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Flexible Metal Hoses: An essential guide

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.

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.

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.

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.

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