Q&A On Water Test
Scale and hardness :
What is scale formation ?
Scale is the accumulation of minerals on the water side of boiler heating surface. Ions calcium (Ca2+) and magnesium (Mg2+) account for the majority of scale formation minerals, although scale-forming minerals such as silica can also contribute.
What is the meaning of hard and soft water ?
What is the meaning of hard and soft water ?
Hard water is water that contains large quantities of dissolve minerals in water (such as calcium and magnesium).
Soft water is water that contains very small quantities of dissolve minerals.
The hardness of water is measure in parts per million (ppm).
What is the normal value of hardness ?
Normal value of hardness is less than 1 ppm. So almost zero.
Where does calcium and magnesium come from ?
As water moves through soil and rock, it dissolves very small amounts of minerals and holds them in solution, which make city (domestic) water hard. Hard water express a high concentration of ions calcium (Ca2+) and magnesium (Mg2+) dissolve in water.
What are the consequences of high concentration of calcium and magnesium ?
Scale on surface is the result of high concentration of calcium and magnesium.
Scale has 2 consequences:
First, it reduces heat transfer efficiency. It acts as an insulating material and reduce heat transfer. This results in more fuel burned to generate the desired amount of steam. For example, 1/16'' thick scale increase fuel consumption 15 %, 1/8'' thick scale increases fuel consumption 20 %. Scale also narrows the inside of the tube and restrict the flow of boiler water through it, further reducing heat transfer.
Second, boiler's tubes can overheat and fails (fish-mouth fail) as heat is not transferred to water anymore. So if the boiler water does not remove heat from the heating surface as designed, the heating surface overheats. For example, if a boiler tube without water is placed in the furnace of a boiler that is fired, the tube overheats and distorts quickly. However, a boiler tube placed in the furnace or boiler with boiler water passing around or through it (depending on the type of boiler) is not damaged. The tube does not overheat because sufficient amount of heat is transferred to the boiler water.
How to prevent scale formation ?
There are two ways to proceed; external and internal water treatment :
External water treatment - Zeolite water softener :
Zeolite water softener removes calcium and magnesium salts from water to reduce it to zero hardness. In the process (ion exchange process), a zeolite softener removes hardness and replace it with highly soluble sodium ions to prevent scale buildup on heating surface.
Internal (chemical) water treatment – Making nonadhering sludge (precipitation) :
To prevent calcium and magnesium carbonate (that may have pass the water softener) from forming scale within the boiler, it is necessary to precipitate these impurities in the form of a sludge (so chemicals change calcium and magnesium to sludge).
Adding caustic soda (another name to say sodium hydroxide) and/or phosphate to the boiler water can prevent scale formation by changing calcium and magnesium into nonadhering sludge. The sludge is then remove from the boiler by mean of the bottom blowdown.
Where do we generally take our sample test water for hardness ? And what higher than normal value could mean ?
Water is normally test at two spots for hardness; at the outlet of water softener and from a condensate tank.
Water softener : If hardness is greater than the control limit (normally zero), check softener operation and brine tank.
Condensate : Hardness inside the condensate return tank could indicate a leak into the condensate (probably form a heat exchanger breach) and compromise boiler feedwater (as condensate water does not return inside the softener, but directly to the deaerator and boiler).
Conductivity, TDS, Priming, Carryover
What is conductivity of water and what is the relation between conductivity and TDS ?
The greater the amount of total dissolve solids (TDS) present in the water, the greater will be the conductance of the water. So to determine the amount of dissolve solids in the water, the ability of the water to conduct an electric current is measured.
In other words, the TDSis a measurement of the concentration of dissolved impurities in boiler water. As conductivity is the ability of a material to allow the flow of electricity. The amount of solids in water affects the ability of water to conduct electricity. The more TDS in the water, the higher the conductivity.
Do we measure TDS directly ?
No, total dissolved solid is measure by the mean of conductivity, so it’s an indirect measure.
It’s like oxygen, we don’t measure it, and we measure the sulphite concentration, which absorb oxygen.
On the other hand, hardness is measure directly.
How do we measure conductivity of boiler water (and thus TDS present) ?
The TDS present in boiler water can be measured with a conductivity meter. A conductivity meter is an instrument that measures the electrical conductivity of a water sample to determine total dissolved solids present.
Where dissolve solid come from, as water have already be filter with external treatment (softener, reverse osmose) ?
Dissolved solids are introduces by feedwater, even though the water has been treated to remove substances that contribute to scale and deposit formation. No water treatment process can remove all of these substance from the boiler feedwater. Solid also come from the condensate return line (metal particle, etc.).
What are the consequence of high conductivity test result ?
Solids such as sodium salts and suspended matter do not readily form scale. However, as boiler water boils off relatively pure steam, the remaining water is thickened with solids. If the concentration of solids is allowed to accumulate, priming and carryover could occur. Sludge could also cause harmful deposits, leading to overheating of the metal.
Priming is a condition that occurs when large slugs of boiler water are carried into the steam lines.
Carryover is a condition that occurs when small particles of boiler water are carried into the steam lines.
