Table of Contents
Boiler Explosion Impact
The power liberated by the explosion of a Lancashire boiler of size 2.28 mt OD x 9.1 mt long weighing 15422 Kgs, containing 27.2 Kgs of steam and 19096.5 Kgs of water, and with a pressure of steam at the instant of explosion of 7 KSCG, is sufficient to project it to a height of 3.29 Kms.
Safety Measures
To lessen the risk of breaking of gauge glass, the water cock shall be reopened, after the steam cock.
Fuel Loss Due to Leakages and Deposits
Leakage of one drop of oil per second would result in a fuel loss of 4000 litres per annum.
3 mm of soot on a heating surface has the insulating effect of a 16 mm layer of asbestos, decreasing the boiler efficiency by 9% and increasing fuel consumption by 2.5%.
1 mm thick scale on the waterside of the boiler increases the fuel consumption by 5% – 8%.
Boiler Blowdown
Blowdown of the boiler should be based on the TDS of the boiler water. Excessive blowdown in a boiler can lead to fuel loss.
Condensate Return & Feed Water Temperature
With a good percentage of the condensate return, the fuel requirement of the boiler will be reduced. For every 6 deg C rise in the feed water temperature, there will be approximately 1% saving of the fuel in the boiler.
The condensate is almost entirely pure water, which needs no treatment. Hence, with a good percentage of condensate returning to the boiler house, the expense involved in water treatment will be reduced by an appreciable amount.
Steam Pressure and Heat Transmission
In indirect heating system, for better heat transmission, use steam at the lowest pressure since it has the highest latent heat of steam.
Dry Saturated Steam vs. Wet Steam
The best steam for industrial process heating is dry saturated steam and not wet steam since
– Moisture overloads the steam traps and other condensate handling equipment.
– Trapped moisture particles reduce the total heat in the steam since they carry no latent heat.
– Trapped moisture increases the resistant film of water on heat transfer surfaces, and thereby the rate of heat transfer is slowed down.
Limitations of Superheated Steam
Superheated steam is not desirable for industrial process heating since
– Temperature in the plant cannot be effectively controlled (unlike saturated steam, whose temperature depends only on pressure).
– It gives up its heat at a rate lower than the condensation heat transfer of saturated steam.
– It has a lower heat transfer coefficient. Therefore, to deliver the same heat transfer rate as saturated steam, SH Steam requires more heat transfer area.
Benefits of Direct Steam Injection
Where the dilution of tank contents and agitation is acceptable, the heating of a liquid by direct injection of steam is often desirable, and its advantages are as follows:
– No condensate recovery system is desirable.
– The heating is quick.
– The process is thermally efficient since the sensible heat of the steam is also used with the latent heat.
– Agitation of the contents can be created by blowing steam.
Steam Leaks and Air Film Resistance
A 3 mm diameter hole on a pipeline carrying steam at a pressure of 7 KSCG would waste 32.65 KL of fuel oil per year. Steam leaks on high-pressure steam lines are prohibitively costlier than on low-pressure steam lines. In many of the industries, this is a neglected area.
Air is considered to be a worse conductor of heat than the best lagging. The removal of air from inside heating surfaces is the most neglected, yet it is one of the most important. It is estimated that a 0.25 mm thick air film offers the same resistance to heat transfer as a 330 mm thick copper wall.
Advantages of Air Preheating
Preheating of secondary air in the case of solid fuels and the entire air in the case of other fuels confers many benefits. Even in the case of solid fuels, the entire air is heated in pulverised fuel firing:
– 1% gains in thermal efficiency for every 30°F fall in flue gas exit temperature by using it to preheat air;
– Preheated air gives higher flame temperatures and hence increased heat transmission through radiation.
– Easier to burn lower-grade fuels;
In the case of a mechanical stoker, the preheat of air should not be too high as to increase the flame temperature to such a limit as to destroy the grate.
Importance of External Cleanliness
External cleanliness of the boiler, superheater, economiser, reheaters, and air heaters has a direct bearing on.
