Hydrocarbon Processing - July 2021 - 79

Valves, Pumps and Turbomachinery
Improving final element safety and
reliability. Final element safety and reliability
can be improved by making use
of fully integrated assemblies, partial
stroke testing and increased diagnostics.
Documenting and taking credit for unplanned
trips can simplify compliance
with test requirements.
Fully-integrated assemblies. Rather
than having a user specify several individual
valve components from different
vendors, a single provider is now offering
a complete final element assembly
appropriate for a user's specific safety
application (FIG. 3). This valve assembly
is specifically engineered to satisfy the
application requirements and is provided
holistically as a fully tested and certified
unit. The assembly is maintained from a
total safety lifecycle perspective, rather
than as a general service isolation valve.
Each assembly is furnished with a single
serial card delineating the details of every
part, as well as a single SIL certificate and
safety manual covering all components of
the final element assembly.
These improvements in design and
testing as an engineered solution and
safety lifecycle approach to the final element
provide a significantly improved
PFD rate. In some cases, the failure rate of
the assembly will be up to 50% less than
the combination of the same components
purchased individually, allowing a plant
to significantly extend proof test intervals.
Partial stroke testing. A partial stroke
test (PST) utilizes a custom solenoid or
positioner to allow a valve to partially
stroke toward its safe state (i.e., open or
closed) during the test. While this does
not constitute a full test since the valve is
not completely stroked, it does show that
the valve is not frozen in place and the solenoid
and actuator components are functioning
as designed. A PST is considered a
diagnostic and partial test and can be used
to improve a valve's overall reliability rating.
The challenge of a PST is to avoid upsetting
the process. If the valve strokes too
far or moves too quickly, it will often introduce
enough of an upset to shut down
the process, so PST components must be
carefully designed to avoid this situation.
Increased diagnostics. Significantly
improved diagnostics are available with
smart valve instrumentation, such as a
digital valve controller (DVC), which can
be installed on valves with analog or digital
controls (FIG. 4). A DVC can monitor
several parameters, including air supply
pressure, actuator movement, seal performance,
solenoid health and others. It
can also be used to initiate and document
a PST, and the improved diagnostics can
be used to significantly reduce the PFD of
the entire shutdown valve assembly.
Additionally, some DVCs can document
and take credit for unplanned valve
movement. Occasionally, an unexpected
process upset will cause the logic solver to
trip a SIS valve. If the valve position is historized
using data from a digital positioner,
the valve performance can be captured
and documented as the trip sequence
executes. Assuming the valve performs
as expected, this data can be considered
a fully executed proof test, thus resetting
the proof test interval time.
Results. Implementation of these concepts
has resulted in significant savings for
several end users, with the reliability and
integrity of the safety systems improved.
One end user wanted to achieve a 6-yr
proof test interval for a SIL 2 loop. When
they performed a component SIL calculation
on a standard final element assembly,
they could only achieve SIL 1 reliability
at a 6-yr proof test interval, as shown in
TABLE 1. However, a proprietary solutiona
was designed to meet SIL 2 requirements
using the same components and still
achieving a 6-yr proof test interval. This
allowed the plant to safely extend its run
time to 6 yr, generating millions of dollars
in additional revenue.
In another case, a U.S. refinery had to
shut down as a hurricane approached.
Trips and shutdown valve performance
were captured by a proprietary digital valve
controllerb
as the plant was taken out of
service. With this data used as documentation
of proof testing, the plant avoided further
downtime costing $2 MM/d.
Caveats. Implementation of each of
these concepts requires careful design and
engineering, coupled with close attention
to operating procedures. Many chemical
plants and refineries are already running
with minimal onsite staff, exacerbating
these issues. Because SIS installations are
highly specialized, a plant may not have
the specific required expertise on hand
to implement changes and modify operating
procedures, even for improvements
expected to be highly beneficial.
These and other related issues can
FIG. 4. PST-via digital valve controllers-
provides two methods to significantly improve
shutdown valve reliability. These devices can
perform a PST while ensuring the valve does
not fully stroke. The digital valve controller
shown can perform the PST, monitor valve
components and initiate other tests.
usually be effectively addressed by enlisting
the services of a trusted partner,
either an engineering or consulting firm
or a supplier. This requires reliance on a
third-party source, introducing cost and
risks. However, in most cases, the return
on investment can be justified.
Takeaway. Satisfying SIL requirements
with extended proof test intervals can
be very challenging, and many plants are
looking for ways to further extend intervals
safely. SIS final elements offer the
greatest opportunity to extend proof test
intervals through improved SIF reliability.
PST, increased diagnostics, valve trip
documentation and complete valve assemblies
can dramatically improve final
element performance, while continuing
to meet the required SIL, even as production
runs are extended.
NOTES
a Fisher™ Digital Isolation™ Solutions from Emerson
b
Fisher™ FIELDVUE™ DVC6200 digital valve controller
MIKE
HOYME is a Product
Manager for Fisher Rotary Valves.
He is a certified functional safety
professional with 10 yr of
valve engineering and product
management experience. He strives
to create final elements for SISs that
both improve safety and process uptime. Mr. Hoyme
earned a BS degree in mechanical engineering from
the South Dakota School of Mines and Technology.
JUSTIN MILLER is a Senior Sales
Engineer for Emerson's flow controls
products, specializing in the chemical
industry. Previously, he spent 6 yr
in product management where
he supported Fisher sliding stem
valves. He is a certified functional
safety professional with 9 yr of valve application and
product management experience. He strives to help
customers find the best possible solutions to
challenging applications. Mr. Miller earned a BS degree
in mechanical engineering from Iowa State University.
Hydrocarbon Processing | JULY 2021 79
Hydrocarbon Processing - July 2021

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