POWER February 2012 - 38

INSTRUMENTATION & CONTROL
Monitoring Control Loop
Performance
Control loop performance-monitoring software can help to improve loop performance
at electric power plants by automatically collecting data, assessing
several aspects of loop performance, and providing the results in
reports and user interfaces.
By Jacques F. Smuts, OptiControls Inc.; John N. Sorge, Dale Evely, and Mark Faurot, Southern Company; and Aaron Hussey,
Electric Power Research Institute
P
ower plant control systems must
perform well to obtain maximum efficiency,
reliability, regulatory compliance,
and safety from a plant. Poorly
performing controls can cause operational
difficulties such as boiler oscillations, increased
heat rate, accelerated equipment
wear, increased emissions, slower load
ramp rates, reduced generation capacity,
steam temperature excursions, and unit
trips following upsets.
To maintain optimum plant performance,
control loop performance should always be
tuned for maximum performance. This can
be achieved only by monitoring loop performance
and taking the appropriate corrective
actions when poor performance is detected.
However, the diagnosis and resolution of
control loop problems are difficult, particularly
in large and complex process facilities
such as power plants.
Several control loop performance-monitoring
(CLPM) software products have become
commercially available over the past
decade. These products automatically monitor
the performance of control loops. They
potentially identify several aspects of poor
control and can generate a prioritized list of
problem loops with diagnoses of the individual
problems so that these can be corrected.
CLPM Software
Control loop monitoring solutions are available
from distributed control system (DCS)
vendors and third-party software companies,
including ABB, AspenTech, Capstone Technology,
Control Arts, ControlSoft, Control
Station, Emerson, ExperTune, Honeywell/
Matrikon, PAS, and RoviSys. The products
vary in the range of analyses they perform,
presentation of results, reporting features,
and data collection methods, but they all aim
to identify and report on poorly performing
control loops. One such software product,
PAS PSS Loop Analysis, was tested by the
Electric Power Research Institute (EPRI)
38
and Southern Company on Alabama Power's
Plant Gaston Unit 3.
Although there are differences in the features
and presentation methods from one
CLPM product to another, most provide the
following core functions:
■ Automatically collect process data for
analysis.
■ Assess the dynamic performance of control
loops.
■ Diagnose loop performance problems.
■ Represent loop performance with a set
of metrics.
■ Aggregate loop performance metrics
into high-level key performance indicators
(KPIs).
■ Present loop performance in user interfaces
and/or reports.
The first requirement for monitoring the
performance of control loops is access to uncompressed
process data sampled sufficiently
fast to capture the loop dynamics (Figure
1). A good way of collecting process data for
loop monitoring is through object linking and
embedding for process control (OPC) with
real-time data access (DA) because it is supported
by all modern control systems, and it
gives the loop-monitoring software the most
control over sampling rate and data resolution.
Careful consideration should be given
to the rate at which loop performance assessment
software acquires data from the data
server to avoid overloading it.
To improve network security, most plants
segregate process control computer systems
and regular users into individual networks
separated by firewalls. Process data has to
flow from the process control network to
the application servers, and client computers
must have access to connect to these software
applications. Software vendors provide various
data connectivity solutions to get process
data to their application servers and application
data to their users.
Because CLPM software will likely have
multiple users, a client-server architecture
is essential. The server is responsible for
collecting data, running the analyses, his1.
Keeping things under control. The basic steps in control loop performance monitoring
are shown. Courtesy: OptiControls Inc.
Collect data
Assess loop
performance and
calculate metrics
Diagnose
performance
problems
Calculate
high-level KPLs
Report results
www.powermag.com
POWER | February 2012

http://www.powermag.com

POWER February 2012

Table of Contents for the Digital Edition of POWER February 2012