10 common causes of carry over in power plant:-
Today we discussed about the a very important topic which related to the boiler operation that’s topic is carryover. Carryover is the process during the boiler operation when the contaminant leaves a boiler steam drum with the steam. It can be in solids, liquid or vapor form. It is undesirable in a boiler. Carryover is not a serious issue during the boiler operation but it can be harm when it happened in regular way. It dose effect the steam generation in many ways. Carryover results from incomplete separation of water from mixture of steam and water. Steam separation units are placed inside the boiler drum. Defects on these separation units may lead to carryover. Foaming of boiler water is also responsible for carryover. Boiler design, type of mechanical steam separation equipment, load fluctuation, boiler drum level, amount of space available for steam separation from the steam-water mixture some causes of carryover. Carryover is the common problems now these days it caused corrosion and water hammering in the system. In simple way, when drum water passes through the steam system along with the steam. This water naturally contains many type of impurities which also get carried along with the steam. In this post we briefly discussed the causes and effects of carryover on a steam system.
10 common causes of carry over in power plant:-
Here we discussed 10 common causes which is responsible for carryover
1.High Boiler Steam Drum Level Operation Causes of carryover:-
when boiler in running condition it should be insure that all the tripping of boiler are In the healthy condition. Boiler drum level high high and low low level tripping one of them . High water levels are decided by the boilermaker. At on job site conditions the drum levels are to be clearly marked and set by competent persons. While the low water levels inside the drum affect the circulation, the high level inside the drum affects steam purity. The carryover is more with high water level. Lowering the water inside the boiler drum settings should be examined ensuring that roof tubes are within the water level. In many cases, Boiler users have reduced the carryover in the boiler simply by altering the boiler drum level settings and by tuning the control valve action.
suddenly on off operation of boiler feed pumps a cause for carryover in flue tube boiler. It can be practiced only when the drum size is big as in an oil-fired boiler with internal furnace. In external furnace cases, the steam space is less, thus feed water regulation would give purer steam.
2.Boiler Feed Water Chemistry Causes of carryover:-
Boiler feed water is very important factor of any boiler industries. Boiler makers designed each and every boiler for operating predetermines boiler water chemistry. The boiler water if contains high TDS (total dissolved solids) the carryover of solids will be more. Since the efficiency of separator which is fixed inside the boiler steam drum, the only way to bring carryover is to bring down the Boiler water TDS (total dissolved solids) . In boiler water, the presence of free NaOH must be eliminated by practicing coordinated phosphate control. High alkalinity and presence of suspended impurities & TDS increase carryover chances.
When the suspended impurities are more, there is a blanket formation over the water-steam interface and this prevents the easy passage of steam bubbles from the boiler water. The formation of the impurities on the surface of boiler water inside the steam drum is called the foaming .The steam bubbles out of the skin formed by the impurities throwing the suspended impurities to the steam driers.
The silica in boiler water is to be controlled more stringently based on pressures. Higher chances of Silica carryover is at the pressures above 28kg/cm2 inside the boiler
3.Improper Chemical Dosage inside the boiler drum or deaerator Causes of carryover
For maintainaning the feed water chemistry inside the boiler proper chemical dosing inside the drum ( tri sodium phosphate ) and inside the Deaerators ( hydrazine ) should be dosed . Insure that the adequate quantity of chemical should be dosed. Whenever chemical quantity increases inside the boiler drum or Deaerators it also disturbed the boiler water chemistry. Insure that stroke length of dosing pumps should be set according to requirement.
A good water treatment is important to keep a check on carryover. Antifoam agents chemical are used to control the carryover. Use of sludge conditioners also help in preventing carryover besides preventing deposits.
4. Wrong Erection Of Steam Drum Internals Causes of carryover:-
Whenever a customer order for the steam drum then vendor send the proper engineering for drum erection, if wrong erection of drum take place on the site. During the wrong operating of boilers or the drum internals get disturbed during maintenance. The drum internals fail due to corrosion in service due to poor water chemistry or improper boiler storage at idle times. Regular check by the manufacturer or competent boiler consultant would be worth.
5.Boile Operating lower pressure Causes of carryover:-
Boiler operating is lower pressure than the design pressure also a causes of carryover . when the operating the boiler at pressures significantly lower than design pressures is a cause of carryover. Lower the steam pressure of boiler more will be the specific volume of the steam inside. Hence, the steam travelling velocity inside the drum internals would be more at lower pressures. This will hamper the separation of water droplets from steam which results in carryover.
6.Fluctuating Boiler Loads Causes of carryover:-
Fluctuating boiler loads is very common factor increase the chances of carryover in boilers. Whenever we increase the loads suddenly on turbine side then steam demand suddenly increases, causes the drum pressure drops down. As a result, the drum water will suddenly expand resulting in a phenomenon called ‘priming’. Due to priming, slugs of water will be carried along with the steam resulting in carryover.
7.Boiler Steam Drum Size Causes of carryover:-
Boiler steam drum size is also factor for carryover. each and every boiler maker designed the boiler drum according to steam demand and steam purity consideration .where 3 element of controlling the boiler steam drum level is used their steam drum size is less as compared to single element controlling. The drum internals include baffle plates, cyclone type separators, Chevron separators or screen driers, perforated boxes. The drum diameter should be selected so that the distance from water-steam is adequate for highly purifying steam generation. If any of the boiler steam drum not working properly its highly chances to carryover
8.Improper Boiler Stem Drum Internals arrangement Causes of carryover:-
Improper boiler steam drum internals arrangement is a major cause for carryover. There is always good scope for improving the steam quality by critically reviewing the present arrangements. Every boiler maker have different design consideration regarding the boiler steam drum internals arrangement.
Drum internal design should aim at fulfilling the following criteria-
• minimizing the turbulence produced by the risers & down comers by adopting baffles.
• Creating a twisting path for steam before it enters drier section.
• Provide adequate drier section to trap water droplets with the effect of centrifugal force and drain out without allowing re-entering into the steam.
• Maximize the distance between the water-steam interface and the driers.
• Distribute the dosing chemicals over the entire drum length
• Extract the blow down water across the entire drum length
• Distribute the feed water without creating turbulence inside the steam drum.
in low pressure boilers the fire tube boilers are provided with dry pipes at steam pipe inlet. High-pressure boilers are to be provided with multi stage separators such as cyclones, chevron separators. Some boiler makers use demisters pads with higher drum sizes.
9.Improper Blow Down of boiler Causes of carryover :-
CBD (Continuous blow down ) is the best method to maintain the constant boiler water chemistry .if improper blow down given through the CBD then TDS ( total dissolved solids ) remaining inside the drum so it highly chance the carryover if any fluctuation happen during the boiler operation .
10. Improper Preservation of boiler during long trum shutdown Causes of carryover:-
whenever boiler is shutdown for long interval of time it should be take care of boiler passivization and adopted correct method of boiler preservation dry or wet method of boiler preservation. Improper preservation of boiler is leads to iron oxide generation in boiler water and this leads to carryover during boiler restarting.
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