application of a new boiler scale and corrosion control automation

Report
Improved Boiler System Operation
with Real-Time Chemical Control
Debbie Bloom, Nalco Company
A Need for Measureable Environmental
Return on Investment …
• Increasingly competitive marketplace
•
•
– Extend equipment life
– Reduce fuel and water costs
– Optimize operational labor costs
Increased environmental awareness
Corporate/government initiatives to
– Reduce greenhouse gas emissions
– Fuel and water consumption
2
Primary Water-Related Challenges For
an Operating Boiler
• Mineral Scale
–
–
–
–
Dissolved minerals exceed solubility
Typically magnesium, calcium, iron, silica based
Impedes heat transfer
Commonly treat with phosphate, polymers, chelants
and by improving feedwater quality
• Corrosion
– Causes metal loss, perforation of equipment surfaces
– Causes iron deposits in boiler
– Commonly treat with oxygen scavengers and pH
control agents
3
Traditionally, scale and oxygen control
chemicals have been measured and
controlled in the boiler water
• Analytical detection not low enough for
•
•
feedwater
Sample already existed
Variability of the feedwater system
4
Until Recently, Control of Boiler
Chemistry was Test and Adjust
•
•
•
•
Gather sample
Test
Adjust chemical feed
“Repeat as necessary”
Why Feedwater instead of Boiler Water?
• A boiler typically has a very long holding time
– BD sample has little direct correlation to the feedwater at any time
• Every boiler will have unique lag time
– Based on design, feedwater quality and operating conditions
• Lag time is always VERY LARGE relative to
dosage control
Site
Campus A
Campus A
Chemical Co. B
Chemical Co. B
Paper Mill C
Paper Mill C
Boiler
Type
Holding Time
Half-life
Pressure
Cycles of
First 1% or 50% Last 1%
psig/barg Concentration (hrs)
(hrs)
(hrs)
Firetube
Watertube
HSRG
Power
Recovery
Recovery
125 / 9
125 / 9
1000 / 69
1000 / 69
1250 / 86
1250 / 86
10
10
29
29
45
45
0.1
0.1
0.1
0.3
0.2
0.4
4
6
10
18
16
26
25
41
67
120
109
174
6
Scale Control
Automated Scale Control Utilizes a
Stable Inert Trasar
Provides a stable inert monitor of system performance
• Inert tracer chemistry survives in
Patented LED fluorometer
boiler system (FW & BW)
– Good for boiler systems up to
1000 psig/69 barg
– Works for both on-line and
grab sample monitoring
– Provides indication of carryover if seen in the condensate
– Provides positive feedback
that chemical treatment is fed
8
Corrosion Control
Corrosion/ORP Basics
• Corrosion is an electrochemical process
• Corrosion involves both oxidation and reduction
(REDOX) reactions
• ORP = Measures the net voltage (mV) produced
by all REDOX reactions taking place
• ORP is a good indicator of feedwater corrosion
Reducing Conditions Minimize Corrosion
(More Negative ORP)
O RP ( m V) 400F, 204C
M o re R e d u c in g
400
O xid izin g
200
0
-2 0 0
-4 0 0
-6 0 0
0 .1
R e d u cin g
1
10
100
D is s o lv e d O x yg e n (p p b )
1000
11
Many Factors Affect the ORP Fingerprint
of Each System
Mechanical
Operational
Chemical
• System design
• Deaerator venting,
• Dissolved oxygen
• Oxygen
metallurgy
• Deaerator tray
alignment
• Feedwater heater
• Economizer leaks
• Pump leaks
steam supply
• Steam load changes
• Start up and shut
down
• Condensate vs. make
up ratio
•
•
•
•
Process leaks
scavenger/passivator
chemistry and dosage
limitations
• Scavenger mixing,
residence time
• Condensate treatment
recycle
Temperature
• pH
Feedwater demand • Process contamination
Economics
leaks
• Corrosion products
Comparison of RT ORP to AT ORP
• Room temperature ORP probes:
– Can become polarized (inaccurate) over time
– Are less sensitive
– Require cooling of the water sample
• Changes water
chemistry
• Lag time reduces
responsiveness
13
Comparison of AT ORP to Conventional
Measurement and Control Techniques
• AT ORP:
– Addresses multiple MOC corrosion mechanisms
simultaneously
– Works with any metallurgy
– Works with any scavenger/passivator chemistry
• AT ORP is much more
•
sensitive
AT ORP has a fast response
14
Opportunities for
Energy Savings
Opportunities for Energy Savings
• Dosage adjusted in real-time, minimizing
•
•
potential for scale
Overdosing of solids-contributing chemicals
eliminated – feed just enough
– Sulfite
– Caustic
Accurate cycles determination and
optimization
16
Midwestern University
Background
•
•
•
•
3 water tube boilers with economizers, 175-psig
Natural gas fired
Softened make-up water
Steam supplies absorption chillers, heat, and reheat for
campus, hospital, and laboratory buildings
• Polymer fed relative to feedwater flow/steam load
• Sulfite fed to maintain desired boiler water residual
• Boiler blowdown controlled manually based on
conductivity
18
Manual Control Leads to Human Error
Monitoring Phase – AT ORP Response Prior to Control
time
19
AT ORP Maintains Desired Feedwater
Reductant Levels to Minimize Corrosion
AT ORP (mV)
% Sulfite Pump Output
Time
(2 weeks)
20
Feedwater Product (ppm)
Before / After Improvement in Scale
Inhibitor Feed
21
Product Pump Out %
Feedwater Product (ppm)
Scale Inhibitor vs. Steam Flow
22
Energy and Water Savings ($/yr)
Blowdown Energy Cost
Blowdown Sewer Cost
Make-up Water Cost
Subtotal (Costs)
Net Savings or (Costs), $/yr
Before
After
Installation Installation
38,147
22,577
11,114
10,002
58,263
6,578
3,198
32,353
Difference
15,570
4,536
6,804
26,911
26,910
23
Gulf Coast Refinery
24
Before MOC Review of System . . .
Only 45% of feedwater hardness readings were in control
25
Blowdown was Done Manually
Boiler cycles ranged from 2 to 22
26
After - Feedwater Quality Improved
Hardness was in target zone 89% of time
27
All-Polymer Dosage Controlled by
Fluorometer
Product Dosage (ppm)
Can be automatically increased based on input from
hardness analyzer
28
Improved Cycles Control will Save an
Estimated $406k in Water and Energy
29
Summary
• Economic challenges require a fresh
•
look at ways to reduce operating
costs, protect asset life, and improve
productivity
Numerous benefits to feedwater
automation including:
– Improved asset preservation, increase
boiler system reliability
– Optimized scale and corrosion control,
including optimized feed of internal
treatment and oxygen scavenger
– Process visibility – data management
– Real time, on-line communication
30

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