BOILER WATER TREATMENT TECHNOLOGY DSS HEAD OFFICE - SERPONG 26 27 May 2008
PURPOSE Discuss the BOILER WATER TREATMENT TECHNOLOGY in general Provide an understanding of the problems inherent with the BOILER SYSTEM operation. Provide an understanding of how these problems can be SOLVED
PROCESS PRESENTATION DISCUSSION QUESTION AND ANSWER
PAY OFF More understanding about BOILER Treatment Program Better BOILER Treatment control program implementation in the field to optimize the treatment results and deliver more benefits
OVERVIEW Pre-treatment Boiler System Feed Water System & Boiler Feed Water Treatment Internal Boiler Treatment
HIGH PRESSURE BOILER A boiler which operates at a pressure above 700 psig (USA standard)
OBJECTIVES HPB WATER TREATMENT Increase Unit Reliability Minimize contaminant Decrease downtime Increase safety Decrease operational costs
1. Fire Tube Boiler Type Of Boiler low pressure steam, small capacity
Type Of Boiler 2. Water Tube Boiler (Package) Low & High Pressure Steam, Big Capacity
Type Of Boiler 3. Water Tube Boiler (Field Erected) High Pressure Steam (turbine Drive) Capacity: Big.
Boiler Water Processes Steam Generation (Evaporation) - where boiler water in the steam drum has sufficient energy to convert from liquid to gas (steam). Note that only the pure water is converted to steam, so the dissolved solids are left behind to accumulate in the remaining boiler water.
Boiler Water Processes Cycles of Concentration - the ratio of TDS in the boiler water to TDS in feed water E.g., Feedwater has TDS of 2.0 ppm Boiler water has TDS of 70 ppm Cycles of concentration = 70/2 = 35
Boiler Water Processes Blow Down - Deliberate dumping of water in the steam drum to limit the total dissolved solids in the boiler water. Remember, high TDS higher potential for scale and corrosion to occur. This is how we control cycles This lost water is replaced with an equal volume of fresh, less concentrated feed water.
Boiler Water Problem Corrosion (all type of boiler) Scale (LP Boiler with softener) Deposit (HP boiler, Fe & Cu) Carry Over (all type of boiler)
Boiler Water Treatment Objective Prevent Corrosion BFW : Oxygen Corrosion Boiler Corrosion : Caustic Corrosion Steam and Condensate Corrosion : CO2 Corrosion Prevent Deposit Promote Production of Pure Steam
BOILER FEED WATER Source Returned Condensate Pretreated Make-up Water
WATERSIDE PROBLEMS IN HIGH-PRESSURE BOILERS CORROSION DEPOSIT CARRY OVER
WATERSIDE PROBLEMS IN HIGH-PRESSURE BOILERS Major problems Deaerator F/W System HP boiler Superheater Turbine Steam-using equipments Condensate system Corrosion Oxygen X X X X X X Alk. / CO 2 X X X X Ammonia X X X Deposits Metal oxides X X Carryover Dissolved solids X X X Note : Alk. : Alkalinity, F/W: Feed water, HP : High pressure
CORROSION An electrochemical process where metals are reverted back to their natural (oxidised) state. For example, steel is pure iron, Fe. In the presence of air and water, the iron corrodes to form iron oxide, Fe 2 O 3 (rust).
CORROSION Boiler corrosion usually classified as: Oxygen corrosion Acid attack Caustic corrosion / embrittlement Erosion corrosion (flow accelerated corrosion) Can be either general or localised (pitting) attack
Corrosion Mechanism
Oxygen Corrosion Stopping Corrosion Metallurgy Selection Oxygen Resistant Materials are expensive and poor heat conductor Scavenge Oxygen Take away the active ingredient (both mechanical and chemical) Passivate Metal This what the Second generation oxygen scavengers tend to do
Oxygen Corrosion Source of Oxygen Dissolved Oxygen Feed Water Return Condensate Air in Leakage Feed Water Pump Condensate Pump
Oxygen Corrosion Why Scavenge Oxygen Remove remaining oxygen after mechanical deaeration Prevent oxygen pitting in the preboiler system O2 Attack results in pitting type corrosion Rapid localized metal loss
Rate of Corrosion Corrosivity of Oxygen Influenced by : ph Corrosivity of oxygen decrease as ph increase ph optimum > 9.0 Temperature Higher temperatures reduce O 2 solubility, but significantly increase corrosivity Dissolved oxygen concentration Higher O 2 concentrations increase corrosivity Fluid velocity Enhances effect of other corrodents
Prevent Corrosion Metallurgy Selection Oxygen resistant material are expensive and poor heat conductor Corrosion Control by Remove All Oxygen : Mechanically Dearation Chemically Scavenge Oxgygen Passivate Metal This is what the second generation oxygen scavengers tend to do
