Optimizing Steam and Cooling Systems for Reliability and Sustainabilitydev.chemtreat.com/.../10/Optimizing-Steam-and-Cooli… ·  · 2013-11-01Optimizing Steam and Cooling Systems

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

The Hydrologic Cycle

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 Softening? How does it work?

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++

Service

Backwash

Regeneration

Rinse

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

ASME Limits

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

Condensate Treatment

Note the

groove

caused by low pH

corrosion

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

Questions

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

Ammonia Evaporative Condenser

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

EXCESSIVE AND POOR USE OF OXIDIZERS

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

Questions?