Page 88 - Industrial Plant 2015

Fig. 2 - The the special control panels, but involved Cear
electrochlorination engineers from the early stages of the project in the
package working on electro-instrumental engineering, selection of
platform components and materials most suited to the
application, construction and programming of the
PLC (Programmable Logic Cntroller) and HMI
(Human Machine Interface) software, and the
commissioning and start-up activities in the platform, exhaust valve that changes the air; then the
completed in 2013.
A necessary choice:
overpressure execution
The main requirement of our customer was to have
the power and control system installed on each
electrochlorination skid. This requirement involved
the need to think of an Ex p pressurized PLC panel
therefore able to operate in the hazardous zone.
The Ex p plc panel was designed to allow the control
units and switchgear, as well as complete automation
systems suitable for the safe area, to be mounted
into the pressurized cabinet.
The overpressure (Ex p) ensures that the pressure
inside an enclosure is sufficient to prevent the
entrance of flammable gas, vapour, dust or fibres
and prevent possible ignition. Another process
maintains a constant flow of air (or an inert gas) to
take away any potentially explosive atmosphere.
Thanks to this type of protection, even non-
explosion-proof devices could be operated in
potentially explosive areas, as the underlying idea is suitable for operation in zone 2.
to prevent an explosive atmosphere from entering a
sealed protective enclosure by generating a
permanent overpressure against the surrounding
atmosphere.
To obtain this condition, the cabinet is equipped with A particular feature of the two electrochlorination
an additional external panel, in Ex d execution,
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containing the components necessary to activate
the overpressure process, which is achieved with
the release of a protective gas (normally consisting of
instrument air) within the closed control panel.
The release of this gas is preceded by a “washing
phase”, during which the protective gas entering the
casing to be protected flows under the control of an
“maintenance phase” of pressurization, which allows
the control panel to operate safely, thus allowing the
full commissioning of the equipment.
In case of failure or malfunction, the control panel is
also equipped with signal alarms to put the
equipment out of service until the necessary
pressurization is restored.
In the specific case, were made two power and
control panels marked Ex pz (ia) zone 2 3G IIB T5
that, according to the Atex/94/9/EC Directive, adopt
a system of protection for overpressure joined to the
intrinsic safety equipment which makes them
suitable for use in zone 2, guaranteeing a maximum
surface temperature of less than or equal to 100 °C.
The cabinets were made of stainless steel Aisi 316,
with an enclosure rating IP 66, to resist environmental
corrosion and meet the requirements of the
standards, while the internal electrical components,
thanks to the Ex pz execution, permitted the
installation of components for safe area.
Being automation panels, they are also provided
with an external HMI touchscreen operator panel,
Double system to optimize
production
packages is that, being placed near to each other,
they would have to work, alternately or together
depending on the production needs of the plant. In
this way, we could achieve the goal of optimizing
production while also ensuring the continuity of
operations (figure 2). Obviously, this operation must
always take into account the presence of an
environment at risk of explosion, and therefore,
during the design of the automation system, we
considered various precautions so as not to
compromise operational security.
So that the two panels could operate safely, in a
precautionary way, and in line with the intrinsic safety
requirement for this application, Cear engineers
developed an analysis of Ex i loop safety parameters
to verify that the energy that passes through the
loops to the field does not cause sparks in the event
of a short-circuit.
In addition, we did not overlook the element of