Instrumentation In Hazardous Area

[YSH]

Dear Experts,

Though I am clear about the concepts of hazardous area classification and the protection methodologies, I have many questions. This is specifically for instrumentation (not electrical, like motors etc).

Now when we consider instrumentation on closed vessels having flammable liquids, (eg. tunning fork type liquid level switch or a differential pressure transmitter type LT) there two points to remember,
1. The actual sensor part which is wetted and falls into Zone 0 and carries some electrical energy for actual measuring principle, capacitance measuring, piezo resistivity etc.
2. The enclosure part which lies in Zone 1.

Now my questions are,

-why do we need the Ex ia in this particular case, as the actual electronics is in Zone 1, Ex d type protection can do.
- Why do Ex-ia devices also have Ex d type heavy enclosures?
- In instruments which required higher power 110 VAC, say a vibration fork for flammable liquid or a capacitance type LT, how are these used to work in closed vessel holding flammable liquid. how is the electrical energy going inside the vessel via the probes controlled. Does in case the fork is intrinsically safe and the electronics part is in explosion proof enclosure? What about the process seal which isolates the probe and the electronics, what if it fails, leading to flammable vapour entering the high powered enclosure.

I am working a vibrating fork type level switch for liquid with the following marking,

ATEX II 1/2G Ex d IIC T6/IECEx Zone0/1

Since it says 1/2G it can be used in Zone 0. Also the protection methodology is Ex d which means it cannot be used in Zone 0. The documentation says it can be used in closed vessels with flammable liquid but there is no mention of how the energy going via the fork into the vessel is limited. Now this is far more confusing than I thought. Please help me to clear it.

Also I have referred many standards but none of them speak of the probe and the electronics parts separately. If anybody knows of any documentation, please let me lnow

[Safeti]

An
explosive atmosphere
for the purposes of Directive 94/9/EC is defined as a mixture
i)
of
flammable substances
in the form of gases, vapours, mists or dusts;
ii)
with
air;
iii)
under atmospheric conditions
22
;
iv)
in which, after ignition, the combustion spreads to the entire unburned mixture (It has
to be noted that sometimes (mainly with dusts) not always the whole quantity of the
combustible material is consumed by the combustion).
An atmosphere, which could become explosive due
to local and/or operational conditions, is called a
potentially explosive atmosphere
. It is only this kind of potentially explosive atmosphere which
products falling under the Directiv
e 94/9/EC are designed for (s
ee as well chapter 4.3 ‘Risk
Assessment’).
It is important to note, that products
are
not covered by Directive 94/9/EC
where they are intended
for use in or in relation to mixtures which might potentially be explosive, but one or more of the
defining elements i) to iv) above are not present
.

[aseptman]

Any relevant standards? Pl upload

[Safeti]

if the working pressure of the vessel is higher than 1 bar and/or no air is present in the vessel, ATEX directive shall not be applied.
If ATEX is to be applied then you have to consider the risk assessment and rate your device for the worst case.(Area 0, protection mode i). Wetted part element if not in contact with air does not fall in the directive.