Chemical Engineering October 2010 - 45

Before improvements
After improvements
Fisher Controls International
Reactor
level
~ 50% less
variation in
valve position
and flow to
downstream
reactor
~ 120 days
replace the control valve with one that
is properly sized and has better throttling
control performance and sufficient
on-off characteristics (in this
case a tight shut-off). And, the valve
positioner should be tuned properly
for the optimum response.
In an effort to provide more immediate
improvement in the reactor level
control, the following interim improvements
were made to the control valve:
reduce the slack in the actuator coupling
system; and improve the tuning
of the digital valve positioner. To reduce
the slack in the actuator coupling
system, new keys were machined with
a tighter fit in the keyways. Then, additional
and larger set screws were
installed in the coupling to press on
the keys. Figure 8 shows the changes
made to the actuator coupling system.
The valve positioner was tuned to provide
a faster response time, improve
its travel-gain performance and to reduce
the tendency to hunt or oscillate.
The interim efforts to improve the
control valve performance were beneficial
and helped to improve the reactorlevel-control
performance and reduce
the frequency of unplanned reactor
trips due to level control disturbances.
Figure 9 shows the reactor's level control
performance before and after the
interim improvements were made.
References
1. Tolliver, Terry, Process Analysis for Improved
Operation and Control, Fisher-Rosemount
Systems Advanced Control Seminar, 1996.
2. Bialkowski, W. L., Dreams Versus Reality, A View
From Both Sides of the Gap, Keynote Address,
Control Systems '92, Whistler, British Columbia,
1992, published Pulp Pap. Can. 94(11), 1993.
3. ANSI-ISA-TR75-25-02-2000 Control Valve
Response Measurement from Step Inputs
4. ANSI-ISA-75-25-01-2000 Test Procedure for
Control Valve Response Measurement from
Step Inputs.
5. EnTech Control Valve Dynamic Specification V3.0.
6. ANSI/ISA 51.1-1979 (R1993) Process Instrumentation
Terminology.
7. Fitzgerald, Bill, " Control Valves for the Chemical
Process Industries " , McGraw-Hill, New York, 1995.
325 Waverly Avenue * Mamaroneck, NY 10543
1 800 AT ELCAN * 914-381-7500
sales@virto-elcan.com * www.virto-elcan.com
Circle 48 on p. 66 or go to adlinks.che.com/29255-48
ChemiCal engineering www.Che.Com oCtober 2010 45
The complete recommended solution
was to replace the original 18-in.
valve with a 12-in. high-performance,
segmented-ball, throttling control
valve with a spring and diaphragm
actuator. This control valve system,
similar to the one shown in Figure 10,
is specifically designed for throttling
applications. A high-performance,
digital valve positioner was included
and was properly tuned to achieve
the optimum performance. The new
control valve provided even better reactor
level performance and allowed
a 10% production-rate increase and a
significant economic benefit.
■
Edited by Dorothy Lozowski
Author
James Beall is a principal
process control consultant at
Emerson Process Management
(12301 Research Blvd.,
Research Park Plaza, Bldg.
III, Austin, TX, 78759; Phone:
903-235-7935; Email: James.
Beall@Emerson.com) with 29
years of experience in process
control. His areas of expertise
include process instrumentation,
control strategy analysis
VirtoElcan_2:Layout 1 11/10/09 2:15 PM Page 1
and design, control optimization, control valve
performance and advanced process control.
FIGURE 9.
The reactor
level control
improved
with interim
control-valve
improvements
FIGURE
10.
A high-performance,
segmented-ball controlvalve
system, similar to
the one pictured here,
provided better level
performance
is now
The BEST screening machines
just got BETTER!
* Eliminates Blinding
* Superior throughputs
* Increased efficiencies
* Exceptional Performance on
Dense Powders Damp feeds
Liquids/Slurries Sticky materials
* Screening down to 10 microns
http://www.virto-elcan.com http://adlinks.che.com/29255-48 http://www.Che.Com
Chemical Engineering October 2010

Table of Contents for the Digital Edition of Chemical Engineering October 2010
