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.
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.
External corrosion
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”