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!