metal hose

Why Stress over Residual Torsional Stress?

It’s no secret that torque is a killer for metal hose. If you’ve ever attended Hose Master University, you’ve probably heard our corporate trainer’s well-known adage- “Don’t twist the hose!” Torsional stresses are something that you want to avoid completely in all metal hose assemblies. When Hose Master inspects corrugated metal hoses and looks for torsional stress, it’s typically after installation while the hose is in service. However, we should also be concerned with torsion that can happen before installation during the manufacturing process. This is referred to as residual torsional stress.

Residual torsional stress (RTS) is problematic because it accelerates fatigue to the metal, thus reducing the ultimate service life of the hose. A key indication of RTS is when a hose has a visibly twisted seam weld. Despite the issues created by RTS, it is inherent in many hose forming processes used today The only way to prevent RTS in metal hose is through avoiding the use of manufacturing methods that cause it. Let’s explore some of the ways residual torsional stress can be created during manufacturing.

That’s a Tricky Question…

A question we are frequently asked by our customers is “Can I use a metal hose for food-related applications?” It’s a simple question, but the answer isn’t always so simple.

The quick answer is no, metal hose generally cannot be used for the transfer of food-grade materials. This has nothing to do with the capabilities of the manufacturer or the quality of materials used in the hose. On the contrary, while the steel used may indeed be “food-grade,” the corrugations in the hose can potentially trap food media and make it difficult to clean the hose to the proper standard. Instead, you typically see PTFE hoses in these types of applications. The only exception to this would be if the media is at a temperature high enough to kill off any bacteria, which may allow for a metal hose to be used in some cases.

However, just because metal hose generally cannot be used to transfer food-grade materials does not mean that there are not applications for metal hose elsewhere in production! You just have to know where to look for them…

Metal Hose for Food Industry Applications

Clean in Place Systems

A key component of food production that requires the use of metal hose is clean-in-place (CIP) systems. Clean-in-place systems serve as a way to clean and sanitize the internal surface of a piping system as part of the routine cleaning of the production line or when production is being changed over to a different product. These systems utilize steam and chemicals (such as sodium hydroxide) to clean the piping system, which can be hard on any non-metal hoses that are used in the system. Instead, non-metal hoses are removed to be cleaned separately and metal hoses are installed in their place during the cleaning process. These metal hoses are better suited to handle the high heat and caustic cleaning solution running through the system, making metal hose an optimal solution for this application.

In modern steelmaking, heat rules. Heat changes coal into coke, melts ore into liquid iron, and converts iron into steel. All of these products must be transported from one process to the next, and hydraulic power units (HPUs) are employed to provide that power. Hydraulic hoses provide flexible connections between the HPUs and the equipment they power, and this is where problems can arise. Heat and hydraulics do not mix, and hydraulic power systems can experience premature hose failures unless a proactive approach is taken.

Most steel is made using one of two processes. The first is an Electric Arc Furnace (EAF), which uses scrap steel as the main feedstock. The scrap is charged into the furnace, where huge electrodes create an arc of electricity that melts the charge so it can then be refined and processed into the desired alloy. The second process is an integrated mill, where Blast Furnaces supply liquid iron to a Basic

Long Enough? Measuring Metal Hose Assemblies

There are several important factors to note when designing a metal hose assembly: alloy, fittings, media, pressure, and so on. One of the most crucial factors that is often taken for granted in industrial applications is hose length.  Utilizing the incorrect length in an assembly can be detrimental to its cycle life and potentially result in failure in an assembly.  If an assembly is too short, there is potential for the corrugation geometry to be deformed as the assembly is stretched between the connecting points.  Conversely, if an assembly is too long it risks being over-bent as the hose tries to move out of its own way.

It comes as no surprise that metal corrugated hose is the preferred choice for high-temperature applications. But what about low-temperature applications? This is a question we frequently see from our customers. The simple answer is yes- metal hose is a great option for low-temperature applications. However, there are important factors that should be considered before making a recommendation.

Service Conditions

Before recommending a particular hose for a low-temperature application, we first need to identify the conditions that the hose will experience while in service. For

As Albert Einstein once said, “The only source of knowledge is experience.” When it comes to interlocked hose, Hose Master has had a fair share of experience.  While other product lines have been added and developed over the years, Hose Master has been manufacturing and continuously refining interlocked hose since the company opened its doors in 1982.  During that time, they’ve seen hoses both excel in the field, as well as fail from a variety of factors. However, in their decades of experience, the majority of interlocked hose failures can be attributed to one of three failure modes: torque, abrasion, and over-bending.

Torque

Chemical plants are one of the biggest industrial users of corrugated metal hose assemblies. Processes performed in the plants involve some of the most demanding environments:

• Temperatures ranging from extreme heat to cryogenics
• Mixing and transfer of hazardous compounds
• Equipment configurations that result in less-than-ideal piping situations

Metal hose can handle all of these factors and has some other inherent benefits over other hose types when it comes to the kind of applications seen in chemical plants. Let’s dig into some of the main areas of consideration and concern when dealing with chemical hoses.

