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Optimizing Steam and Cooling Systems for Reliability and
Sustainability
Presented By: Kevin Emery [email protected]
804-317-2381
Optimizing Steam and Cooling Systems for Reliability and Sustainability
• Steam systems
Boiler Feedwater Pretreatment
Internal treatment and cycle control
Condensate System Treatment
• Cooling System Failure Modes – Balancing Cycles vs. Risk
Fundamentals of Water Why isn’t water perfect for boilers?
EARTH AIR
MINERALS
Calcium
Magnesium
Iron
Silica
Oxygen
Carbondi-
oxide
Mud
Silt
Clay
Primary Boiler System Contaminants and Issues
Contaminant Issues
Hardness salts Scaling and deposits
Silica Turbine deposits and scale
Alkalinity and CO2 Foaming and carryover,
condensate corrosion
Iron and manganese Boiler deposition
Suspended solids Deposition
Oxygen Corrosion
Boiler Scale What is it?
• Hardness = Calcium & Magnesium
• Calcium forms hard calcium carbonate scale in boiler.
• Magnesium forms magnesium silicate in boiler.
Impact of Boiler Scale Percent Fuel Loss
Scale layer
Inches
Normal
Scale
High Iron
Scale
Iron and
Silica Scale
1/64 1% 1.6% 3.5%
1/32 2.0% 3.1% 7.0%
3/64 3.0% 4.7% _
1/16 3.9% 6.2% _
Impact of Scale
A boiler operating at 450,000 million Btu’s of
fuel for 8,000 hours at the rated capacity
of 45,000 pounds/hour of 150 psig steam.
If 1/32nd
of scale is allowed to form and the
scale is normal. The fuel loss is 2%.
Assuming $5/million Btu’s
450,000M Btu’s/Year * $4 MBtu * 0.02 =
$36,000
Comparison of Common Pretreatment Systems
Zeolite Softening Reverse Osmosis Demin. Units
Removes Calcium and
magnesium and
some iron
97% of salts, silica
and all TSS, gases
not removed
99.9+% of salts,
silica, alkalinity
and CO2
Application Low to medium
pressure boilers,
<600 psig
Across the board,
polishing required
for high pressure
applications
High pressure
boilers or silica
limitations
Cost and
Limitations
Low
Doesn’t remove
alkalinity or silica
High cost
reject water
High cost
Acids and bases
for regeneration
Sodium Zeolite Softener How does it work?
• Calcium and magnesium from water are transferred to the sodium zeolite resin via chemical reaction.
Na
Na
Na
Na
Na
Na Na
Na
Na
Na
Na
Ca++
Mg+
Mg+
Mg+
Mg+
Ca++
Ca++
Sodium Zeolite Softener
• Ca and Mg are exchanged for sodium
Na+
Na+ Na+ Na+
Na+
Na+
Na+
Na+
Na+
Na+
Ca++ Ca++
Ca++ Ca++
Mg+
Mg+
Mg+
Mg+ Ca++
Sodium Zeolite Softening How does poor operation affect the plant?
• Increased scale inhibitor demand
• Increased energy costs
• Tube failures
Typical Softener Problems
• Iron fouled resin: short runs
• Leaking valves: hardness in boiler feedwater
• Inadequate regeneration: short runs
• Loss of resin: short runs
• Channeling: hardness in boiler feedwater
Reverse Osmosis
• Essentially molecular filtration
• ~ 98% salt removal in permeate
• Dissolved gases are not removed (CO2, etc.)
• 10–25% of flow is rejected as brine
Reverse Osmosis
Feed
200 ppm calcium carbonate
Brine
788 ppm calcium carbonate
Permeate or Product
4 ppm calcium carbonate
RO Pretreatment
• Filtering
• Dechlorination
Free available chlorine must be <0.05 ppm for composite polyamide membranes
Activated carbon or sulfite (1.8–3.0 ppm sulfite per ppm chlorine)
• Antiscalants and dispersants
Most units are softened
• pH adjustment
RO Troubleshooting
• Leaking seals and O-rings: too much water hammer
• Fouling
Hardness and metals
Silt and colloids
Organics and microbiological
• Membrane mechanical failure
Telescoping
Reliability in Plant Utility Systems
• The deaerator is used to remove non-condensable gases from the boiler feedwater:
Oxygen
Carbon dioxide
Ammonia
What is Oxygen Corrosion?
• As temperature increases, the corrosivity of dissolved oxygen in water doubles with every 18 F increase.
Can I Mix Condensate and Soft Water?
• This is a common design flaw
• The temperature of the condensate drives the oxygen out of the soft water
• In mild steel tanks the corrosion is severe
Iron transport to the deaerator and boiler
Pitting in the tank
Deaerator What Do I Need To Know To Operate It?
• Temperature in storage section should be less than 3 F below the deaerator heating steam temperature.
