ABB MagMaster User Manual page 7

EXTRACT FROM EECS NEWSLETTER
NUMBER 15 (OCTOBER 1991)
Certification of intrinsically safe apparatus for use
at non-atmospheric pressure.
The certification
electromagnetic flowmeters and like devices
which have probes protruding into the process
fluids have given us cause for concern for some
time. The problem stems from the possibility of
voids which contain flammable atmospheres
occurring within the process fluid. If the user can
guarantee that such voids will not occur, then the
inside of the pipe or vessel can be classified as a
safe area and no further precautions are needed.
If voids may occur but the pressure is not greater
than atmospheric, then the problem is already
taken care of by the intrinsic safe certification of
the circuit of which the probes form a part.
Unfortunately, the pressure in the pipe or vessel is
almost certainly above atmospheric, and this is
where the problem really lies. The intrinsic safety
Standard is written and provides data only for
explosive mixtures at atmospheric pressure. We
do know that as the pressure of an explosive
mixture increases, the minimum ignition energy
(MIC) required to initiate an explosion, decreases.
The amount of information which we have
available on this topic is very limited, but we do
know that the MIC decreases at an alarming rate
as the pressure increases.
EECS put the problem to the members of the
Heads of Testing Laboratories Working Group
(HOTL) and after some discussion, the unanimous
conclusions were as follows:
a. Certificates of conformity with the European
Standard are valid only for apparatus which
operates at atmospheric pressure;
b. the manufacturer must be informed that any
certificate of conformity will not be valid for
cases where the apparatus is exposed to
pressures significantly above atmospheric;
c. a national or inspection certificate may be
issued for the apparatus if testing (with the
appropriate factors of safety) shows that it is
safe for use at the specified elevated pressure.
1 CENELEC/ATEX APPROVED FLOWMETERS...
of intrinsically
We have discussed this recommendation with the
BASEEFA Advisory Council who have agreed that,
if satisfactory measures to prevent the occurrence
of voids cannot be taken, we should advise
applicants of the limitations of certification of
equipment of this type and ask them for
confirmation that any danger arising from this
safe
source has been recognised and catered for.
Reproduced from EECS Newsletter No. 15
courtesy EECS, Buxton, Derbyshire
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