K. Jensen and L. PARRENT, Emerson Automation Solutions,
Hazardous materials and dangerous reactions are
commonplace in petrochemical, refining and specialty chemical processing units.
As an added layer of protection for plant personnel, equipment and the
environment, companies usually install safety shutdown systems that depend on highly
reliable, safety-critical shutdown valves to move the process to a safe state.
This article discusses recent design enhancements in safety-rated
digital valve controllers that can dramatically improve reliability and
simplify the maintenance of safety valves. These new design enhancements incorporate
fewer components, provide increased diagnostic coverage and performance
feedback, and enable partial stroke testing.
shutdown valves. Most industrial plants are controlled by a basic process control
system (BPCS) (FIG. 1).
Should the BPCS fail to function, or if an unsafe condition occurs, then a
safety instrumented system (SIS) acts to isolate the process and move it into a
safe state. The SIS uses its own instruments to independently monitor the
process, and the system has its own means of tripping the unit by shutting down
pumps, stopping feeds, venting high-pressure gases and/or taking other actions.
device in the SIS loop is critical, shutdown valves are the heart of the
interlock system and are usually responsible for more than 50% of reliability
problems. SIS sensors can easily be made redundant; they can usually be tested
while the plant is running and they employ a host of diagnostics, so they tend
to be very reliable. The logic solver is often housed in a conditioned
environment and includes multiple levels of redundancy and digital diagnostics,
so it rarely fails in normal operation.
sensor and logic solver, the safety shutdown valve must employ moving parts,
each of which are subjected to process conditions, and these valves often cannot
be tested while the plant is running. A typical SIS valve lacks diagnostic
sensors and may go years without actuating—however, when necessary, it must
instantly respond to move the process to a safe state. For these reasons,
safety shutdown valves are often the weakest link of the SIS loop and are therefore
an area where significant improvements can be made to enhance the performance
of the entire SIS.
design improvements. A typical on/off safety valve includes
a solenoid to command the valve state and an actuator to physically move the
valve itself. Larger valves require some type of pneumatic booster to move the
valve faster, and they may also include mufflers to reduce noise as the valve moves.
Such a valve assembly has several inherent limitations, including:
To address these concerns, many users are replacing the solenoid,
booster and muffler with a single SIS-rated, high-capacity digital valve
controller (FIG. 2).
These devices create dramatic improvements in reliability, while reducing maintenance
and support costs.
Digital valve controllers have long been used to improve safety
shutdown valve reliability. These devices provide diagnostic information on air
supply, valve stroke time, actuator internal components, packing performance
and more (FIG. 3).
They also provide feedback on the actual valve position, which can be used to
take credit for a valve proof test should the plant unexpectedly trip.
While all these features improve reliability, recent enhancements allow
the digital valve controller to do much more. Larger valves have much larger
actuators, so a standard digital valve controller is often unable to move the
valve fast enough to satisfy the stroke time requirements. In these cases,
pneumatic boosters are employed to amplify the valve controller signal and move
the valve faster. Boosters tend to be loud when they stroke, so mufflers are
also employed to reduce the sound to acceptable levels. Unfortunately, all these
added devices are perceived to reduce reliability, complicate the design and
increase the form factor of each valve assembly.
Recently introduced high-capacity digital valve controllers incorporate
several design features that eliminate both the booster and the muffler. A single
valve controller can now provide all diagnostics and reliability improvements,
while providing enough air capacity to move very large valves quickly. They
also include specially designed mufflers that deaden the sound without adversely
impacting air capacity, and thus, stroke speed. Internal spools are made of
ceramic materials to minimize wear and combat corrosive environments. The
single unit is simpler, has a reduced footprint and is much more reliable. Because
the booster is an integral part of the valve and is included with the unit’s safety
integrity level (SIL) rating, it does not have to be considered as a separate
component for SIS calculations.
Additionally, an enhanced SIS-rated digital valve controller enables partial
stroke testing. One of the most common failures of a safety shutdown valve is
that it fails to move at all. Because the valve may not have moved for years,
it can become stuck in a position due to process buildup, corrosion or other
factors, and then fail to actuate when called into service. While a partial
stroke test does not prove 100% of valve functionality, it does cover most
valve failure modes and can be used to extend proof testing intervals.
Digital valve controllers can execute these tests and capture valve
performance data as the partial stroke test is performed (FIG. 4). Unlike more
mechanical means of partial stroke test execution, the digital valve controller
can monitor valve position and control actuator pressure, thus greatly reducing
the likelihood of over-stroking the valve and impacting plant operations.
The test can be commanded from the control system, or valve accessories
are available that allow a partial stroke test to be executed and controlled at
the valve, where performance can be closely monitored and quickly overridden,
reliability. An enhanced high-volume, SIS-rated
digital valve controller provides significant improvements in safety valve
reliability vs. the typical solenoid/booster/muffler design. This single device
replaces three components and provides a wealth of diagnostic data that can
flag developing problems in advance of outright failure. The controller also
enables partial stroke testing, yet guards against excessive valve movement,
avoiding process upsets or accidental plant trips. Unlike the solenoid/booster
solution, the digital valve controller provides continuous valve position
feedback so that, in the event of a trip, valve performance will be captured
and documented, allowing plant staff to take credit for a full proof test.
The unit can be installed as a retrofit on existing valves or specified
as the control component on new safety shutdown valve installations. Optionally,
the entire valve assembly (digital valve controller, actuator and valve) can be
purchased as a single engineered assembly, which further improves reliability ratings
and extends proof testing intervals.
Takeaway. When specifying a new SIS-rated shutdown valve or retrofitting an
existing valve, end users should investigate the available digital valve controller
options. New designs are simpler than previous models, and provide a
significantly improved level of reliability, while reducing installation and
maintenance support costs. While specifically designed for safety shutdown
valve applications, these same digital valve controllers are often the best choice
for any critical on/off process valve where high performance and reliability
are required, along with a smaller form factor. HP
KURTIS JENSEN is a Portfolio Manager for Emerson. He is responsible for product
development, and serves as an advocate and promoter of innovative technologies
that benefit end users. He has more than 40 yr of experience in field service,
system integration, asset management, cybersecurity, wireless technologies and
field instrumentation. In Iowa, he earned a BA degree in business administration
from Buena Vista University, along with an MBA degree from the University of Dubuque.
LURA PARRENT is a Product
Manager for Emerson and provides technical support for Fisher safety
instruments. She has been with Emerson for more than 4 yr and has previous experience
as an applications engineer. Parrent earned a BS degree in chemical engineering
from the University of Nebraska.