Feed water composition
In order to generate steam on a constant basis, boiler should be continuously supplied with water, called as feed water. Feed water consists of condensate, which gets collected after the steam gets condensed by losing heat to the process or radiation losses, flash steam and make up water. Quantity of makeup water depends upon the quantity of the condensate and flash steam collected. Condensate and flash steam are pure in form without any impurities. This is not the case with the makeup water as it is raw water which can have many impurities depending upon the composition of natural water at that particular location. Both condensate and makeup water are mixed in feed tank from where they are fed to the boiler.
As we can see, the quality of feed water depends solely on the quality of makeup water. If feed water is not treated properly, it can result in multiple problems like scale formation or corrosion. It can also increase the amount of blow down required and hence can result in wastage of fuel. In order to avoid all these later problems related to feed water, the wise decision is to treat makeup water and ensure that different impurities are under their allowed limits.
The different impurities in the feed water can be grossly classified into three classes, namely dissolved gases, dissolved solids and suspended solids. Each of these impurities affects the boiler system in a different way. Feed water is to be treated for each of these types of impurities.
Air naturally has certain amount of dissolved gases like oxygen, carbon dioxide in it. Many times, ground water has traces of hydrogen sulfide in it. All these gases are corrosive in nature and initiate corrosion when they go inside the system. Carbon dioxide mixes with water and forms carbonic. Oxygen results in corrosion and pitting of boiler tubes. To ensure that feed water has a minimum amount of dissolved gases in it; condensate, flash steam and makeup water are mixed through a deareator head. Deareator head ensures that condensate, makeup water and flash steam are mixed rigorously and oxygen and other dissolved gases get separated.
Apart from this, solubility of oxygen in water goes on decreasing as the temperature of feed water increases. This trend can be understood well from the graph below-
As per BS Std 845, a D.O level of 2 ppm in boiler feed water is acceptable. It means, raising the temperature of feed water above 90 degree C can reduce the level of dissolved oxygen below the acceptable levels. Oxygen scavengers can also be added to reduce the dissolved oxygen levels in the feed water. Carbonic acid can be removed from feed water by neutralization with alkalis.
Suspended solids are sediments or organic matter which comes along with the makeup water. If not removed prior to their entry in the boiler, they can result in multiple issues like sludge formation, foaming, corrosion, clotting etc. Methods like clarification, filtration, and chemical treatment are used to limit suspended solids from the feed water.
As we have seen in the previous article, dissolved solids form scales which can be extremely difficult to be removed. Hence, it is quite essential that they are restricted from going inside the boiler. Dissolved solids include impurities like Hardness, sulfates carbonates etc. Natural water also has silica dissolved in it which can form hard scales. Hardness basically means excess amounts of Magnesium (Mg++) and calcium (Ca++) ions in the water. Hard water can be converted into soft water by using a softener which removes these ions from the water by the process of ion exchange. Along with this, water also has chlorides and sulfates which can be removed by the process of de-ionisation.