Integrative notes to the operating manual for magnetic coupling centrifugal pumps conforming to the ATEX 94/9/CE directive TCK... - TBK... – TBAK…
13
When used in presence of a potentially explosive atmosphere, the external atmosphere could penetrate
inside the bearing housing that contains the external rotor of the magnetic coupling. In this conditions a
possible malfunctioning (look at paragraph 6.2) could
cause
to lit the small amount of explosive
atmosphere inside the bearing housing.
With the
use of the monitoring systems indicated above it
was
evaluated as sufficient to lower the probability of similar events at least to be considered a rare
event
that do
es
not need to be mentioned for the pump belonging to Category 2 and 3 according
to ATEX
Directive.
As further precaution to prevent that some of the potentially explosive atmosphere surrounding the pump
succeed to penetrate inside the bearing housing, it is possible to generate a small overpressure inside
the bearing housing. The effect could be obtained
by
connecting the vapour venting plug of the
bearing
housing (Fig. 8) to a source of compressed air, dry and without any gas or other flammable
substances,
feeding with a very low flow, up to a pressure of 0,1 bar maximum, measured at the outlet.
The effect
causes a light leakage of air through the drain plug at the bottom and the shaft bearings. In
this way the
external atmosphere is prevented from penetrat
ing
. If
the pumped liquid is dangerous
for people’s
safety
, the pump or the environment;
system
must be set up
for
carrying
to a
secure place
the leakage from
the drain plug.
POMPETRAVAINI is not responsible on the technical solutions adopted to realize the overpressure
system.
All the monitoring systems of the pump’s running must be chosen in such a way to guarantee a
protection level at least equal or superior to that of the pump. In case their selection or
installation is done by the user, POMPETRAVAINI can not assume any responsibility on
eventual mistakes committed.
The measurement of the eventually presence of vibrations on the pump casing is a useful system of
monitoring the running conditions. The vibrations may be generated by a pump’s problem itself (
e.g
.
f
unctioning in cavitations condition, alignment’s error of the coupling, flow instability, et
c
.)
or
it
may
be imposed to the pump by the specific installation. The components of the magnetic coupling
and
particularly the internal part of the rotor are build o
f
relatively frail material and are assembled with
tight
assembling tolerances. The presence of vibrations could cause impacts or frictions that
compromise
s
the l
ife
of the coupling system and the pump integrity. The installation of a vibrations
monitoring system is useful to signal the possib
ility
of a mechanical problem on the pump
as a
result of
hydraulic malfunctioning or mechanical failures.
The magnetic coupling pumps however follow all the other indications for maintenance, checking and functioning given
in the Operating Manual.
6.2 –POSSIBLE MALFUNCTIONING
The magnetic coupling is an item of a quite complex construction for specific material used and for the tight functioning
tolerances. The foreseeable malfunctioning as indicated by ATEX Directive are reported in the following paragraphs.
6.2.1 – Dry running
The pumped liquid carr
ies
out also the function to lubricate and cool down the internal rotor of the magnetic coupling,
made
of particularly frail materials. The possible dry running of the pump causes a quick increase of the rotor
temperature, which follow
s
a thermal expansion of its components that could lead to more friction or break-up of some
components
due to abnormal increase of
internal strain
.
The pump with a magnetic coupling must
NEVER
run dry.
In case of dry running for a period of time longer than 5 seconds, the coupling could suffer unfixable mechanical
damage
due to
thermal expansion strain. In case of dry running for a short period of time
followed by
flow again the
thermal strain would be the opposite of the
earlier
mentioned and for sure could lead to break-up of some internal
components of the rotor.
A flow monitoring system m
ust be provided
at
the inlet of the pump that could stop the pump if there is no pumped
liquid.
RESIDUAL RISK: in case of lacking of the pumped fluid the rotor temperature grow
s
up immediately up to the break-up of the same and transmitting the heat to the outside of
the pump. In case of dry running the temperature of the pump’s component could widely
overrun the temperature class Tx for which the pump have been classified. Could be
reached temperatures really higher than 300°C that could lead to ignition of flammable
substances, internal or external to the casing, possibly present.
6.2.2 – Loosening of the assembling screws of the coupling
The loosening of the assembling screws of the magnetic coupling, because of presence of vibrations
due to
alignment
error, cavitation, et
c
. could cause
it to become futher loose
, with consequent generation of vibrations and impact with
the can or other internal components (Fig.9).