What are the causes of priming and carryover ?
Both of these conditions could have mechanical causes, as :
- High water level in the boiler,
- Opening a main steam stop valve to quickly
- Excessive steam load
- Peak loads that exceed boiler capacity
But also, chemical causes, as :
- High concentration of dissolved and suspended solid (high TDS)
- High concentration of chemicals in the boiler water
- Impurities in the boiler water that cause a high surface tension and then foaming (such as oil, fat or other organic product)
What could be the consequences of priming and carryover ?
Priming is a very dangerous condition because is can lead to water hammer. Water hammer in the steam lines can result in the steam header or line rupture, so extreme water hammer can cause steam lines to burst.
TDS is influence by what ?
- Surface blowdown rate
- Percentage of make-up water (new water in steam drum)
- Feed rate of chemical treatment
How de we take care of TDS level in boiler water ?
Oxygen (O2), Carbon dioxide (CO2), Corrosion, Pitting and Sulphite (SO3)
What are the principal gases dissolve in water ?
The gaseous impurities of concern are oxygen (O2) and carbon dioxide (CO2). These are dissolved in the water and can cause corrosion of the boiler metal and pipelines.
What are the consequences of O2 and CO2 in a boiler ?
Oxygen produces a special type of corrosion known as pitting. Pitting corrosion is cause by dissolved oxygen in water and is characterized by a wide and deep pitting type wear, located under a visible tubercule. This corrosion will generally occur in the entire feed water system and in the internal boiler section
Carbon dioxide corrosion produces grooves in the metal and especially attacks pipe threads.
How can we remove O2 and CO2 in water before it enter the boiler ?
These gases can be removed from water before it enters the boiler by use of a deaerator (mechanical treatment). In the deaerator, the water is heated to the boiling point, causing the gases to be driven off through a vent in the top.
Most deaerators are designed to remove oxygen down to levels of 7 ppb.
How do we remove remaining oxygen after mechanical deaeration ?
Chemicals added directly to the boiler that can be used to absorb dissolved oxygen and neutralize carbon dioxide. These chemical are called oxygen scavenger, because they absorb the dissolved oxygen. Sodium sulphite is an oxygen scavenger that is commonly used to treat boiler water. Sodium sulphite combines with oxygen to form sodium sulfate, which accumulate at the bottom of the boiler. The sodium sulfate is then discharged from the boiler through the bottom blowdown valve.
How do when know there is enough sulphite in the boiler water ?
Sodium sulphite is fed to a boiler to prevent pitting due to dissolved oxygen. Usually, if there is an excess of sulphite maintained in the boiler water, then the complete removal of any dissolved oxygen will be assured. So we don't test O2 or CO2 per say, but just the presence of chemical (sulphite) in the water. When there is sulphite, there should be no O2 and CO2.
PH, Condensate, Amine
What low and high pH can cause in a boiler ?
Acidic water (low pH, below 8) removes metal, leading to the weakening of the tubes and tube sheets, causing them to leak.
High alkalinity (high pH, over 13) can lead to caustic embrittlement. Caustic embrittlement can cause metal to crack along the seams and at the ends of tubes in a boiler. Note that the term alkalinity has two meaning when referring to boiler water (we alkalinity below)
What is condensate return line corrosion ?
Caused by acidity of condensate which results from decomposition of bicarbonates dissolved in make-up water (alkalinity). Will cause localized attack and thinning of the immersed section of condensate piping. Leaks will generally occur near pipe treads where pipe wall is reduced. Corrosion residues can also cause fouling of steam traps and deposits in boilers.
How do we control the pH of condensate ?
pH is typically controlled by adding sodium hydroxide (also called caustic soda) to maintain proper alkalinity (lower the pH) within the boiler and may be fed directly to the boiler drum or to the feedwater before it enters the drum. Phosphate compounds may also be used to increase the pH value.
How do we prevent corrosion of the return lines of the condensate ?
Filming amine may be feed to the steam heather. They will form a film (coating) around the pipe surface of the return line and thus prevent corrosion. These chemical are known as amine. Sometime, if steam is used to humidify the air building, amine can’t be used, as it will be mixed with the air building and can cause cancer.
How do we measure pH ?
Boiler water Ph can be measured using a pH meter. A sample of the boiler water is drawn into a clean sample jar. A test probe is calibrated and then dipped into the sample jar. The pH is read on the meter.
Alkalinity P, M, T
The term ‘’alkalinity’’ can be confusing to the boiler operator because the term has two meanings when referring to boiler water.
The first meaning of alkalinity is the condition where water has a pH above 7.
The second meaning of alkalinity is the presence of materials dissolved in water that make the water alkaline.
The dissolved materials that are of primary relevance to boiler operation are carbonates, bicarbonates and hydroxides. Their presence is undesirable because they contribute to chemical reactions that form carbon dioxide. The carbon dioxide dissolves in condensate and forms carbonic acid, which cause corrosion in the condensate return lines.
Alkalinity is normally measured by a titration test of the boiler water sample and is recorded as P, M or T alkalinity.
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