent bellows over-extension or over-compression while restraining the full pressure loading and dynamic forces generated by the anchor failure.”

The number of limit rods is calculated based on the pressure thrust that needs to be contained. EJMA recommends a minimum of three rods.

Tie Rods

Tie rods are similar to limit rods, though they cannot accommodate axial movement. Limit rods can. The sole function of tie rods is to continuously restrain the full bellows pressure thrust in normal operation while allowing only for lateral deflection.

If only using two tie rods—and placing those two rods 90 degrees from the direction of rotation—you could accommodate angular rotation.  This could be a cost-effective alternative to a hinged expansion joint.

Control Rods

Control rods are commonly confused with limit and tie rods, but it’s important to understand precisely what is specified as the accessories serve different functions.

These accessories are only used in universal expansion joints to distribute movement between the two bellows. Unlike limit and tie rods, they are not designed to carry the pressure thrust in any circumstance. 

Shrouds

 Also called covers or guards, shrouds encase the bellows, protecting it from external damage and external flow. Sometimes, a shroud will be needed just during installation to, say, protect the expansion joint from getting hit with a hammer. Other times, the shroud will be needed throughout operation.

Liners

Liners, also called sleeves, are metal devices installed on the inside of a bellows to prevent flow-induced vibration which leads to stress cracking and failure. All Penflex bellows and expansion joints are designed in accordance with the allowable values specified in the latest EJMA standards. These let us know how thick liners need to be to prevent flow-induced vibration.

Liners also prevent the build-up of material on the inside of the bellows. This would be a consideration when working with some media, like concrete for example, that hardens below a certain temperature. Shielding the inside of the bellows also protects them from caustic media.

Together with shrouds, liners are the most commonly specified accessories we see.

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For information about Penflex Expansion Joints, please click here or to print this bulletin, please click here. We’ve also pulled together a 40-minute webinar that covers expansion joint basics. Take a look!

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Some applications lend themselves to either a metal hose or to an expansion joint, but more often than not, there is a preferred option. A few key application criteria will let us know when to use an expansion joint instead of a hose.

Axial Movement

Expansion joints are very good at absorbing axial movement, or the compression and extension of the bellows along a single axis. Whether the component is installed horizontally or vertically, it doesn’t matter.

This kind of movement is typically associated with thermal growth and contraction and found in piping and ducting systems across a wide range of industries.

Externally Pressurized Expansion Joints have a unique design whereby media comes into contact with the outside rather than the inside of the bellows. This external pressurization allows the components to absorb larger amounts of axial movement than more traditional expansion joint designs.

Metal hose is not designed to accommodate axial movement. When a hose is compressed axially, the braid falls out of tension and can no longer serve in its pressure carrying capacity. Once this happens, even at moderate working pressures, the hose is liable to squirm, braking through braid and resulting in a failure.

Vibration Damping

Expansion joints are sometimes a better option in applications with extreme vibration, like what you might find in exhaust systems or piping systems with pumps.

As they can accommodate movement in all directions—axially, laterally, or angularly—they can better absorb vibrations that might otherwise prompt a hose failure or cause stress on the piping system as a whole.

Exotic Material Requirements

Certain alloys, like 800H, Nickel 200, Titanium Grades 1 and 2 and Duplex 2205, may be desired for extremely hot or highly corrosive applications. They are, however, not readily available in a metal hose product and an expansion joint may be the only viable option in some scenarios.

Paper mills, for instance, often use titanium expansion joints to defend against the corrosion so inherent in their operations.

Space and Size Limitations

Expansion joints may be the right solution simply because they take up less space than a hose or because the diameter required lies outside the typical range of hose sizes.

It would be much easier to find a 30” expansion joint than a metal hose with a 30” ID. Penflex manufactures large bore hose up to 24” in diameter but expansion joints can be made up to 120” in diameter.

In some situations, the decision to use an expansion joint instead of a hose will be an easy one. Other times, the answer may not be so clear. Rather than deciding between an expansion joint and a metal hose, the answer may in fact be a combination of the two!

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For information about Penflex Expansion Joints, please click here. We’ve also pulled together a 40-minute webinar that covers expansion joint basics. To watch, visit our video library and scroll to the bottom for the training session with Sales Engineer Igor Smola.

To speak with one of our Sales Engineers about you may need for your applications, click here.

Liquid Penetrant Inspection (PT) is a nondestructive testing (NDT) method to identify surface defects so small they might be missed by the human eye.

When it comes to metal hose assemblies and expansion joints, PT allows us to find discontinuities that could undermine structural integrity or serve as points of chemical attack.

Since PT works on any non-porous material and can be done in several different ways, as well as being an easy process requiring minimal equipment, it is a widely used inspection method.

Step-by-Step: Liquid Penetrant Inspection

Regardless of which type of penetrant is used, the steps are similar.

1. 1. Pre-clean surface. The metal surface must be free of all contaminants that could prevent penetrant from seeping into defects.
   2. Apply penetrant and wait. Whether the penetrant is brushed or sprayed on—or the component is dipped into it—any cracks in the metal surface will “pull” penetrant inside during “dwell” time.
   3. Clean surface of excess penetrant. This needs to be done with care so as not to remove penetrant from the defect.
   4. Apply developer. A wet or dry developer is applied to the entire metal surface to draw penetrant from defects and bring it to the surface.
   5. Assess results. Penetrant magnifies any discontinuities—in essence being the “X” that marks the spot—making those surface defects easier to see. The contrast under blacklight using fluorescent penetrant or the contrast of red dye on white developer using a visual penetrant makes the discontinuities all the more apparent.

PT with Fluorescent Penetrant

Fluorescent penetrants contain dye that glows when exposed to UV blacklight—we use a ZYGLO fluorescent penetrant with Level 3 sensitivity. Fluorescent penetrant is a more sensitive system than one using visible penetrants.

In the image below, you can see defects on a flanged bearing exposed under ZYGLO testing. Any components that do not pass PT are scrapped and new components are made and tested to complete an order.

PT with Visible Penetrant

Visible penetrants contain red dye that is easily distinguished against the background of a white developer.

Using PT to Proactively Improve Processes

Beyond identifying surface discontinuities, the irregularities uncovered can serve as an indication of poor welding technique. For instance, a visible penetrant was used to uncover the cracks in a failed hose assembly later magnified under a microscope below.

We can see a lack of fusion, incomplete penetration of the weld joint, a concave weld profile, improper weld size, and off-center weld placement, all of which resulted in the weld becoming a pivot for vibration.

If further analysis is done off the back of liquid penetrant inspections, proactive steps can be taken to improve welding procedures.

Liquid Penetrant Inspection at Penflex

Penflex carries out PT inspections at our lab in Gilbertsville, PA. We are SNT-TC-1A qualified and comply with ASME V, Article 6, ASME B31.3 and ASME B31.1 PT requirements. We also meet AWS D1.1 NDT requirements. Our on-site CWI, CWE, and NDE Dave Gregor will tell you a bit more in the video below.

If you have any questions about this, or any other NDE testing we do, please contact us.

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