Flexible Metal Hoses: An essential guide
A metal flexible hose is a type of piping used to connect two distant points to transport or transfer fluid. In Oil & Gas applications hoses are used when there is a considerable relative movements. A variety of fluids and fluidized solids can easily be transferred through flexible hoses to other locations. These are most commonly known as hosepipe. Along with loading and unloading services in processing plants, these are widely used by homeowners as garden hose. Normal Flexible hoses are made of non-metals like soft plastic material or synthetic rubber. However, flexible hoses of chemical industries that are designed to absorb pipe movements are made of metallic materials.
Flexible hoses are moade by extrusion or vulcanization process. To add strength to the non-metallic flexible hoses, they are reinforced using a crisscrossed grid of fibers combined together through braiding, spiraling, or knitting. These reinforced hoses can be long enough. Basically, flexible hoses have four parts; inner tube, reinforcement, End fittings and protective outer cover.
A corrugated hose is constructed with a bellow of very long length. Fundamentally, the behavior of a corrugated flexible hose is the same as the bellow expansion joint. The flexible hose has to resist the hoop pressure stress, but cannot sustain the longitudinal pressure stress. Also, it has a tendency to squirm under internal pressure. To resist the longitudinal pressure stress and prevent squirm, corrugated hoses are often constructed with braids wrapping around the outside surface as shown in Fig. 4. The braided cover also protects the corrugation from scratch and wear. The braided hose, similar to a tied expansion joint, cannot accommodate any axial movement. On the other hand, the un-braided hose can sustain very small internal pressure.
Due to the lack of a limiting mechanism, a corrugated tube connector metal flexible hose is prone to abuse. It should not be bent beyond its acceptable range. For braided hoses, the situation is even more critical.
As the corrugations are not visible from the outside, a braided hose does not show immediately when damaged. Therefore, for manual handling in such situations as loading/unloading and switching operations corrugated hose is not suitable. The corrugated flexible hose has a continuous metal wall thus making it pressure-tight. It is suitable for handling any type of gas and liquid as long as it is compatible with the hose material.
An interlocked hose is constructed with links that are kept tight with packing material. There are clearances provided between the links that afford the capability of accommodating some axial movement. As the hose is being bent, the clearances gradually close. The hose becomes stiff and cannot bend any further at a certain point when the clearances are completely closed, . This sudden stiffening effect serves as a warning to the handler, preventing the interlocked hose from being over bent. This automatic warning feature makes the interlocked hose especially suitable for manual handling.
The packing mechanism at the interlocked links does not offer a perfect seal. Therefore, the interlocked hose is satisfactory for carrying low-pressure air, steam, and water, but is generally not suitable for conveying gases and “searching” liquids such as kerosene and alcohol. The outside of the interlocked hose is relatively smooth, making it easy to handle without any covering.
The inner cone with outer thread connector metal flexible hose assembly is normally not analysed. In most of the situations, the end displacements from piping or equipment connections are calculated from stress analysis software and those values are transferred to the vendor for their consideration. Accordingly, the hose length and installation space are determined.
Pipe Supporting for optimum flexible hose working
A piping system which utilizes fexible metal hose to absorb pipe movement must be properly anchored and guided to assure correct functioning and maximum service life of the metal hose assembly. The following basic principles should be observed:
The direction of pipe motion must be perpendicular to the centerline (axis) of the hose.
To prevent torsional stress, the pipe shall be anchored at each change of direction where a flexible metal hose is employed. Typical examples of correct and incorrect guiding are shown below in Fig. 5.
Flexible Hoses are used to accommodate piping and equipment displacements. Hoses being extremely flexible, installations is very easy. However, a few general precautions should be exercised during installation to avoid hose failures.
While installing flange connector metal flexible hose, the allowable minimum bend radius is the most fundamental limitation. For interlocked hoses, the limiting radius depends largely on the clearances between links. It has less to do with the stress and fatigue, so it generally has only one limiting radius for all applications. For corrugated hoses, on the other hand, the limiting radius depends on the stress at the corrugations. For pressure hoses with braided reinforcement, the corrugation stress comes mainly from the bending of the hose. Therefore, the corrugation stresses can be controlled by setting a limitation on the bending. In other words, the installation is acceptable if the hose is not bent beyond the limiting radius. Similar to the situation discussed in the bellow expansion joint, the mode of failure of the hose corrugation is due to fatigue. Therefore, the bend radius limitation depends also on the number of operating cycles expected. Most manufacturers provide two limiting radii, one for static application involving a one-time fit-up installation, and the other for operational movement involving many cycles of intermittent flexing. The whole design and installation process actually ensure that this minimum radius is maintained during the initial layout and throughout the operation.
