This graph can be used to verify that the instrument air requirement is accurate and determine whether there is a volume compliance issue as the
result of a clogged input filter, restricted air line or improperly sized compressor.
Continuous monitoring of instrument air pressure is vital to actuator performance during normal operation. Should the air supply drop below
the minimum requirement, an alarm can be triggered in advance to notify operators of a potential failure. Instrument air is just as important during
valve testing in ESD applications. During a test performed at 60 PSI, the slew speed graph might indicate a 125 mS break (i.e., the time it took to
move the valve). However, if the instrument air increased for any reason to 120 PSI, the break time might increase as well, since it took longer to
exhaust the pressure at 120 PSI than at 60 PSI. This situation could cause an erroneous alarm indicating the valve failed the PST when, in fact,
there was no deviation in valve performance—simply a change in the instrument air.
provide authenticated evidence of these
actions for regulatory validation.
Intelligent control monitors are
designed to provide valve control, monitoring and diagnostic information,
including performance comparison
graphs in real-time, either via a field
network, local RS232 connection or
Bluetooth communication. Utilizing
HART with FTD-DTM software, data
exchange, storage and networking is
possible with any HART-based HOST
Managment System.
The intelligent field device, with
associated interrogation and reliability
software tools, enables plant personnel
to download data from the instrument
and upload this information to a valve
population management tool for use in
establishing, justifying and predicting
maintenance programs. The controller
stores all relevant information for the
valve package in non-volatile memory,
eliminating the possibility of lost data
and providing a complete history of the
equipment from the day it was installed
until the time it is decommissioned.
Field technicians can perform non-intrusive auto-calibration and configuration of the intelligent discrete con-
troller using a wireless PDA. This is
especially valuable when the valve is in
an area inconvenient or hazardous for
plant personnel to access. Utilizing an
auto-cal function, deadband is automatically set to match the hysteresis of the
valve actuator and consistent calibration characteristics can be written into
the user’s control scheme. When a valve
must be replaced, the technician simply
downloads all of the stored data from
the intelligent field device to a PDA,
then uploads it to the new device.
The ability to access a database with
complete information on the valve, actuator and controls from an intelligent
field device is a huge breakthrough for
end users. Not only does this feature
enable components and vendors for the
valve package to be immediately identified, it also allows technicians to upload
all stored information should it become
necessary to completely replace the discrete controller.
Performance Issues
Today, maintenance technicians needn’t
assess each valve’s current operating
condition with a trip to the field. Rather,
intelligent controllers using open com-
munications standards allow individual
valves to monitor their own health and
signal for help when they start exhibiting sub-optimal performance.
The graphs shown throughout this
article depict typical valve performance
issues addressed by intelligent controller
diagnostics utilizing timing, position
and pressure variables. The diagnostic
plots were developed from an advanced
discrete control monitor with pressure
profiling capability. The same relationships hold for double-acting actuators,
but for the sake of clarity, spring-return
actuators are shown—typically the open
cycle only.
User Benefits
The evolution of discrete valve control
technology, as evidenced by the development of intelligent linear/rotary control
monitors, is keeping pace with user
demands for lower total cost of ownership (TOC), reduced maintenance, higher reliability and increased process
uptime. These devices benefit process
operations by enabling real-time reporting, optimum asset management,
increased plant efficiency and reduced
