Corrosion and tube failure caused by water chemistry
Recommended ranges( Co-ordinated phosphate treatment for w/t boiler )pH | - 9.6 to 10.3 |
PO4 | - 4 to 20 ppm |
N2H4 | - 0.01 to 0.03 ppm |
TDS | - < 150 ppm |
Cond pH | - 8.6 to 9.0 |
Cl | - 20 ppm |
O2 | - 10 ppb |
Si | - 10 ppb |
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- Fill one 10 ml tube with distilled water
- Fill one 10 ml tube with boiler water tests.
- Add 0.5 ml sulphuric acid (RE 131) to each
Add 0.5 ml Ammonium Molybdnate (RE130) to each
Add 0.5 ml Aminonapthol Sulfonic acid (RE 132) to each
Stir well between each addition - Wait 3 minutes for calorimetric compaisonAlternately Vanado-molybdnate test
- Place 5 ml boiler water in 10 ml tube
- Place 5 ml distilled water in other 10 ml tube
- Top both to 10 ml with Vanado-molybdnate reagent
- Place in colour comparator and compare after 5 mins
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- Bi carbonates do not show up in the phenolpthalein sample as they have a pH < 8.4. Bi carbonates can not occur in boiler but if suspected in raw feed then the following test.
- Take phenolpthalein sample, add 1 ml methyl orange
- If yellow, bi carbonates are present
- Add H2SO4 untill red
- Total 0.02N H2SO4 x 10 = Total Alk in CaCO3
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- Only necessary where N2H4 used in blr
- Pour condensate sample into two 50 ml colour comparator tubes
- Add 2 ml Nessler reagent to one
- Wait 10 mins
- Use colour comparator
Boiler water treatment
Alkalinity
Treatment
For pressures below 20 bar dissolved O2 in the feed does not cause any serious problems so long as the water is kept alkaline
However cold feed should be avoided as this introduces large amounts of dissolved O2 are present, for pressures greater than 18.5 bar a dearator is recommendedFeed Treatment Chemicals
Sodium Hydroxide Calcium Bicarbonate (CaCO3 + Na2CO3) Magnesium Bicarbonate Magnesium Chloride. . . Sodium Phosphate Calcium Carbonate Calcium Sulphate Magnesium Sulphate All in this column precipitated as hydroxide or phosphate based sludges All in this column form sodium salts which remain in solution - Sodium Hydroxide
- TriSodium Phosphate Na3 PO4 (Cophos III)
- Alkaline
- When added to water decomposes to NaOH and Na2 HPO4
- As water evaporated density increases and NaOH and Na2 HPO4 recombine
- Phosphates can form Phosphides which can coat metal to form a protective barrier, with excessive phosphate levels, this coating can be excessive on highly rated boilers operating at higher steaming rates
Oxygen Scavengers
- Hydrazine N 2 H 4
- Oxygen scavenger, continously injected to maintain a reserve within the boiler of 0.02 to 0.1 ppm and a feed water O2 content of less than 10 ppb
- At temperatures greater than 350oC , will decompose to ammonia and nitrogen and will aid in maintaining balanced alkalinity in steam piping.Steam volatile, neutralises CO2
- Inherent alkalinity helps maintain feed water alkalinity within parameters of 8.6 to 9.0.
- Used in boiler operating above 32 bar, will not readily react with O2 below 50oC hence risk of copper corrosion occurs with the ammonia stripping off the continuously reforming copper oxides.
- Supplied as a 35% solution
- Carbohydrazide (N 2 H3)2CO
- Is a combined form of Hydrazine
- It is superior to hydrazine in performace and is designed to minimise the vapours during handling
- Carbohydrazide and its reaction products create no dissolved solids
- Is an oxygen scavenger and metal passivator at both high (230'C) and low (65'C) temperatures
- Can be used with boilers up to 170 bar
- Erythorbic Acid (Sur-gard) R1-C(OH)
- An effective oxygen scavenger and metal passivator
- It is the only non-volatile scavenger which can be used with spray attemperation
- does not add measureable solids to the boiler water
- May be used in boilers up to 122 bar
- Officially recognised as a Safe Substance
- As with hydrazine a small amount of ammonia is created in the boiler, it is not recommended for layup.
Polymer Treatment
Polymer is a giant molecular built up by stringing together simple molecules
For auxiliary blrs this is a superior form of treatment to the old alkaline and phosphate treatment. The correct level of alkalinity must be maintained as too low a level neutralises the electric charge of the polyelectrolyte. Too high causes caustic alkalinity.
Amine treatment
Compounds containing nitrogen and hydrogen.Neutralising amines
Hydrazine N2H4
Bramine ( cyclohexalamine )
(Bull & Roberts amine treatment)Filming amines
Shows neutralising tendencies, main function however is to coat piping with a molecular water repellent protective filmLimits of density/pressure
Sludge conditioning agents
Treatment in boilers (non congruent)
LP tank blrs (<14 bar)
Na2CO3- precipitates salts, provides alkalinityMgSO4- Sludge conditioners
Medium pressure tank blrs (<17.5 bar)
Na2CO3(Medium to High pressure water tube <60 bar
Na2CO3 + Na2HPO4 +sludge conditionersHP to UHP blrs (42 to 80 bar)
Due to level of decomposition of Na2CO3 . NaOH preferred for better controllability Na2HPO4Permissible limits
Shell | WT | WT | WT | WT | WT | ||
TEST> | PPM | <17.5b | <17,5b | <32b | <42b | <60b | <85b |
Hardness | CaCO3 | <=5 | <=5 | <=5 | <=1 | <=1 | <=1 |
P.alk | CaCO3 | 150-300 | 150-300 | 150-300 | 100-150 | 50-100 | 50-80 |
T.D.S. | CaCO3 | <=7000 | <=1000 | <=1000 | <=500 | <=500 | <=300 |
Cl | CaCO3 | <=1000 | <=300 | <=150 | <=100 | <=50 | <=30 |
PO4 | PO4 | 30-70 | 30-70 | 30-70 | 30-50 | 30-50 | 20-30 |
N2H4 | N2H4 | -- | -- | -- | 0.1 -1.0 | 0.1 - 1.0 | 0.1 - 1.0 |
SO3 | SO3 | 50-100 | 50-100 | 50-100 | 20-50 | -- | -- |
SiO2 | SiO2 | -- | -- | -- | -- | -- | <=6.0 |
Fe | Fe | -- | -- | -- | -- | -- | <=0.05 |
Cu | Cu | -- | -- | -- | -- | -- | <=0.02 |
pH | pH | 10.5-11 | 10.5-11 | 10.5-11 | 10.5-11 | 10.5-11 | 10.3-11 |
Limits for feed water | |||||||
Cl | CL | <=10 | <=5 | <=1.0 | <=1.0 | <=1.0 | <=1.0 |
O2 | O2 | ---- | ---- | <=0.006 | <=0.003 | <=0.015 | <=0.01 |
NH3 | NH3 | ---- | ---- | ---- | ----- | ----- | <=0.5 |
Fe | Fe | ---- | ---- | ---- | ----- | <=0.01 | <=0.01 |
Cu | Cu | ---- | ---- | ---- | ----- | <=0.01 | <=0.01 |
pH | pH | ---- | ---- | ---- | 8.5-9.2 | 8.5-9.2 | 8.5-9.2 |