Corrosion of O 2 Influenced by ph
Corrosion Control System
Corrosion Control Mechanical Dearation : Purpose? Primary Means of O 2 Removal Types Mechanical - spray, tray and spray / tray type. Can remove up to 7 ppb oxygen. Deaerating Condensers - 7 ppb D.O. Vacuum deaerators - 10 ppb D.O. Temperature in mechanical deaerators - 104-170 0 C (power plants) Deaerator Maintenance - ph, spray nozzles and cracks inspection
Corrosion Control Chemical Oxygen Scavenger : Purpose Remove of trace amounts O2 remaining after dearation Kind Of Oxygen Scavenger First Generation : SULPHITE, HIDRAZINE Second Generation : Eliminox, Sugards,
DEAERATOR Remove Oxygen and Other Dissolved Gases from Feed-water Why is Deaeration Necessary? Oxygen Corrosive to Mild Steel Cheaper to Remove Most of the Oxygen with Steam than to Remove Chemically
DEAERATOR Evaluate ON-LINE performance: Dome Storage 1. Check Temperature and Pressure Gauges of the DEAERATOR DOME & STORAGE SECTION 2. Check to See That the DEAERATOR is Venting Steam 3. Check The Feed Water Temperature & Flow Rates against Design Specifications 4. Check The Steam Temperature & Flow Rate 5. 5. Check Oxygen Levels in in the Deaerated Water
Product Selection Considerations Regulatory Issues Solid Contributions : Cycles Attemperation Pressure/temperature use Desired attributes :Scavenger vs passivator Handling Costs
Chemical Oxygen Scavenging - Requirements Oxygen Scavenging - Chemical reaction with trace quantities of dissolved oxygen Passivation - Effective passivation of all system metals Carbon steel Stainless steel Copper alloys (all)
What is PASSIVATION? Formation of a very dense, protective film at the metal surface which inhibits further corrosion of the base metal. Under reducing conditions the preferred iron oxide - Magnetite (Fe 3 O 4 ) in carbon steel - colour changes from red to grey/black Cuprous oxide Cu 2 O in copper surfaces. Colour changes from black to red Lower rate of metal loss after passivation Lower corrosion rate during short upset conditions
Oxygen Scavengers Commonly Used Sulfite Hydrazine Hydroquinone DEHA (Diethyhydroxylamine) MEKO (Methylethylketoxime) Carbohydrazide Erythorbic acid
Hydrazine Hydrazine Reaction N 2 H 4 + O 2 2H 2 O + N 2 3N 2 H 4 + heat ------> 4NH 3 + N 2 (At temperature > 200 o C) Advantages All volatile No effect on cation conductivity No CO 2 contribution Does not add solids Disadvantages Suspected carcinogen Requirement reportable spill quantity = 2.2 lb (1 kg) Flammable liquid Exposure to hydrazines may cause harmful health effects
ELIMINOX TM Carbohydrazide = (N 2 H 3 ) 2 CO Safe, non-carcinogenic No reportable spill quantity Not a hazardous waste >20 years of use in high pressure boilers Industry acceptance Outperforms Hydrazine Better passivation at lower temps Faster O 2 scavenger at lower temps Performance better passivator 50-85% reduction in Fe & Cu transport All volatile and Non solid contributions Excess breaks down to Hydrazine, then to NH 3 in boiler THE MOST SIMILAR CHEMISTRY as Hydrazine alternative
MECHANISM REACTION of ELIMIN-OX
Reaction N-1250 (Eliminox Reaction) O NH2-NH-C-NH-NH2NH2 Direct + 2O 2 2N 2 + 3H 2 O + CO 2 ** < 150 o C Indirect + H 2 O < 150o C 2N2 H 4 + CO 2 ** + 2O 2 2N 2 + 4H 2 O >205 o C 2NH 3 + N 2 + H 2 ** 1 ppm Elimin-Ox contribute 29 ppb (or 0.029 ppm) CO 2 & NO IMPACT to STEAM & CONDENSATE corrosivity
Comparison With Hydrazine N-1250 REACTION RATE vs HYDRAZINE ELIMIN-OX reaction rate 500 FASTER than HYDRAZINE ELIMIN-OX reacts STOICHIOMETRICALLY with O 2 at all temperature
Passivation N-1250 as Passivator Continuously formation of insoluble, nonporous material on metal surface, i.e. magnetite Iron will self passivate in water, if no contaminants are present Reaction is very slow below 212 o F Reaction becomes fast only above 400 o F Natural passivation process can be accelerated by N-1250 Iron III hydroxide/oxide hydrate formation by oxygen followed by reduction to magnetite by scavenger Metal oxidation by scavenger Activation of oxygen as passivating agent by scavenger
Passivation Reaction 12 Fe 2 O 3 + (N 2 H 3 ) 2 CO 8Fe 3 O 4 + 3H 2 O + 2N 2 + CO 2 8CuO + (N 2 H 3 ) 2 CO 4Cu 2 O + 3H 2 O + 2N 2 + CO 2
