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2.2 Safety Instrumented Systems (SIS)
A SIS is a set of devices and software that perform one or more
Safety Instrumented Functions (SIFs). Each SIF has a stated Safety
Integrity Level (SIL) that is related to the acceptable levels of
probability that the SIF will NOT work when demanded (when
needed).

Instruments and control systems are tested and certified against
IEC standards, to determine the level of risk reduction they offer,
when used in a SIF.
MHI's Safety Control System

DIASYS Netmation4S has a unique design that allows integration of
the DCS and SIS. This enables a flexible arrangement in terms of
both cost and security. Furthermore, integration of DCS/SIS brings
several merits, such as common software tools and spare
components.
Multi-level Hardware Redundancies
To reduce or mitigate the impact of device failures on the safety of
the process, redundancies are introduced at various levels of a
control loop. This example shows how a critical protection function
is implemented through triple redundant Transmitters,
Controllers, and IO modules. When there is a failure or
disagreement between redundant devices, majority voting (e.g.
2oo3) is used to determine output.

2.3 Lock-Out-Tag-Out (LOTO) Feature
LOTO is a safety feature is available in DCS operation station, that
allows lock-out of a field device for maintenance and make it
impossible for others to operate the device without removing the
LOTO first.
2.4 Setpoint Supervision
While operators are allowed to enter set points as per process
requirements, strict upper and lower limits are placed to restrict
the operator from entering parameters which can destabilize the
process.

2.5 Preventing Override of Protection Functions
Plant operators are not allowed to override safety interlocks or
setpoints from operator stations. Any modification to those
setpoints and logics are accessible only to the Engineer through the
Engineering Station (located in a separate Engineering Room)

As process plants continue to increase in size and complexity,
instrumentation and control systems continue to help construction,
operation and maintenance personnel to better monitor, analyse
and control plant processes, thereby making them a safe place to
work.

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