Hydrocarbon Processing - July 2021 - 78

Valves, Pumps and Turbomachinery
A dangerous undetected failure is a
major incident waiting to happen. The
SIF cannot function, and the operating
unit is unaware of that fact. The operating
unit will only discover the problem when
a trip condition occurs, and the SIF fails
to function.
Proof testing and diagnostics are
key. Proof testing is a plant's main defense
against dangerous undetected failures.
To detect and minimize the probability
of a SIF not functioning, plants
must fully proof test all equipment on a
routine basis. To achieve and maintain
a SIL, the plant can either test more frequently,
use components with lower failure
rates or use components with a higher
level of integrated diagnostics.
Many refining and chemical plants
have SIL 2 and SIL 3 loops and run up
to 5 yr between outages, creating extreme
challenges for the safety system. Since a
proof test will often trip a unit or even
an entire plant, it usually can only be implemented
during a full unit shutdown.
Therefore, the operating unit must either
devise a means to dramatically improve
the reliability of the SIF components,
or it must shut down more frequently to
perform proof testing.
Safety equipment reliability. A SIF's
SIL is determined by the PFD of the
entire safety loop. This is determined
by calculating the PFDs of each component
and adding them together. When
one evaluates the typical contribution of
each component of the safety loop, final
elements are found to be responsible for
over 50% of the SIF loop failures (FIG. 2).
Sensors inherently have more diagnostics
and can usually be tested while
the plant is running, and it is relatively
simple to implement redundancy and
more advanced voting schemes to increase
reliability. Furthermore, most sensors
have no moving parts and, thus, have
no wear parts, so long operating lives with
minimal required maintenance is typical.
Most final elements are valves and
have constant contact with the process
media. These valves typically have minimal
diagnostics, can go years without
operating and cannot be fully tested
without either shutting down the plant
or isolating the valve needing testing
through a bypass arrangement. Like
most mechanical components, valves
work best when operated periodically,
which is often not possible with traditional
installations. If a plant is looking
to improve SIF reliability and extend
Proof test interval
PFDavg
TABLE 1. End user's SIL calculation comparison
Traditional fi nal element
6 yr
Risk reduction factor
SIL
0.01
96
1
Proprietary solutiona
6 yr
0.005
189
2
proof test intervals, the final element is
the first place to start.
Focus on final elements. A typical final
element is composed of several components
that might include a valve, actuator,
solenoid, regulator, mounting brackets
and couplings-and possibly positioners,
I/P transducers and booster valves. Each
of these components must function individually
and in concert to ensure the safety
function is achieved, whether that is to
open or close the valve. Except for the positioner,
many of these components offer
few, if any, diagnostics, so failures tend to
go undetected.
Engineering a final element can be a
complicated task. It is usually some type
of shutdown valve composed of several
components, and each part must be carefully
engineered to handle the process
conditions and actuating torque, while
working reliably despite very infrequent
use in typical safety applications.
Normal and upset process conditions
must be carefully evaluated and understood
when choosing the shutdown valve
and its actuator components. In addition,
the proper combination of solenoids,
mounting brackets, couplings and other
critical hardware must be specified and
carefully matched to the selected valve.
When unsure of the sizing, the actuator
will often be over specified above required
safety margins, needlessly increasing the
weight, size and cost of the final element.
Therefore, careful design is required, often
requiring the assistance of specialists.
FIG. 3. Proprietary final element solutionsa
are
FIG. 2. An analysis of typical SIF loops shows that the final elements (typically valves) contribute
approximately 50%-70% of overall safety loop failures.
78 JULY 2021 | HydrocarbonProcessing.com
custom engineered shutdown valves provided
as fully integrated, tested and certified final
element assemblies, with the valve specifically
designed for the application and each
component carefully matched.

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Hydrocarbon Processing - July 2021

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