improved boiler system operation with real-time chemical control debbie bloom, nalco company
TRANSCRIPT
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
6
SiteBoiler Type
Pressure psig/barg
Cycles of Concentration
First 1% (hrs)
Half-life or 50%
(hrs)Last 1%
(hrs)
Campus A Firetube 125 / 9 10 0.1 4 25
Campus A Watertube 125 / 9 10 0.1 6 41
Chemical Co. B HSRG 1000 / 69 29 0.1 10 67
Chemical Co. B Power 1000 / 69 29 0.3 18 120
Paper Mill C Recovery 1250 / 86 45 0.2 16 109
Paper Mill C Recovery 1250 / 86 45 0.4 26 174
Holding Time
Scale Control
Automated Scale Control Utilizes a Stable Inert Trasar
• Inert tracer chemistry survives in 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 carry-
over if seen in the condensate– Provides positive feedback
that chemical treatment is fed
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Patented LED fluorometer
Provides a stable inert monitor of system performance
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
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Reducing Conditions Minimize Corrosion
(More Negative ORP)
-600
-400
-200
0
200
400
0.1 1 10 100 1000
OR
P (
mV
) 40
0F, 2
04C
Dissolved Oxygen (ppb)
Oxidizing
Reducing
Mo
re R
edu
cin
g
-600
-400
-200
0
200
400
0.1 1 10 100 1000
OR
P (
mV
) 40
0F, 2
04C
Dissolved Oxygen (ppb)
Oxidizing
Reducing
Mo
re R
edu
cin
g
Many Factors Affect the ORP Fingerprint of Each System
Mechanical
• System design metallurgy
• Deaerator tray alignment
• Feedwater heater
• Economizer leaks
• Pump leaks
Operational
• Deaerator venting, steam supply
• Steam load changes
• Start up and shut down
• Condensate vs. make up ratio
• Process leaks
• Temperature
• Feedwater demand
• Economics
Chemical
• Dissolved oxygen
• Oxygen scavenger/passivator chemistry and dosage limitations
• Scavenger mixing, residence time
• Condensate treatment recycle
• pH
• Process contamination 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
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-100
-600
-700 -600
-500
-400
-300
-200
-100
0
-200
-300
-400
-500AT O
RP
(m
v) R
T O
RP
(mv)
Time (hrs)
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
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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
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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
19time
Monitoring Phase – AT ORP Response Prior to Control
AT ORP Maintains Desired Feedwater Reductant Levels to Minimize Corrosion
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% S
ulfi
te P
um
p O
utp
ut
Time(2 weeks)
AT O
RP (m
V)
Before / After Improvement in Scale Inhibitor Feed
21
Feed
wat
er P
rodu
ct (p
pm)
Scale Inhibitor vs. Steam Flow
22
Fe
edw
ater
Pro
duct
(ppm
)
Prod
uct P
ump
Out
%
Energy and Water Savings ($/yr)
23
Before Installation
After Installation Difference
Blowdown Energy Cost 38,147 22,577 15,570Blowdown Sewer Cost 11,114 6,578 4,536Make-up Water Cost 10,002 3,198 6,804Subtotal (Costs) 58,263 32,353 26,911Net Savings or (Costs), $/yr 26,910
Gulf Coast Refinery
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Before MOC Review of System . . .
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Only 45% of feedwater hardness readings were in control
Blowdown was Done Manually
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Boiler cycles ranged from 2 to 22
After - Feedwater Quality Improved
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Hardness was in target zone 89% of time
All-Polymer Dosage Controlled by Fluorometer
28
Can be automatically increased based on input from hardness analyzer
Prod
uct D
osag
e (p
pm)
Improved Cycles Control will Save an Estimated $406k in
Water and Energy
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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
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