The article describes various types and sources of flexible metal hoses (FMH) vibration. Depending on the direction of vibration displacements, basic variations of sleeves vibration are identified: transversal, longitudinal and torsional. The distinguished forces, that excite vibration in FMH, acting on it, are divided into static and dynamic loads. The most common type of vibration - transverse vibration of flexible sleeves is considered in more details. Also, the ripples - one of the main causes of transverse vibration, which significantly degrade hydraulic performances of pipeline communications, are investigated. The paper presents the analysis of characteristics of the bending and longitudinal stiffness, which implies that the stiffness increases with increasing internal pressure, the diameter of the sleeve and the number of braids. To determine frequency characteristics of FMHs, the bar, with reduced parameters of elasticity and mass, has been chosen as FMH mathematical model. The research results of an influence of various factors on the metal sleeves eigenfrequencies have been studied.
The first step in alloy selection is to determine the source of any potential corrosion. While corrosive attack may be initiated by the media running through the metal hose, it is also possible that corrosion can initiate from external sources.
If a hose assembly is used in a potentially corrosive environment, then it should be made using an alloy that is resistant to the corrosive agent unless it can somehow be shielded from exposure to that corrosive. This can be tricky, as many covers do not provide adequate corrosion protection, and may even exacerbate the problem. For example, there have been instances where flexible polyvinyl chloride (PVC) covers have been applied onto stainless steel-corrugated dock hoses as a means to protect them from the salt water environment. Over time, these covers can begin to degrade, releasing chloride-containing compounds that can attack the stainless steel hose. External corrosion can also be caused by media that drips or sprays onto the exterior surfaces of the connector.
If the media being transferred through the hose or expansion joint is corrosive, then proper alloy selection is critical. Here, it is important to remember that although the product being conveyed may not be corrosive, it may contain impurities that can cause problems. A good example here would be steam transfer. Boiler water may contain various water treatment chemicals such as anti-scaling or anti-foaming agents, and water-softening chemicals, all of which can be corrosive if allowed to concentrate in the system. Natural gas may also contain sulfur-based impurities that can attack commercial stainless steels. This ‘sour gas’ can lead to critical safety issues if system corrosion results in gas leaks. A detailed analysis of the medium may be required in order to identify any corrosive impurities that may be present.
Once potential corrosive agents have been identified, the next step is to determine which alloys will best withstand any corrosive attack. Most alloy producers provide detailed specification sheets for the alloys they offer that give valuable insight as to the suitability of a given alloy when exposed to certain chemicals. However, in corrosive applications, industry resources that show real-life test results might provide more reliable data. Various databases are published by organizations which perform corrosion testing on alloys, analyzing their resistance to different chemicals under various operating conditions.
Some of these resources are referenced in industry standards and specifications. When using these databases, not only will you need to know the name of the chemical being transferred, but also the temperature and concentration percentage at which it is being conveyed, as these variables can have a dramatic effect on the corrosion rate. For example, sodium hydroxide is generally non-corrosive at low temperatures and concentrations, but becomes aggressively corrosive to stainless steel as the temperature and/ or concentration increases. This is also true for many water-treatment chemicals. Conversely, some chemicals may exhibit reduced corrosion at high concentrations, so caution is key. There are a few important considerations when consulting these corrosion resistance charts. First, they typically do not include any corrosion resistance data for name-brand chemicals or mixtures of multiple chemicals. If name-brand chemicals are being transferred, the chemical manufacturer should be consulted for corrosion resistance data. Secondly, certain corrosion resistance information may be product specific. In other words, corrosion charts that can be found in the back of catalogs for fittings, valves, pipe, etc. should not be used as a reliable corrosion guide for union connector metal flexible hose.
While these charts are fine to use as a guide for the products in the catalog, they can be misleading. Although a chart may give an ‘acceptable’ rate of corrosion for those specified products, that same rate may not be acceptable for a flexible metal hose, which is formed using relatively thin-walled corrugated tubing. Incidentally, be wary of corrosion-resistance information found online and make sure that all data is published by a reliable source. Caveat emptor: Buyer beware, especially when the information is free. It is important to remember that, if a metal hose or expansion joint is attacked by a chemical, it is seldom because the alloy is defective. In most cases where corrosion is present, either the incorrect alloy was selected, or the alloy was exposed to unspecified chemicals to which it was not chemically resistant.
“If the media being transferred through the hose or expansion joint is corrosive, then proper alloy selection is critical. Here, it is important to remember that although the product being conveyed may not be corrosive, it may contain impurities that can cause problems”
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