Boiler troubles – Internal troubles _

           The problem of makeup feed water for boilers is common in almost all boiler of industries and has been a subject of detailed study by several boiler engineers and operator. In treatment of boiler feed water total impurities, elimination is not possible (since it is eliminate by ion exchange is up to ppm level ).these impurity of ppb level are not possible by treatment. Further these impurities during steam formation get accumulate in boiler  ,if proper continuous blow down not done. Generally reducing of water loss operational persons avoid to keep CBD (continuous blow down ) in open  and want control by given IBD (intermittence blow down )for once in a shift.

     Due to excess accumulation of impurities various problem gets in internal of boiler troublesome for operation and also leakages  in boilers takes places. In general not only important to treatment of water but also very important to  control of impurities by operational teams.

MAJOR  TROUBLE  IN INTERNAL BOILER 

CARRY OVER

 

        It is condition in which surface tension of boiling water increase by high TDS(total dissolved solids) and TSS (total suspended solids ). If higher the TDS carry over of ultimately takes places .

It is control by adding chemicals and also keep open CBD condition at least crack open must.mall

In this tds get carry over with small droplets of steam.

The solid and liquid contamination in steam expressed as steam purity and steam impurity quality.it is expressed as ppm (part per million )of steam purity or steam impurity .

 

Steam using in power generation is usually superheated for achieving greater efficient of turbine and other engine where is steam using e

 

·       Priming :

 

Priming  is  a very rapid boiling of water occurring in the boiler in a such a way that some water particles are carried away along with steam in the form of spray into the steam outlet. Priming is mainly attributed to the presence of suspended impurities and to some extend to dissolved impurities in the water. Thus it may be caused by imperfect filtration or  precipitation of insoluble salts from the improperly softened or unsoften feed water. Algae and vegetable growth in tanks can also lead to priming. feed water containing even a small quantity of scale forming salts may cause priming particularity ,if the total solids in the feed exceed 300 ppm. priming may also be caused by :                                                                                                            

·         Steam velocities high enough to carry droplet of water into the steam pipe.

·         Very high water level in the boiler

·         presence of excessive foam on the surface of the water which substantially fills the foam space

·         sudden steam demands leading to sudden drop of pressure in steam line followed by  ebullition, which causes a large mass of water to be interspersed with fine bubbles.

·         faulty boiler design.

 

 

Priming can be minimized by

 

a)       Good bolier design providing for proper evaporation of water maintaining  uniform heat distribution and adequate heating surfaces and also providing with anti priming pipes and dash –plates , etc.

b)      Maintaining low water levels.

c)       Avoiding rapid changes in the steaming rate caused by sudden steam demands.

d)      Maintaining as low concentration of the boiler water(with respect to  dissolved impurities) as is consistent with scale and corrosion preventation.

e)      Minimaxing foaming .

 

Foaming is the formation of small but persistent bubbles at water surface.this bubbles are carried along with steam leading to excessive priming.

            According to Bancroft, foams are formed when there is a difference in concentration of solute or suspended matter between the surface film and the bulk of the liquid. Substances which increase the viscosity of the film favour production of foam. The bubbles may also be protected by finely divided solids forming a protective “shell” around  each of them. Any material which lowers the surfaces tension of the water will collect at the interface and thus increase the foaming tendency of the liquid.

                    In steam raising, the bubbles of steam may be stabilized because of the accumulation of soluble salts in water. Clay or organic matter in raw water, oil or grease in condensed make-up water, and finely divided particles of sludge may also cause foaming.

              If the steam bubbles does not collapse on reaching the surfaces of the water, the foam may be drawn into the preheater or steam lines . the accompanying liquid film may carry along with it dissolved salts, suspended solids or other stablilizing materials which will be deposited on the cylinder walls , turbine blades, or in steam lines. If the stability of the bubble is such that it breaks near the steam outlet, tiny droplets of liquid from the collapsing film may be swept along into the steam lines giving “ wet steam’’.

             Foaming (and the consequent priming ) can beast  be prevented by removal of the Foaming and stabilizing agents from the water. Clay and others suspended solids as well as droplets of oil and grease can be removed by treatment of feed water                                                                                with agents such as hydrous silicic acid and aluminum hydroxide. The concentration of salts and sludge of the boiler can be controlled by intermitted or continuous blow-down.

Spiral or cyclone baffles or a series of baffle plates near the steam outlets help to prevent water droplets from entering the steam lines .

 Foaming can also be controlled by adding antifoaming agents . some of these act by counteracting the reduction in surface tension while others reduce foaming by simple mechanical action . For instance, castor oil spreads on the surface of water and prevents foaming. This is used only for low pressure boilers. Addition of a small concentration of polyamide antifoaming alters  Some the surface tension and leads to the formation of only large unstable bubbles at the same heat input. Antifoaming agents may also act by reducing the charge on the protective film or its components. A may he destroyed by the addition of another good foaming agent. The foam is finally destroyed by mutual antagonistic effects of the differences in charges ( positive and negative ) on the colloidal particles of the two foams . This can be illustrated by the fact that the foam obtained by a solution of an anionic detergent (e.g. , Aerosol OT) and that obtained from a cationic detergents ( Ethyl cetab ) will destroy each other on mixing the two solutions.

                    For avoiding priming and foaming , the following limits on total dissolved solids (in the boiler water) are usually suggested.

                

 

                 Type of boiler                            Total dissolved solids,ppm

                 Water tube                                 2000 to 5000

                 Lancashire                                   1000 to 1500

                 Vertical                                         < 3500

In the order to keep down the feed water concentration within safe limits , it is customary to remove a portion of the concentrated boiler water by blow-down and replace it with fresh feed water. Blow-down may be done periodically or continuously . However , blow-down will result in loss of heat .

      

                                                                                                                                                                                                                                      

 

 

 

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