essentials for a sound boiler water treatment...
TRANSCRIPT
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Essentials for a Sound Boiler Water Treatment Program Presented by Steve Connor & Debbie Bloom April 23, 2014
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Mechanical Aspects
What needs to be removed from
the water?
Most Economical Long-Term Approach
Tolerances are Based on
Pressure & Temperature
Chemical Aspects
The Necessity of Water Treatment
Today’s Topics
2
Essentials for a Sound
Boiler Water Treatment Program
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Boiler Water Treatment
3
Water is an excellent heat transfer medium, but it must be properly treated
in both steam and hot water systems or serious problems will ensue, robbing
energy and reducing the life of the asset.
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Boiler Water Treatment
Steam Systems Hot Water Systems
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Necessity of Boiler Water Treatment
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• Scaling • Porous Deposition • Corrosion • Efficiency Loss • Life of the Asset • Steam Quality
• % moisture • Steam Purity
• Silica • Sodium ion
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What Needs To Be Removed
Dissolved Minerals • Hard Scale Formers
• Calcium • Magnesium
• TDS • Calcium • Magnesium • Sulfate • Sodium • Silica • Chloride • Alkalinity
• Dissolved Gas • Free Oxygen • Free Carbon Dioxide
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Waterside Scale
Waterside Corrosion
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Tolerances are Based on Pressure & Temperature
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Feed Water Boiler Water Oxygen Silica Iron & copper Total Alkalinity Total Hardness Free OH Alkalinity pH Specific Conductance Non-Volatile TOC (Total Organic Carbon)
Oily Matter ASME Guidelines:
Broken down by feed water and boiler water, and then by firetube and watertube boilers.
Water Constituent Tolerance: 0-300 operating psig
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Mechanical Chemical
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Most Economical Long-Term Approach
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Most Economical Long-Term Approach
Chemical Treatment Only • Phosphate Treatment • Polymers • Hybrid
Phosphonate/Polymer • Sulfite Treatment • Precise Control &
Monitoring • Extensive Blowdown • Wastes Water, Chemicals
& BTU’s
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Mechanical Chemical
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Most Economical Long-Term Approach
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Mechanical Removal
Removes • Calcium • Magnesium
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Water Softener
(Resin Tanks)
Brine Tank
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Cation Resin Bead Composition
Chemically engineered polystyrene beads (resin) with an affinity for positive ions.
Will exchange weaker positive ions for stronger positive
ions • Sodium (Na+) for Calcium (Ca++) and Magnesium (Mg++)
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Hard Water Soft Water
Cation Exchange Process
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Service Cycle • Na+ ions are exchanged for
Ca++ and Mg++ ions
Ca++ & Mg++ Na+
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Cation Exchange Process
Exhausted Condition • Cation beads can be used in the
service cycle until they are saturated with Ca++ and Mg++
• Requires regeneration
Ca++ or Mg++
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Cation Exchange Process
Regeneration or Brining • Sodium Chloride (NaCl) solution
(brine) is passed through the resin to drive off the Ca++ and Mg++ ions to be rinsed away with the Cl- ions
Resin bead will be replenished with Na+
Ca++ or Mg++
Na+
Cl-
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Removes • Bicarbonate & Carbonate
Alkalinity • Sulfate • Nitrate
Dealkalizer
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Mechanical Process
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Alkaline Water Dealkalized Water
Anion Exchange Process
Service Cycle • Chloride (Cl-) ions are exchanged
for Carbonates of Alkalinity (-)
Alkalinity Cl -
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What Needs To Be Removed
Dissolved Minerals • Hard Scale Formers
• Calcium • Magnesium
• TDS • Calcium • Magnesium • Sulfate • Sodium • Silica • Chloride • Alkalinity
• Dissolved Gas • Oxygen
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Waterside Scale
Waterside Corrosion
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Oxygen Corrosion
50 F
86 F
122 F
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RULE OF THUMB
• Almost 2 times more corrosive at 122oF than at 86oF
• Dissolved Oxygen is ~ 10 times more corrosive than CO2
Reference Chart Courtesy of: http://www.engineeringtoolbox.com/oxygen-steel-pipe
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Mechanical Removal
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Deaeration Removes • Oxygen • Free Carbon Dioxide
Heat Agitate Liberate
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Mechanical Removal
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Column Spray Tray
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Packed Column Deaerator
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Water In Venting Out
Column Packing Rings
Steam In
Boiler Feed Tank
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Spray Deaerator
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Tray Deaerator
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Steam
Water
Vent
Trays
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Mechanical Removal
Automatic blowdown system senses anything that adds conductivity • Calcium • Magnesium • Sodium • Silica • Sulfate • Chloride • Iron
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Conductivity Sensor
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TDS Control with Blowdown Heat Recovery
Water inlet To drain To drain
Separator Heat exchanger ?
