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Bredel Hose Pumps
invests in advanced grinding machinery
Specialist pump manufacturer Bredel Hose Pumps B.V. designed the
high-pressure hose pump in 1973, and since then has successfully completed
more than 65 000 installations in industries as diverse as chemical
processing, food processing, brewing, paper manufacturing, mining and
waste & water treatment. The company recently invested €2.2 million in
custom-built automatic grinding equipment. In this article from the
Netherlands, Tiemen Postma, Development Engineer Rubber & Hose Technology,
explains the importance of the grinding process for the reliable operation
and longevity of Bredel’s hose pumps.
In a hose pump, a composite, reinforced hose is enclosed within a casing
that is flanged at both ends. The flanges are connected to the suction and
discharge lines of the system (Figure 1). The principle of operation is as
simple as it is reliable. Within the casing is a rotor with two pressing
shoes opposite each other across its centre line and mounted on a shaft
supported on its own bearings. As the rotor rotates the hose is totally
compressed by the shoes and the product within the hose is pushed forward
(Figure 2).
The hose: the heart of the matter
So, the hose (Figure 3) is the heart of the hose pump. If it does not
function correctly the pump will not have optimal performance. In
cooperation with the company’s hose manufacturing division, Bredel has
some 30 years of experience in developing, designing and producing hoses
for the Bredel hose pump.
Traditionally, development of new rubber-related products is done by
‘trial-and-error’ methods. This can often be a time-consuming process. To
shorten development time, and therefore save material and money, Bredel
Hose Pumps developed a finite element analysis (FEA) simulation model (Figure
4). The model can for instance be used to determine the optimal geometry
by looking for the best stress-strain distribution and a minimum of
hysteresis (heat development).
Of course, rubber is a material that is hard to model due to its
non-linear behaviour. Bredel realizes that such a FEA model is only a
reflection of reality and will have its discrepancies. Therefore practical
tests are still necessary to back up the calculations and guarantee a
satisfactory service life of the hose.
Fully automated grinding machinery
Numerous hoses are available on the market in various qualities for
industrial hose pump applications. One of the first obvious differences
between these hoses is the outer surface: ground or non-ground. From the
mechanical point of view, Bredel believes strongly that ground hoses are
of higher quality. Since quality is an important issue for Bredel Hose
Pumps, all hoses are ground before they leave the factory.
Up till now the hoses were ground according to traditional methods. It is
only thanks to the experience and the dedication of the operators of the
company’s original machinery that the hoses were ground within
satisfactory tolerances. Convinced as the company is of the significance
of the accurate grinding of hoses, an investment of €2.2 million was made
to design and build completely new, fully automated, grinding machinery
according to its specifications (Figure 5). In this article it will be
outlined why it is important to grind the hoses before they are mounted in
a hose pump.
Constant quality control
The production of a patented Bredel hose is a multi-step process. First of
all, the specific rubber compounds are prepared by mixing them in an
internal mixer controlled by a computer. This way the right amount of
ingredients is added and the right mixing conditions are assured. Control
is carried out afterwards by checking some key properties of the rubber
compound, these being the processing behaviour (Mooney viscosity), curing
behaviour (rheogram) and mechanical properties – the hardness and density
of the vulcanized compound. Only after the rubber compound has passed
these tests is clearance given for its use in the production department.
The construction of the hose starts with extruding a homogeneous resilient
rubber inner layer upon typically a 40-metre mandrel. The compound used
for the inner layer determines the area of application of the finished
hose, since this material needs to be chemically resistant towards the
medium to be pumped. The available compounds and their area of application
are listed in Table 1. Then the reinforcement layers are fitted by winding
dyed nylon cords around the inner layer, each of the four layers separated
by a thin layer of rubber preventing the cords of the individual layers
touching each other (Figure 6). This reinforcement is needed to increase
the maximum operating pressure and to secure the hoses’ recovery. The
construction of the cords and the cord layers is of great importance for
the functioning of the hose. Due to the high strength and stiffness of the
cords compared to the rubber, inaccurate construction can cause
dimensional changes of the hose under dynamic loading and internal
pressure. These unnecessary deformations of the hose cause a shorter
lifetime, capacity loss or backflow. The final hose construction step is
the extrusion of the outer layer, which is tough, but still has very good
dynamical properties. During all these construction steps there is regular
checking on the geometry to be sure that the production is still within
the specified tolerances.
