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Energy Seminar
Emerson Process Management
June 22/23, 2010
Final Control Element Best
Practices for Efficient
Energy Use
Mike Lewis
Novaspect, Inc.
Emerson Process Management
Energy Management Seminar
Agenda
Process variability defined and its effect on
energy waste
Control valve shut-off defined and its effect on
energy waste
How to engineer improvement
Variability, defined by a Real Life
Example
Acceptable shower
temperature
Probability
of
Occurrence
Perfect shower temperature
Cardiac arrest
2nd degree
burns
Mean Value =
Shower Temperature
Causes of Variability
Loops
The Cause
20%
Design
Increases
Variability
30%
Tuning
As Many As 80% of
Loops Actually
Increase Variability
30%
20%
Source: Entech---Results
from audits of over 5000
loops in Pulp & Paper Mills
Control Valve
Performance
A Typical Control Valve Specification
You specify …
– Fluid properties
– Sizing requirements
– Design pressure and
temperature
– Allowable leakage when
closed
– Failure mode
– Connecting pipe size
– End connections
We engineer …
– Valve size
– Valve trim Cv versus % open
characteristic
– Valve type
– ANSI P/T rating
– ANSI leak class
– Actuation system
– Materials of construction
– Special characteristics for
noise, cavitation, flashing,
corrosion
An Industrial Example
Main Steam Temperature Control
ΔT = 50 F
1005 F
0.75% NPHR
Setpoint = 955 F
MS design temp
0.30% load !!
PV Distribution
+/-1-Sigma
+/-2-Sigma
+/-3-Sigma
Control Loop Objective …
Reduce Process Variability
Upper
Specification
Limit
Set Point
PV Distribution
2-Sigma
2-Sigma
Set Point
Reduced PV Distribution
2-Sigma2-Sigma
SUPERHEAT TEMP.
Main Steam Temperature Control
Decreased Variability = Increased Profit
Upper Limit
NPHR
= 0.75%
Reduction
Set Point
Increased Temp. Set Point
Reduced Process Variability
Provides the Opportunity for
Setpoint Change
= ( NPHR) X Fuel cost X KW-HR generated/year = Savings
= .75% x 11,000 BTU/KW-HR X $2.22/MM BTU X 320,000 KW X 8760 hours / year =
$516,517 per operating year !!
Dynamic Valve Performance
We’ve demonstrated value in reducing variability
in critical control loops
Poor control valve dynamic performance
contributes to variability
Let’s discuss …
– A specification for performance
– Designing for performance
– Testing for performance
– Maintaining performance
A Dynamic Control Valve
Specification
Combined backlash and stiction should not exceed
1% of input signal span
Speed of valve position response to input signal
changes from 1% to 10% shall meet specific Td, T63
and T98 times
Overshoot to step input changes of 1% to 10% shall
not exceed 20%
Loop process gain should fall between 0.5 and 2.0
… Entech “Control Valve Dynamic Specification” March 1994
Achieving Dynamic Performance by
Design
Friction
Positioner design
Machining accuracy
Positioner gain adjustability
Clearances
Flow geometry designed for
stability
Positioner tuning matched
to the valve assembly
Air delivery system
Plug/stem connection
Transducer design
Lost motion linkages
Soft part flexibility
Actuator spring flatness and
stiffness
Testing for Performance
Open-Loop
Fixed position –
constant load
Signal generator
flow
Control valve
FT
Transmitter
Pump
Open Loop Valve Performance
Open Loop Step Test
4" Segmented Ball Valves with Metal Seals and Standard Actuators/Positioners
Tested at 600 gpm in the 4" Test Loop
70
65
Fisher V150HD / 1052(33) / 3610J
0.5% Steps
1% Steps
2% Steps
5% Steps
10% Steps
60
55
(%)
50
45
40
35
0.5% Steps
1% Steps
Neles R21 / QP3C / NP723
70
65
2% Steps
5% Steps
10% Steps
60
55
(%)
50
45
I/P Input Signal
Actuator Travel
Filtered Flow Rate
40
35
0
50
100
150
200
250
Time (seconds)
300
350
400
450
Testing for Performance
Closed Loop
Load disturbance
z
flow
Control Valve
FT
Transmitter
Controller
Pump
Closed-Loop Valve Performance
Closed-Loop Random Load Disturbance Summary
Tested at 600 gpm in the 4" Test Loop
Controller Gain, Kc for the Fisher 4" ED / 667(45) / 3582
1
0.1
6
V
DG
F
5
Valtek 4" Mark I / Spring Cylinder(50) / Beta
Fisher 4" ED / 667(45) / DVC5010 G Tuning
Fisher 4" ED / 667(45) / 3582
Manual
Variability
(%)
4
V
3
V
V
V
V
2
DG
F F
F
F
DG
F DG
F
DG
V
F
V
DG
F
DG
F
Minimum
Variability
1
Faster Tuning
Slower Tuning
0
1
10
Closed-Loop Time Constant, (seconds)
Sustaining Performance Through On-Line
Diagnostics
G
H
D
E
A
F
B
C
Plugging of I/P transducer
Travel Deviation
Insufficient Air Supply
Calibration Changes
Diaphragm Leaks
Piston Leaks
O-ring Failures in Actuators
Packing condition
Friction and Deadband
External Leaks
Insufficient Seat Load for Shut-off
Many others
Control Valve Shut-off
Decreasing leakage
Increasing first cost
Increasing maintenance cost
An Industrial Example – a Feedwater
Heater Emergency Level Valve
Shell & tube heat exchanger ….
In heater: 31 psia, 215 F., 183.1 BTU/#
In the condenser: 1” Hg abs., 79 F., 47.1 BTU/#
Leakage worth 136 BTU/#
Difference in leakage between an ANSI Class II and Class IV is
1653-33=1620 #/hr
Result: 220,320 BTU/hr
At 3415 BTU/hr/KW: 64 KW!
At $1.58/MBTU coal cost: $4,284 / op. year!
Typical Power/Boiler Plant Energy
Efficiency Opportunities
Aux boiler mode steam
Air preheating
Aux steam header
pressure balancing
Blowdown and sampling
Condenser performance
Feedwater heater
efficiency
Superheat attemperation
Reheat attemperation
Emergency heater drain
valve leakage
Sootblowing steam
system
Station heating
Steam and water loss
Turbine cycle condition
Throttle pressure
Throttle temperature
Other Energy-related Variability
Examples
Fuel/air ratio control
Load change responsiveness
Steam header pressure balancing
Ramp rate improvement
Burner light-off
Drum level stability
Conditioned steam temperature stability and
turndown
The Takeaway
The undesirable behavior of control
valves is the biggest single contributor
to poor control loop performance and
energy waste … spend your money in
the basement!