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Mechanical Chemical
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Most Economical Long-Term Approach
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Treatment for Steam Boiler Systems
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Guidelines
• ASME • ABMA • OEM guidelines • Water treatment
companies
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ASME Suggested Water Chemistry Limits Industrial Watertube, High Duty, Primary Fuel-Fired - Feedwater
Drum Operating Pressure psig (MPa) 0 - 300 (0 - 2.07)
Dissolved Oxygen ppm (mg/L) O2 – measured before chemical scavenger addition <0.007
Total iron ppm (mg/L) Fe ≤0.1
Total copper ppm (mg/L) Cu ≤0.05
Total hardness ppm (mg/L) as CaCO3 ≤0.5
pH @ 25oC 8.3-10.5
Chemicals for preboiler system protection NS
Nonvolatile TOC ppm (mg/L) C <1
Oily matter ppm (mg/L) <1
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Table 2
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ASME Suggested Water Chemistry Limits Industrial Watertube, High Duty, Primary Fuel-Fired – Boiler Water
Drum Operating Pressure psig (MPa) 0 - 300 (0 - 2.07)
Silica ppm (mg/L) SiO2 ≤150
Total Alkalinity ppm (mg/L) as CaCO3 <1000
Free OH Alkalinity ppm (mg/L) as CaCO3 NS
Specific Conductance µmhos/cm (µS/cm) @ 25oC <7000
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Table 2
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Scale and Deposits Boiler Corrosion
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Carryover
Improperly Controlled Water Chemistry
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Scale and Deposits Form in High Heat Flux Areas Where Steam is Generated
Effect of Scale & Deposits: • Calcium, magnesium, and iron inhibit
heat transfer • Raise tube metal temperature
• Blisters/overheat – mostly water tube boilers • Efficiency loss – firetube boilers
• Iron also causes under deposit corrosion
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We Can Minimize Scale and Deposition and Impact Boiler Cycles of Concentration by…
• Minimizing feedwater contaminants entering boiler • Using best treatment program for your system • Accurately controlling blowdown rate
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• Phosphate based • Very old, reliable chemistry • Dosed based on hardness
• Can be used with very high feedwater hardness • Requires higher blowdown rates • Amount of phosphate scale formed depends on
hardness present
Boiler Treatment for Scale / Deposition - Precipitating Programs
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Threshold Scale / Deposition Inhibitor
• Good fit for low-pressure boilers • Able to handle moderate to severe hardness upsets • Hybrid program that combines phosphonate (HEDP) and
polymer • Functions by distorting the crystal structure and
preventing scale formation
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• All-polymer or chelant programs • Dosed based on hardness • Conditions sludge and solubilizes
hardness • Clean boiler internals when fed at recommended
dosage • Reduces blowdown requirements
Boiler Treatment for Scale / Deposition -Solubilizing Programs
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Mild Steel Corrosion
• Mild steel used in boiler systems because of mechanical strength, high thermal conductivity and low cost
• Corrosion minimized over pH range of 8.5 to 12.5
Rel
ativ
e C
orro
sion
Att
ack
Approximate pH Value at 25oC/77oF
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Chemical Oxygen Scavenger
• Chemical removes trace oxygen left by deaerator/FW tank
• Sulfite is typically scavenger of choice in low-pressure systems
• Non-sulfite scavengers exist • Fed to deaerator/FW tank • Performance traditionally
determined by scavenger residuals
• Oxygen is not the only corrodent in the FW system 39
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Treatment for Hot Water Boiler Systems
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Hot Water Systems
• Hot water boilers are used for many applications from 140oF to over 400oF
• Requirements to maintain all systems are similar
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Corrosion in Hot Water Systems
Water loss/make-up should not be more than 0.1 to 0.5% of system volume monthly (1.2 to 6% system volume yearly) • Special emphasis on finding and fixing leaks • As water make-up increases and water temperatures increase, oxygen
pitting becomes the most likely cause of corrosion-related failures
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Scale / Deposition in Hot Water Systems
• Softened make-up is recommended for all hot water boiler applications
• Primary species of concern: CaCO3, MgSiO3
• Scale potential is a function of: • Temperature: higher is worse • Holding Time: lower is worse since constant influx of make-up water