• Ensure that the deaerator is properly vented
• Inspect internals annually for oxygen pitting
Dissolved Oxygen Control
• Common chemical oxygen scavengers and metal passivators Sulfite
Hydrazine
Carbohydrazide
Erythorbate/ascorbate
Methylethylketoxime/MEKO
Hydroquinone
Diethylhydroxylamine/ DEHA
Oxygen Scavenger Reaction Rates
• Catalyzed sulfite is the fastest
• Ratio feed to BFW • Allow at least 2 minutes • Prevent ingress of oxygen in
standing equipment such as pot feeders, sample ports, etc.
• Circulating some of the boiler feedwater back to the deaerator can improve reaction rates and prevent oxygen ingress
The Deaerator What if it doesn’t work properly?
A feedwater line subjected to excessive oxygen because of poor deaerator control.
Indications of Trouble
• High oxygen scavenger use
• Sudden changes in pressure
Maintain at least 2 psig unless under vacuum
18- to 24-inch discharge plume
• Flooding of the tray or spray section
• Poor inspections
Internal Treatment
“The last stand” Program selection based on contaminants and
pressure
Iron deposition is a common contaminant in high purity systems
Hardness contamination can be a concern from the pretreatment system or condensate leaks
Provide buffering to prevent corrosion
Internal Treatment
• Prevent corrosion
Low/high pH conditions
– Destroy magnetite layer
– Generate corrosion products
– Generate hydrogen (low pH)
Underdeposit/concentrating film (formation of acid phosphates/NaOH)
– Localized metal loss
– Gouging
Boiler Water Treatment
• Prevent deposition
Iron oxide/scale/deposits lead to heat transfer losses, underdeposit corrosion, tube failures from overheating
On line cleanup of a dirty water tube boiler can create chip scale and tube blockage.
Treatment
Programs
Precipitating
Phosphate
Chelant
Polymer
All
Polymer/Organ
ic
Coord.
Phos/ EPT
Application • Unsoftened
makeup with
90%
condensate
return
• High purity
makeup
• Softened
makeup
• Consistent
quality
deaerated
water
• Low iron
• Low hardness
• Consistent
quality
• Low hardness
• <25 cycles
• Phosphonate/
Polymer for
Iron
Contamination
or variable
makeup
• High pressure
• High purity
makeup
• No polymer
after 1,250 psig
Pressure <900 psig <600 psig <900 psig >900 psig
Feed Point Drum or BFW After BFW pump
in BFW
DA Storage
BFW
DA Storage
BFW
Attention Level Low High Low High
Boiler Blowdown Why do we blowdown?
• To remove the dissolved solids that have concentrated in the boiler due to evaporation
• To prevent high alkalinity and steam contamination due to foaming
• To remove sludge
• Cycles = Boiler feedwater /boiler blow down
• Avoid >50 cycles in industrial boilers
How do Cycles of Concentration and Blowdown Relate?
5 10 15 20 25 35 40 45 50 55 60 65 70 75
CYCLES OF CONCENTRATION
Pe
rce
nt
of
Fe
ed
wa
ter
that
mu
st
Blo
wd
ow
n 25
20
15
10
5
0
How does Blowdown Affect Plant Operations
• Too much blowdown
Increased fuel costs
Increased water costs
Increased chemical costs
• Over cycling the boiler
High TDS – Scaling and deposition
– Underdeposit corrosion
Polymer breakdown
High alkalinity – Carry over
– Turbine fouling
Boiler Bottom Blowdown
• Purpose: To remove precipitated solids from the bottom of the boiler
• Procedure: Open valve 3–4 seconds only; repeat 2 times
• Frequency
Daily to once per shift with normal operation
3–4 time per shift during hardness problems
Limiting Factors for Boiler Cycles
• Steam Purity and Separation Equipment
Silica is a major concern as is boiler alkalinity
• Iron contamination from the condensate system
• Percent condensate return and feedwater quality.