Passivation Passivation Better than Blank at High Temperatures Increasing Passivation 200 250 300 350 Temperature ( of) Blank Hydrazine Carbohydrazide Erythorbate
Passivation PASSIVATION PERFORMANCE ELIMIN-OX (CHZ) has EXCELLENT PASSIVATION performance at all Temp. (starting from LOW to HIGH temp. section). While HYDRAZINE has GOOD PASSIVATION Performance at HIGH temp. section only 50-85% reduction in Fe & Cu transport
Reaction and/or Breakdown Products Chemical/Formula % C (wt.) Reaction and/or Breakdown Products Hydrazine N 2 H 4 0 Nitrogen, water, ammonia Carbohydrazide (N 2 H 3 ) 2 CO 13.3 Hydrazine, nitrogen, water, ammonia, carbon dioxide Hydroquinone 65.5 Benzoquinone, light alcohols, ketones, C 6 H 4 (OH) 2 low molecular weight species, carbon dioxide Diethylhydroxylamine (CH 3 CH 2 ) 2 NOH Methylethylketoxime (CH 3 )(CH 3 CH 2 )C=NOH 53.9 Acetaldehyde, acetic acid, dialkylamines, ammonia, nitrate, nitrite 55.2 Methylethylketone, hydroxylamine, nitrogen, nitrous oxide, ammonia, carbon dioxide Erythorbic Acid 40.9 Dihydroascorbic acid, salts of lactic and C 6 H 8 O 6 glycolic acid, carbon dioxide
Scale Problems Loss of Boiler Efficiency Scale reduces heat transfer Boiler Tube Failure Scale elevates tube temperature, causing tube overheating Under-deposit Corrosion Caused by high localized concentration of corrosive molecules
Effect of Scale on Tube Temperature
Under-Deposit Corrosion Magnetite NaOH Steam Out NaOH NaOH NaOH Water In Fe 3 O 4 porous deposit NaOH
Types of Caustic Damage in Boilers There are two forms of damage caused by caustic soda to high pressure boilers, namely: Caustic corrosion Caustic embrittlement
Caustic Corrosion Usually found only in high pressure boilers Problem usually due to deposits Localized in boiler Also called crater attack or caustic gouging No embrittlement of metal
Requirements for Caustic Corrosion Two conditions are necessary for caustic corrosion to occur: The presence of a corrosive material in the boiler water (caustic soda) A mechanism for concentrating this material
Concentrating Mechanisms The following conditions can result in dangerously high localized caustic soda concentrations Porous metal oxide deposits Metal oxide deposits Operation above rated capacity Excessive rate of load increase Excessive localized heat input Localized pressure differentials Restrictions in generating tube(s)
Prevention of Caustic Corrosion Prevention of caustic corrosion is achieved by minimizing or eliminating the presence of free caustic soda in the boiler water. Coordinated phosphate Congruent sodium phosphate Phosphate-low hydroxide (tri-ad) Equilibrium phosphate control All-volatile treatment
Coordinated Phosphate Control of ph comes from hydrolysis of trisodium phosphate in water Na 3 PO 4 + H 2 O -> Na 2 HPO 4 + NaOH Molar ratio of sodium : phosphate is 3 : 1 in water Feedwater contamination usually dictates causticconsuming chemicals, such as disodium and trisodium phosphate Does not ensure absence of caustic under concentrating conditions
Coordinated Phosphate Advantages Can be used in boilers up to 2400 psig FDA/CFIA Approved
Coordinated Phosphate Disadvantages Does Not Prevent Mineral Scale Does Not Prevent Iron Deposition Requires Auxiliary Polymer Hardness upsets can cause acidic conditions (corrosion) Requires Frequent Testing and Careful Control
Congruent Phosphate This program was developed to prevent free caustic in boiler water during concentrating conditions At sodium:phosphate ratio of 2.85 in boiler water, precipitated solids have same concentration Safe range is between ratio 2.3-2.6 Control is based on ph and PO 4 values
Types of Internal Treatment Coordinated Phosphate CONGRUENT SODIUM PHOSPHATE PROGRAM ( Na : PO4 molar ratio = 2.6 : 1 ) TO RESPOND THE SOLUBILITY DIFFERENCES OF CONCENTRATED SODIUM PHOSPHATE & CAUSTIC EXPERIMENT RESULTS : At high temp. sodium phosphate soln. with a Na : PO4 molar ratio > 2.85 => Dangerous free caustic region ( > 3.0 ratio ) With a soln. Na : PO4 molar ratio : 2.85 => the Na : PO4 molar ratio is identical
EXPERIMENT RESULTS : At higher temp., the congruent point at Na : PO4 = 2.6 CONGRUENT CONTROL PROGRAM : Operates between safe Na : PO4 molar ratio : 2.3-2.6 Types of Internal Treatment Coordinated Phosphate ADVANTAGES : 1. EASY / SIMPLE - Properly blended by Nalco 2. NO FREE CAUSTIC 3. PREVENT CAUSTIC CORROSION 4. PREVENT TURBINE DEPOSIT 5. ABILITY TO BUFFER HYDRATE ALK.