The hose is now ready for the vulcanization. Before the actual
vulcanization reaction the hose is wrapped with a bandage to prevent the
rubber from flowing away at the beginning of this reaction. Since the
vulcanization of rubber is a form-determining reaction, the surface
roughness of the bandaging is printed upon the outer surface of the
finished hose. Finally the hoses are sawn into the right lengths.
A part of the finished vulcanized hose will go to the laboratory for the
final control checks. At this stage the mechanical properties of the
rubber, the bonding between the cords and the rubber as well as all the
geometrical characteristics of the hose are checked. Only when the results
of this control conform to the specifications are the hoses sent to the
grinding department.
Other hose manufacturers producing reinforced hoses often do so in a
somewhat different fashion. But the most important similarity in all hose
manufacturing processes, as far as the scope of this article is concerned,
is that bandaging is used during the vulcanization process and that a
print is therefore left behind on the finished hoses.
The right balance in compression
For the working of a hose pump in general and its positive displacement
characteristic, the hose needs to be compressed to full closure. One can
imagine that the force needed to squeeze the hose flat steadily increases
during this movement.
However, the force increases dramatically when the hose is compressed
beyond the point of full closure. This is shown in the force-displacement
curve (Figure 7) made by compressing a SP(X)40 hose using a hydraulic
press. In the vicinity of complete closure, around 26 mm in this example,
a millimetre extra compression leads to an increase of a few kilo-Newtons
in compression force. Due to the incompressibility characteristic of
rubber, high stresses are formed inside the hose. With increasing
compression internal frictions also increase, resulting in a higher hose
temperature and a shorter fatigue lifetime. The pressing shoes and the
rotor deliver the counterforce needed to compress the hose. Too much
compression can give an unacceptable load on the bearings of the rotor.
Also the pump casing and the rotor shaft can be damaged.
On the other hand, incomplete closing of the hose causes (high velocity)
backflow. Especially with abrasive media, severe wear of the inner layer
can occur. From experience it is known that a minimum over-compression is
far less harmful for the hose than under-compression. Under the influence
of internal (system) pressure, rotor speed and temperature, different
displacements are necessary to completely close the hose. For this shims
can be put underneath the pressing shoes to increase the total diameter of
the rotor, just to the point that there is no more backflow. Shimming
curves per pump type are available for the right selection. Since after
grinding twice the tolerance on the wall thickness of the Bredel hoses is
smaller than the thickness of one shim (0.5 mm) the shimming does not need
to be adapted after hose exchange.
Smallest possible tolerances required
Keeping the importance of the right amount of compression in mind and then
going back to how the produced hoses look, the question must arise if such
a hose can function optimally. Looking at the outer surface of a newly
produced hose the print of the wrapping is visible. Height differences of
this print are measured to values exceeding 1.0 mm. Secondly there is a
tolerance on the inner as well as on the outer diameter of a hose. Adding
up the tolerances gives a significant difference between a hose produced
in the low tolerance area and one produced in the high tolerance area. The
solution for interchanging all these tolerances with a smaller new one is
as simple as it is effective: grinding the outer surface of the hose to a
uniform wall thickness (Figure 8).
The effects of the grinding process on the dimensional tolerances and the
over-compression of the hose are shown in Table 2. In order to prevent
backflow for any produced hose that is used, the compression should be
high enough to compress a hose with a high tolerance inner diameter and a
low tolerance outer diameter. The worst-case scenario is when a hose is
used with the opposite dimensional tolerances. It shows that
over-compression of non-ground hoses can have values from 2.4 up to peaks
of 3.5 mm due to the height of the wrapping (Figure 9). When the data of
Table 2 are coupled to figure 7, it is found that the compression force of
the ground Bredel 40 mm hose is approximately 4 to 5 kN. However, the
compression force on the non-ground hoses can vary between 4 and 16 kN.
This is very harmful for hose life. That ground hoses have a surface with
a considerable reduction in friction is an extra, but significant,
benefit.
Bredel Hose Pumps emphasizes the importance of grinding the rubber hoses
used for hose pumps. This way only one geometrical tolerance is introduced,
which can be controlled very accurately. A regular outer surface is
created as well as a uniform wall thickness ensuring a constant
compression force without peak values. The constant load on the hose and
the pump result in their longevity, a synergy that symbolizes the quality
of the Bredel Hose Pumps products.
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