Introduction to Carryover
When any solid, liquid or vaporous contaminant leaves the boiler drum along with the steam, the phenomenon is called as carryover. Carryover though does not sound like a serious issue, can affect performance of a steam system in many ways. Carryover results from incomplete separation of water from mixture of steam and water. Carryover, a commonly faced problem in many boiler systems results in many other issues like corrosion and water hammer.
In common situations, drum water passes in to the steam system along with the steam. This water naturally contains certain amount of impurities which also get carried along with the steam. This article briefly describes causes and effects of carryover on a steam system.
Causes of Carryover
A large number of factors result in carryover in boilers. Some of these factors are-
1. Operating pressure
Operating the boiler at pressures significantly lower than design pressures is a cause of carryover. Lower the steam pressure more will be the specific volume of the steam inside. Hence, the steam velocity in drum internals would be more at lower pressures. This will hamper the separation of water droplets from steam which results in carryover.
2. Load Characteristics
Fluctuating boiler loads increase the chances of carryover in boilers. When steam demand suddenly increases, 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.
3. Drum Size
This factor is by design. Three-element drum level control had made it possible to design smaller diameter drums. 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 interface to driers is quite adequate. Failure to address this would result in carryover.
4. Drum Internal Geometry
There is no single standard method to arrange the drum internals. The boiler configuration calls for extensive variation of drum internal arrangement between boilermakers and even within the product range of single manufacturer. Figures show several internals arrangement followed / encountered by me.
Improper internals arrangement is a major cause for carryover. There is always good scope for improving the steam quality by critically reviewing the present arrangements.
Drum internal design should aim at fulfilling the following criteria -
- Reducing the turbulence created by the risers & down comers by adopting baffles.
- Creating a tortuous path for steam before it enters drier section.
- Provide adequate drier section to trap water droplets and drain off without allowing re-entrainment into the steam.
- Maximize the distance between the water-steam interface and the driers.
- Distribute the chemicals over the entire drum length
- Extract the blow down water across the entire drum length
- Distribute the feed water without creating turbulence
Low-pressure boilers, generally, 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 boilermakers use demisters with higher drum sizes.
5. Condition Of Drum Internals
Whatever be the proper engineering & supply made by the boiler vendor, it is possible that things go wrong in erection at site. Even in operating boilers, 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.
6. Feedwater Chemistry
The chemistry of feedwater should meet certain minimum requirements laid out by the boilermakers. The feedwater if contains oil or organic matter, the same leads to foaming. The carryover of solids is very high.
If the feedwater has dissolved iron, which may be either from condensate or from the make up water or generated within the boiler will again lead to foaming.
If the feedwater contains suspended impurities then also carry over will be high. Dissolved oxygen in feedwater leads to corrosion of economiser and this leads to generation of iron oxide, which again leads to increased suspended iron in the boiler water.
7. Boiler Water Chemistry
The chemistry of Boiler water should meet certain minimum requirements laid out by the boilermakers. The boiler water if contains high TDS the carryover of solids will be more. Since the efficiency of separator which is fixed, the only way to bring carryover is to bring down the Boiler water TDS. In boiler water, the presence of free NaOH must be eliminated by practicing coordinated phosphate control. High alkalinity and presence of suspended impurities increase carryover.
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 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. Silica carryover is more at pressures above 28kg/cm2.
8. High Drum Level Operation
The operating level, alarm levels & trip levels such as NWL, Low water level, very low water level, High water levels are decided by the boilermaker. At site conditions the levels are to be clearly marked and set by competent persons. While the low levels affect the circulation, the high level affects steam purity. The carryover is more with high water level. Lowering the NWL 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 NWL settings and by tuning the control valve action.
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.
9. Improper Chemical Dosage
A good water treatment is important to keep a check on carryover. Excess chemical dosage upsets boiler water chemistry. Antifoam agents are used to control the carryover. Use of sludge conditioners also help in preventing carryover besides preventing deposits.
10. Blow Down Practice
Continuous blow down is the best way to maintain a constant boiler water quality. Proper blowdown practices help reduce the carryover.
11. Offline Preservation
Improper offline preservation leads to iron oxide generation in boiler water and this leads to carryover during boiler restarting.