Handling Considerations

Mishandling of hoses is one of the main contributors to premature failure. Because chemical plants have so many different inputs and outputs, hoses are often used to facilitate the transfer of chemicals from trucks, trains, or barges to the plant and even within the plant from one unit to another. Chemical blending manifolds are a great example of this, where a single hose assembly may be used for various connections at different times depending on what operations the plant is performing.

The need to easily connect and disconnect these hoses quickly and often makes cam and groove couplings a popular choice for chemical plants. When moving hoses from one outlet to another, it’s tempting for users to abuse the “arms” on the fitting and over-bend the hose or torque it into position. Always try and keep the hose as straight as possible, and avoid twisting it. Additionally, hoses are made to flex, but extremely tight bends (especially near the end fitting) can damage the hose and cause it to fail prematurely. Operators should keep this in mind to prevent deformation of the hose when making connections (guidance on using bend radius information can be found here).

Metal Hose Attributes

There are several intrinsic features of metal hose assemblies that make them well-suited for chemical plant service. Chiefly among them is that they are not susceptible to permeation. This is a huge benefit for both operator safety, and plant safety. The metal core is puncture-resistant, and in the event of a leak, the hose will typically develop a small crack or pin-hole and does not burst apart!

Metal hoses also have a more compact end fitting configuration. Because end fittings are welded onto the end of the hose instead of a barbed or crimped mechanical attachment they don’t take up as much of the hose’s flexible length. This results in more working live length compared to non-metallic assemblies, which further facilitates handling and easier installation by the operators. It also means that metal hose is easily customized without the need for adapters. Stainless steel fabrication techniques provide the ability to use a wide array of fitting configurations, and can be tailored to prevent media entrapment, resist end-pull, or to accommodate high system pressures.

Finally, one of the handling benefits of metal hose is its light weight. Calling metal hose lightweight might sound contradictory, but pound for pound, metal hoses generally offer higher working pressures than rubber or composite chemical transfer hoses. This gives metal hose a wide range of potential applications, and also translates into easier handling and installation by operators.

After personnel safety, avoiding unplanned downtime is the main priority for all industrial operations.  Plants typically keep an inventory of maintenance items like hoses on hand to swap out as needed to minimize lost production time.  Unfortunately, this inventory is not always stored or cared for properly.  I personally have visited power plants where they kept replacement hoses, pumps, gaskets, and flanges on the ground outside.  The end result of this kind of storage often defeats the purpose of having inventory parts because they can fail or lose significant service life before they’re ever even used.  While these storage concepts apply to all maintenance components, let’s discuss metal hose storage specifically.

External Considerations

The storing of hoses outside may come as a bit of a surprise (or may not) but it’s actually relatively common.  Rain or dust seem like insignificant elements to stainless steel but they can actually facilitate a great deal of damage, especially over time.  With rain, the phrase “evaporation equals concentration” helps to illustrate this point.  Everything that is picked up by the rainwater on its way down (including nearby plant gasses) is delivered in a diluted state, but as the water slowly dries up, it leaves behind a concentrated residue that can cause corrosion (especially if the hose is in a position to collect water that can then pool on the interior).

Dust and particulate matter can do this too, especially inside the plant.  Maintenance storage cribs and spare parts inventories can often be found near the equipment they’re meant to service.  Heavy dust and particulate matter from process equipment can pick up other chemicals and off gases that are present in the plant, and carry them down onto the outside of uncovered hoses.  This new mixture can cause unintentional chemical reactions that can corrode the exterior of the hose. I know of a specific instance in a coal-fired power plant where a baghouse collecting ash was improperly releasing a large amount of particulate…which then combined with lime dust and landed on nearby hose assemblies causing the exterior to become embrittled and fail. Even in cases where corrosion isn’t an issue, these fines can buildup on the outside of the hose in-between the corrugations and underneath the braid.  This can be difficult or impossible to clean out, and can affect the hose corrugation’s ability to flex, or can become entrapped in the braid causing increased wear.

Hose Options for Storage

Fortunately, there are simple remedies for most of these issues. It’s always up to the end-user how they want to properly address their plant processes: be it either with a modification of the hose itself, or by rearranging how they store the hoses in the plant. Let’s break down each one separately:

Addressing New Issues

Steel mill operators don’t like to have downtime problems, in fact they can’t afford to.  They want to run as much as possible, and as efficiently as possible.  Production equals dollars.  As problems pop up that cause unplanned downtime or upset production (and subsequently get addressed) over the years, they’ve driven the industry to continue to change and evolve as a whole.  So the mills of today don’t have the same issues that mills did in the past.  You can’t as easily say “Hey, we saw this exact same problem up the street on their furnace!” the way you may have been able to 50 years ago.

That doesn’t mean that mills still don’t run into issues, they just tend to be a bit more personalized. And when you have a unique issue, you tend to get a unique solution.  A mill will do its best to solve its own problems, yet each mill has their own idiosyncrasies.  When

"Which way do I measure this metal hose?"

A common question among some customers who use metal hoses is: "Which way do I measure this metal hose?" Well, there's a few different options.

1. The first method is to measure the overall length of the assembly.
2. Or, the live flexible length of the hose assembly can be measured.

Live Length vs Overall Length

Traditionally, the live length - or the amount of flexible hose between the fitting - is used to determine whether there is sufficient hose length to accommodate a certain