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.