Boiler water chemistry - Properties of feed water explored
Introduction
In our previous article, we have already taken a look at boiler water testing and learnt about a few common tests. The purpose of these tests is to monitor and maintain its various constituents of the feed water. All the constituents should be maintained in their appropriate chemical range. This can be done by adding of various chemicals or by treating the boiler water using specialized instruments.
Let’s take a look at the boiler water chemistry and the effects resulting from the inability of maintaining the correct chemical equilibrium of the various properties.
Properties and their effects
1. pH
Controlling the PH of water is the first step towards boiler treatment.
We all know that if PH is below 7 the solution is acidic and above 7 it’s alkaline.
For boiler water PH is maintained between 11-11.8 and that of feed water between 7-9.
Effects
If either of the values increases, water will accordingly become acidic or alkaline, leading to detrimental effects caused due to acidic attack or caustic embrittlement. This may damage the boiler completely.
2. Hardness
The main purpose of controlling hardness is to prevent scale formation building on the interior surface.This can be done by keeping hardness values low.
Standard hardness value is kept less than 1 Mg of CaCo3/liter.Hardness can be removed by using various phosphoric acid based chemicals in the blow-down process of boiler.
Effects
In case layers of scales keeps on building inside the boiler, the insulation layer will reduce, which in turn will prevent the heating of water and generation of steam.This will lead to overheating and damage to the internal parts of the boiler, reducing the overall efficiency.
3. Oil and fats
Oil-in-water emulsion is quite common on ship. Proper care should be taken to prevent any ingression of oil in the feed water.
This is done in order to prevent carbonization of oil and formation of scales due to boiler heat.
Accumulation of oil will reduce the capability of heat transfer surfaces, thus reducing the overall efficiency.
Effects
Carbonization and scale formation will lead to overheating of the boiler material and parts, which may lead to serious damage to the boiler.
4. Dissolved oxygen
When oxygen reacts with metal of the boiler,it causes corrosion. Thus it is imperative to remove this dissolved oxygen.This can be done by adding additives such as hydrazine to water.
Standard value of hydrazine in water should be 0.1-03mg/l.
Effects
Prolonged corrosion leads to damage of boiler material and even rupture of internal parts.
5. P alkalinity
This is standard value of hydroxyl ion(OH) and carbonate ion (CO3) in water. The higher the value of P alkalinity, higher the probability of scales formation.
The conventional boiler water test finds only the presence of OH and half of CO3.
Standard value of P alkalinity in Boiler Water is between 50-300mg CaCO3/l.
Effects
Higher value of P alkalinity leads to scale formation and overheating of the inside of the boiler causing internal damage.
6. Chloride ion
It causes generation of scale formation. The more the chloride ions, the more the scale formation. Therefore,it is necessary to limit it as low as possible. The problem with chloride ions is that high degree of dissolution makes it difficult to remove them at a later stage.
It also serves as a standard of boiler water concentration together with total solids.
Formation of chloride ions can be done by controlling the amount and time of boiler blow down.
Standard value of chloride ion is less than 300 mg Cl/l.
7. Phosphates
Proper check on phosphates reserve of boiler water should be kept because it reacts with calcium and forms calcium phosphates, which is a major scale forming component
When calcium phosphates is formed it precipitates on the water surface and can be easily removed by blow down.
Phosphates also contributes to the prevention of alkali corrosion and caustic embrittlement.
Standard value of Phosphate is 20-40 mg PO4/l.
8. Total Solids and electrical conductivity.
The increase of total solids promotes deposition of scales in the boiler as well as in the equipment consuming steam due to carry over.
More amount of dissolved solids also leads to more scale formation and carrying over.
Effects
Carry over of solids is a dangerous issue as it can leads to steam using machinery such as high speed turbines etc.
References
Mohammed Abdul Hamid Notes - Singapore Maritime Academy
