Integrative notes to the operating manual for magnetic coupling centrifugal pumps conforming to the ATEX 94/9/CE directive TCK... - TBK... – TBAK…
12
6 – THE MAGNETIC COUPLING
The magnetic coupling is made of 3 different main parts: the internal rotor with bushings, the external rotor and the
sealing vessel or can (Fig.8).
The internal rotor bear
s
a series of permanent magnetos placed on its circumference, in number, dimensions
and position variable according to the coupling size and torque to be transferred. To the internal rotor is connected the
pump shaft. The internal rotor bearing is on bushings made of special material, corrosion resistant but particularly
sen
sitive
to impact and vibrations. The running of the pump involve
s
the reciprocal rubbing between static and
rotating elements of the bushings of the internal rotor with consequent heat development, that is removed by the
circulation of the pumped liquid around the rotor.
The rotor is sealed inside the pump by the sealing vessel or can, that physically divide
s
the inside and outside of
the pump containing the pumped liquid. The distance between the internal rotor and the can is really tight to allow a
better efficiency of the coupling and reduce the eddy current generated by the
turning
magnet, because it develop
s
heat. On the outside of the can there is the external rotor, with its permanent magnets, positioned
coupled
with the
ones of the internal rotor, to create a series of magnetic bonds able to transfer the movement to the pump shaft. The
external rotor is connected to a shaft on ball bearings moved by the pump motor. Also the distance between the
external rotor and the can is really tight for better performances of the coupling.
As a
ll the parts of the coupling assembly are buil
t
with heat conductive materials, we can assume that the external
surface temperature of the can is always some degree above the temperature of the pumped liquid. From the tests
it was measured that usually it is of 20° K above the temperature of
t
he pumped liquid, because
it
receive
s
part
of the heat generated by the turning of the internal rotor
,
hand
ed
over to the liquid film that cover
s
the bushings
and used as lubricant and cooler;
and
part of the heat generated by the eddy current produced by the permanent
magnets.
To guarantee better start up conditions to the pump and long lasting magnetic coupling
,
the use of
a soft start-up system
is strongly recommended
. The use of this kind of system reduces the risk of losing
of the synchronism between the two rotating elements of the magnetic coupling.
6.1 – CHECKINGS ON THE RUNNING
The correct running of magnetic coupling pumps ask for a special attention to the coupling qualities that has
particularly frail components.
The pump start must be executed according to the in
structions
reported in the Operating Manual supplied with the
pump. A
n incorrect
execution of the pump start-up procedure could lead to a mechanical damage of the coupling
components with consequential mechanical problems and impossible running of the pump.
The effect of rotation transfer uses the synchronism between the internal and external magnets. If, for an unforeseen
reason
, the synchronism
is
interrupted, the pump suffers violent vibrations with the possibility of mechanical
breakings of the components. Hardly c
an
it recover
back to
correct running.
During functioning, the internal rotor develop
s
heat f
rom
friction between its components and for the eddy current of
the magnets; the heat is
transferred
to the liquid that flow around,
hence
it is necessary to continually monitor the
presence of liquid inside the pump with a flow measurement instrument o
r
something equivalent. If the liquid flow is
not there
the pump must be stopped immediately.
All the magnetic coupling pumps utilized in
an
environment with the possibility
o
f presence of a potentially
explosive atmosphere must be equipped with a probe to measure the temperature on the outside surface of the can.
The pumps are already arranged for the installation and this fitting could be supplied by POMPETRAVAINI or installed
by the user. In case of measurement of a temperature increase on the can surface above the 20°K more than the
pumped liquid temperature; the pump must be stopped and the reason of the temperature increase checked. If the
pumped liquid has a boiling temperature within the interval of the 20°K, this interval must be reduced as to operate the
pump stop when the can temperature is at least 5°K below the boiling temperature of the liquid. If the can could reach
the boiling temperature of the liquid, the latter could generate vapour bubbles and so ma
k
e the pumps running dry.
If - from the analysis of the pump requisites for specific operati
ng
conditions - the user need
s
more
accurate
information about the temperature increa
se
of the magnetic coupling
, he
could ask POMPETRAVAINI.
EXAMPLE:
The pumped liquid has a temperature of 60°C and a boiling temperature of 90°C. Adding an increment of 20°K due to
the heat developed by moving part of the rotor, the applied thermocouple should indicate always a surface
temperature of the can below 80°C. In case that it will read a higher value of the temperature, then it will a symptom of
some functioning problem of the magnetic coupling a
nd
the pump must be stopped.