– Efficiency
– Generation capacity
– Pressure maintenance and
– Draft loss
Impact of High TDS
Maintaining high TDS levels in the boiler drum results in water carryover and foaming. This leads to wet steam and lower heat transfer efficiencies to the process, thereby increasing the fuel consumption.
Importance of Monitoring Operation Parameters
Boilers by themselves do not operate efficiently but need to be operated efficiently in a manner that they deliver the best efficiency. Boiler operation parameters should be monitored continuously to correct operation. Therefore, the first parameter to be measured is steam fuel ratio.
Reasons for High Stack Temperature
High stack temperature may be due to following reasons:-
– Short circuit of flue gases due to fallen or damaged baffles in the flue path
– Impairment of heat transfer due to scale deposit on waterside
– Soot deposit on the fireside of the heat transfer surface
– Insufficient heat transfer area provided due to faulty design.
– Insufficient heat transfer area due to removal of part of the heating surface due to damage.
A 17 deg C rise in stack temperature means a 1% rise in fuel cost. In any case, the stack temperature must be 30 deg C and 85 deg C higher than the saturated steam temperature of oil and coal-fired boilers, respectively, corresponding to the specified working pressure.
Radiation Loss and Piping Insulation
Bare steam pipelines, flanges, and hot processing equipment give up heat to the atmosphere by radiation. It has been estimated that a bare steam pipe of size 150 mm in diameter and 100 meters in length carrying saturated steam at 8 KSCG could waste 25 KL of furnace oil in one year.
Pipe Sizing and Redundant Piping
A right-size steam pipe should be selected for the amount of steam it has to carry at a particular pressure. If the size of the pipe were small, then it would result in
– Steam starvation at the using end
– High pressure drop.
If the size of the pipe is too large, then it will result in
– Increase in capital cost of installation
– Increase in radiation loss from the larger surface.
Eliminate redundant pipework
– Redundant pipework needs to be eliminated since it will invariably be at the same temperature as the working portion of the system and losing heat at a similar rate. Reductions of 10 to 15% are possible on older sites.
Boiler Feed Tank Sizing
The boiler feed tank should be sized to be 1.5 times the peak steam demand. Large feed water tank presents two problems.
– The incremental heat addition by condensate return is reduced.
– Increased radiation loss due to holding large quantities of hot water.
Combustion Efficiency and Air Leakage
Prevent leaping out of flame outside the furnace and other openings. An increase of air leakage by 5% leads to an increase in the excess air level and also a loss of boiler plant efficiency of 0.5% – 1%.
Right Boiler Sizing and Combustion Settings
Avoid over-sizing the boiler. When an oversized boiler is installed in a plant, it results in greater stack loss and thereby the overall efficiency of the boiler is reduced.
Selection and design of boiler auxiliaries to be optimised from the point of view of power consumption.
Maintaining proper combustion with derived air ratio. Black or white smoke emitted from the boiler chimney indicates inadequate combustion leading to excess fuel consumption. Check boiler parameters and settings
If too much air than what is required for complete combustion were allowed to enter, additional heat would be lost in heating the surplus air to the chimney temperature. If less air than what is required for complete combustion were allowed to enter, then this would result in incomplete combustion.
For proper combustion, optimum excess air (normally 20%) and carbon-di-oxide between 12.5% – 13.5% (or oxygen between 3% to 4%) level has to be maintained which would consume the least fuel and correspondingly maximising the efficiency of the boiler. 5% to 10% fuel savings are possible by controlling the excess air alone. 100% excess air reduces boiler efficiency by 5%.
| Parameter | Required | Actual | Impact |
|---|---|---|---|
| Line Size | 80 NB | 100 NB | Material costs 20% ↑ |
| Heat losses 15% ↑ | |||
| Insulation Costs 25% ↑ | |||
| Line Size | 100 NB | 80 NB | Velocities 60% ↑ |
| Pressure drop 300% ↑ | |||
| Steam Starvation – Process / Product get affected |
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