may increase the scaling load on the system • Water Chemistry: pH and dissolved ion levels • Dispersant in Program
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Chemical Programs
1. Nitrite/Azole program • Temperature <300oF and oxygen <1-2 ppm • Maintain Nitrite levels at 1000 ppm or higher • Azole ≥10 ppm
2. Combination Molybdate/Nitrite/Azole program • Temperature <300oF and oxygen <1-2 ppm • At least 1000 ppm Nitrite • Molybdate greater than 100 ppm • Azole ≥10 ppm
3. Molybdate/Azole program • Molybdate at 300 ppm or higher (500 ppm if temperature is >250oF) • Molybdate requires approximately 1 ppm oxygen to provide adequate
corrosion protection
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Sulfates & Chlorides
• Corrosion rates sensitive to increased chlorides and sulfates
• Both degrade nitrite and molybdate performance • Increase nitrite and molybdate dosage for chloride and
sulfate levels >50 ppm
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Sometimes Sulfite or Alternate Oxygen Scavengers are Used
• Sulfite is not compatible with nitrite • Minimize water loss/make-up • Deoxygenate make-up if rates are high
• Sulfite plus oxygen results in sulfate • Sulfate levels above 300 ppm increase yellow metal corrosion
• Some non-sulfite scavengers can add ammonia • Ammonia is known to increase yellow metal corrosion
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Monitoring and Control
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Boiler
Steam
Boiler Feedwater
Boiler Blowdown
Heat
966,700 lb/Day @ 0 ppm TDS
1,000,000 lb/Day @ 100 ppm TDS = 100 lb/Day Solids
3.33% of Feedwater 33,300 lb/Day @ 3000 ppm = 100 lb/Day Solids
Based on Chemistry
Cycles 30 1003000
ion feedwaterion blowdown
==
30 Cycles = 3000 ppm TDS
BD 3.33% 30
100cycles100
==
Boiler Cycles and % Blowdown
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Determining Boiler Cycles
Conductivity • Good to use if feedwater conductivity is relatively high • Limited use if feedwater conductivity is low and contains ammonia or
amines Trace chemicals can aid cycles determination
• Inert fluorescent dye • Non-volatile • Non-precipitating • Non-reactive
Other Measurements • Silica • Chlorides • Sodium
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Blowdown Control Minimizes Chemistry Swings in the Boiler
With
Without
Time 50
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0%
5%
10%
15%
20%
25%
30%
35%
40%
45%
Cycles of Concentration
% o
f F
eedw
ater
G
oing
to
the
Blo
wdo
wn
1 10 100 5 50
20 to 40 cycles saves 2.5% FW
5 to 10 cycles saves 10% FW
20 40
… And Can Save Water and Energy
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Blowdown Control is Only Half the Issue
• Perform chemical tests relative to treatment • Adjust chemical feed to
maintain dosage • Maintain boiler parameters to
prevent over-cycling • Testing can be automated
with feedwater control
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Midwestern University Case History
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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
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Before / After Improvement in Scale Inhibitor Feed
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Feed
wat
er P
rodu
ct (
ppm
)
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Scale Inhibitor vs. Steam Flow
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Fe
edw
ater
Pro
duct
(pp
m)
Pro
duct
Pum
p O
ut %
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Automation Maintains Desired Feedwater Reductant Levels to Minimize Corrosion
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% S
ulfite P
um
p O
utp
ut
Time (2 weeks)
Cor
rosi
vity
(m
V)
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Energy and Water Savings ($/yr)
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Before Installation
After Installation Difference
Blowdown Energy Cost $38,147 $22,577 $15,570
Blowdown Sewer Cost $11,114 $6,578 $4,536
Make-up Water Cost $10,002 $3,198 $6,804
Subtotal (Costs) $59,263 $32,353 $26,910
Net Savings or Costs $/yr $26,910
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Today’s Take-A-Ways
• Proper water treatment is a combination of mechanical and chemical remediation
• Has significant impact on efficiency and reliability attainment
• Chemical treatment varies based on watertube, firetube, pressure/temperature.
• Corrosion accelerates with temperature • Increasing cycles of concentration saves
fuel dollars • Heat from continuous blow down for TDS
control can be recouped • Hot water systems need to be chemically
treated too • Monitoring and control is ESSENTIAL
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Contact Us
Debbie Bloom Principal Consultant, Technical Expertise Center [email protected] 630-305-2445
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Tom Leunig Product Manager, Packaged
Water Systems [email protected] 414-577-3197