• Polymer Residence Time
The Value of Condensate
• Energy
• Water
Condensate
Return
Makeup
Water can cost over $3.00 per 1000 gallon
Condensate is worth even more because it is pure
Value of Condensate – 10 gpm $54K per year or $5.4
• Energy 180 F – 80 F = 100 BTU/lb 10 gpm = 600 gph = 5004 lbs/hr 5004 lbs/hr x 100 BTU/lb x 24 hours day x 365
days/year = 4383 MM BTU At $4/MM BTU = $17534/year in energy
• Water, Sewer, and Pretreatment 10 gpm x 1440 mpd x 365 = 5256 K gallons/year 5256 x ($3/1000 gallon water + $2/1000 sewer) =
$26,280 5256 x $2/1000 gallons pretreatment costs = $10,512
Condensate Contamination
• Problems
Significant scale potential
Boiler corrosion potential: organics
Foaming and carryover
Product contamination
• Solutions
Find the source and repair, or
Dump condensate
Treatment Options
• Mechanical
Deaeration
Dealkalizers
Demineralizers
Condensate polishers
• Chemical
Filming amines
Neutralizing amines
Oxygen scavengers
Filming Amines
• Advantages
Nonwettable barrier ODA
Protection against oxygen and carbon dioxide
Feed at low level to feedwater or steam
Low cost
FDA/USDA approval
• Disadvantages
No good tests
Gunking
Distribution problems
Cannot use with turbines
Feed separate
Ammonia
• Advantages
Cheap
Rapid pH rise
Feed to feedwater, boiler or steam
FDA approval
• Disadvantages
Hard to control in narrow pH range
Only high distribution
Potential for copper Corrosion
Elevates pH
Neutralizing Amines
• Feed to feedwater, boiler, steam
• Neutralize carbonic acid, pH control
• No oxygen protection
• Wide range for complex system
Distribution ratios
Basicity
• FDA/USDA approvals
Morpholine
• High boiling point 265 F
• Low distribution ratio
Protection initial condensation
• Not good alone in complex systems
• Cannot exceed 10 ppm for FDA
• 2.4 ppm/ppm CO2
Cyclohexylamine
• Low boiling point 205 –206 F
• High distribution ratio
• Good blend for complex systems
• FDA maximum 10 ppm
• 2.11 ppm/ppm CO2
Diethylethanolamine
• Low boiling point 202 –203 F
• Moderate distribution ratio
• Blend with other amines
• FDA 15 ppm maximum
• 2.1 ppm/ppm CO2
Probability of Boiler Failures Variable Level of
Control
Potential for
Failure
Impact
Oxygen Pitting Constant High Immediate
Pitting of
economizer
Polymer Levels Changes
Slowly
Low Gradual
Scaling
Hardness in
Feed water
Constant High Immediate to
Gradual
Scaling
Conductivity Constant High Immediate
Carry over
Deposition
Monitoring Program
• Focus on Critical Few
hardness, conductivity, sulfite, oxygen, silica, pH
Condensate purity
inhibitor
Mass balance (flow and inventory)
• Results Based
Heat Rate
Boiler Inspections
Boiler Efficiency
What Determines Limits for Cooling Tower Cycles?
• Hardness: Scaling index • Cycled pH • TSS
Film fill vs. splash fill Filtration
• Silica and iron in makeup • Corrosion • Discharge limits
Inorganics: Chlorides, Zn Phosphate
• Application Skin temperature Low flow
Safe Limits: Circulating Water
• Silica <180 ppm • TSS <50 ppm: use more polymer above
30 ppm • LSI <2.5 is ideal, but 2.8 is possible • Chlorides <150 ppm • Calcium <1,000 ppm use LSI • Iron <3.0 ppm
Cycles pH Cond 'M' Alk Calcium Magnesium Ortho PO4 Silica Chlorides Sulfates LSI pHs
1.0 7.8 634 111 83 56 3 19 92 67 0.3 7.5
1.5 8.0 951 130 125 84 3 29 138 101 0.7 7.2
2.0 8.2 1,268 173 166 112 4 38 184 134 1.2 7.0
2.5 8.4 1,585 216 208 140 4 48 230 168 1.6 6.8
3.0 8.5 1,902 260 249 168 4 57 276 201 1.9 6.7
3.5 8.7 2,219 303 291 196 4 67 322 235 2.1 6.5
4.0 8.8 2,536 346 332 224 4 76 368 268 2.3 6.4
4.5 8.9 2,853 390 374 252 4 86 414 302 2.5 6.3
5.0 9.0 3,170 433 415 280 5 95 460 335 2.7 6.3
5.5 9.0 3,487 476 457 308 5 105 506 369 2.9 6.2
Wichita Water Cycle Up
2000 gpm tower with 10 delta T $3/1000 water $2/1000 sewer
0.00
20,000.00
40,000.00
60,000.00
80,000.00
100,000.00
120,000.00
140,000.00
2 3 4 5 6 7 8 9 10
Co
st
Cycles
Inhibitor
Water
Sewer
Failure Mechanisms
• Over cycling/high pH: Mineral scale deposition
• Poor biological control: Deposition, under-deposit corrosion, disease
• Under cycling: Water waste, corrosion, chemical waste
• Overfeed of halogen • Low pH: Corrosion • Low inhibitor: Corrosion +
deposition • Low flow: Corrosion+ deposition
Mechanical Oversights
Chemical Feed and Control Systems – Design for high and low load
Location of Tower – Avoid Air Intakes, Dusty Roads, Trees, Exhausts
Exchanger added to end of the line
System changes (water source, temperature, flow)
Metallurgy – Don’t use galvanized towers with high alkalinity waters
Film Fill vs. Splash Fill
No filter to remove TSS
Overfeed of Halogen
• High oxidant feed rates cause copper pitting
• Copper plates out on steel and pits steel
Probability of Failure
Variable
Level of Control
Potential for Failure Impact
pH Constant when required
High Immediate
Inhibitor levels Changes Slowly Low Gradual
Biological control Constant Depends on system
Immediate
Conductivity Constant High Immediate
Cooling System Monitoring Program
• Focus on the critical few pH, conductivity, oxidant, inhibitors Mass balance (flow and inventory)
• Results based Corrosion rates Temperatures and heat transfer Biological growth Appearance Water use and flow Chiller efficiency
• Data Management: Trend graphs, statistics, reports to management