Page 13 - Magazine - 13 Version.cdr

Instrumentation & Controls - Making your Plant a 'Safe Place to Work'

Haldipur Amul
Chief Engineer I & C And Senior Manager
Whenever one speaks of safety, the first things that normally come to anyone's mind are the
safety precautions and personal protective equipment (PPE) being used during site
construction and operation. However, various safety features must be incorporated by the
design engineers during the conceptual, basic, and detailed design development stages of
the project.
From a safety viewpoint, instrumentation and control systems are designed to address the
following critical needs:

1. How instruments themselves may pose a safety hazard
(electrical signals from instruments possibly igniting
hazardous atmospheres), and how such hazards can be
mitigated.
2. How process safety systems and instrumentation and control
systems may be configured to detect unsafe process
conditions and automatically protect the process/ equipment/
human by taking timely action.

1. Area Classifications and Instrument Selection
Instrumentation and control systems generally work on 24V DC or lower voltage levels, which itself is not
dangerous for humans to work on. However, when the process involves some hazardous (flammable)
substances, even this low voltage can pose a hazard if exposed to the flammable atmosphere.
To avoid any unsafe event and/or damage to personnel and/or property, it is mandatory to consider the
appropriate hazardous area classification while selecting field instruments, junction boxes and control systems
as indicated below:
Instruments are designed in such a way to limit current flowing
into instrument to levels that reduce possibility of ignition in field.
Such instruments are classified as Ex ia and are suitable for Zone-0
and Class-1 Div-1 areas.
Another method is to use Explosion-proof instruments (classified
as Ex d) and junction boxes. In this design, energy is contained
within the instrument/junction box using special construction to
avoid products of explosion from coming into contact with
hazardous gases outside the enclosure. Such arrangement is
suitable for Zone-1 area.
2. Process Safety and I&C

Various aspects of process safety and how it is achieved through
instrumentation and control is described below:

2.1 Functional Safety
Functional safety is a part of overall safety of a system or an
equipment that depends on the system or equipment operating
correctly in response to its inputs. The following LOPA (Layers of
protection analysis) diagram depicts how instrumentation &
control systems offer additional levels of protection.

12