SOME IMPROVEMENTS IN THE DESIGN AND OPERATION OF COAL FIRED BOILERS
5.00 The use of accumulators
This section is written in a lighter vein and deals with the psychological handling of persons, in this case boiler attendants.
A clash of personalities developed between our shift boiler attendants.
Standing instructions were that the accumulator had to be maintained at a pressure between a low of 90 p.s.i. and a high of 150 p.s.i. An explanation had to be given if the chart went beyond these limita- tions.
One boiler attendant resented being told by another boiler attendant that he could not handle the boilers correctly and set out to prove that he could control the accumulator pressure.
As will be seen from the accompanying accumu- lator chart the one shift maintained the accumu- lator pressure to a consistent 125 p.s.i. (Chart 7.)
Knowing and understanding the temperaments of the men under the control of the Engineer is abso- lutely essential if the best is to be obtained from those men.
When this accumulator chart was examined a query was naturally raised as to why this pressure was so consistent on one shift, whereas the other shifts showed a normal chart range. When the facts of the case were brought to light, no immediate action could be taken from a psychological aspect as this was something which had to be worked out of this boiler attendant's system.
A personal interest was taken in the respective boiler attendant and his controlling of the boilers was examined several times per shift each day in the boiler house and a sympathetic ear was turned to his troubles. He was given every encouragement.
His duties became extremely onerous for he was continually altering the boiler outputs to suit the accumulator, sometimes at two minute intervals. At the end of his shift he was physically worn out due to his excessive running around controlling the boilers.
After three days he was congratulated on his achievement and told that since he now had proved his point he could settle back and control the boilers to the limitations of the accumulator chart lines.
CHART 7
This he was only too willing to do and he fell back into normal control.
I shudder when the boiler charts of this period are examined for they show excessive variations in output on one shift, in other words they were doing the work of the accumulators and also the thermal efficiency of the plant fell during this period, but this was a small price to pay in order to maintain a happy, harmonious relationship between these boiler attendants, all of whom are good men.
Acknowledgements
Thanks are due to Natal Associated Collieries
(Pty) Limited and particularly to their Fuel Techno- logist, Mr. E. A. Cole, for their close co-operation and assistance in the tests on the additive described in Section 3.00 and for the photographs of clinker.
Thanks are also due to Messrs. Huletts S.A.
Refineries Limited for their permission to use the charts from boilers and accumulators and photo- graphs of the economiser tubes.
References
James, W. G., A.M.Inst.F. and Fisher, A. H., A.M.Inst.F.
A new fuel additive for coal-fired boilers. Journal of the Institute of Fuel. April, 1967, p. 170-172.
174 Proceedings of The South African Sugar Technologists' Association — June 1969
A. Deposit taken f r o m generating tube showing heavy build-up causing chokage through birdnesting. Chord dimension 4 " .
C. Clinker sample clearly showing the folds which f o r m e d at the finger bars while still in its plastic state. Sample 6"
long.
D. Sample taken from 2nd tube on side water wall adjacent to ignition arch showing friable surface next to t h e tube and dense mass at rear of mass (fireside). Sample about 8" long.
E. Reverse view of photograph D showing dense mass of clinker attached to the tube as seen f r o m the fireside.
F. Deposit taken f r o m side water wall tube showing dense but perforated deposit which has broken free f r o m tube after being treated. Sample about 3 f t . long.
B. Vitrified deposit taken f r o m boiler arch b r i c k w o r k . 3"
maximum horizontal dimension (thickness).
G. Sample f r o m boiler being treated. The soft friable nature of the deposit can be clearly seen. Sample 71/2" long.
H Economiser tubes choked w i t h deposit A l l tubes had to be removed f r o m economiser and each one physically cleaned to bare metal. Boilers sootblown daily, econo- miser waterwashed every second day. Before flyash arresters f i t t e d .
Economiser tube after 12 months' steaming w i t h f l y a s h arresters f i t t e d . Normal sootblowing. N o w a t e r w a s h i n g . N o t e the effect of water on the tubes w h e r e s l i g h t build-up has occurred.
Proceedings oj The Sou
Discussion
Mr. Ashe: What was the effect of the chemical additive on the clinker?
Mr. Shum: We used the additive because we had tube fouling and clinker trouble.
The effect of the additive on the clinker was to break it down into smaller portions, and on the tubes it softened the build-up and allowed it to be crumpled by hand.
Mr. IHulctt (in the chair): It is possible that it was a change in the coal supply, not the additive, that caused the improvement.
Mr. Shum: I do not think so because when he stopped using Blitox we got a build-up on tubes and brickwork, and clinker trouble.
A small part of the fly ash is re-used in the pro- portions of 3 fly ash to 1 Blitox to give more bulk and make it more easily distributable when feeding onto the conveyor of coal supplying the boilers.
Mr. Fokkens: We use a very small amount of additive in our boilers with great success.
We sometimes have to fire the boilers with coal but we do not clean them during the year. There is no clinker and no birdnesting.
Mr. Magasiner: A very low pressure drop is re- ported across the superheater. Any superheater of this type is susceptible to overheating. By increas- ing the pressure drop by 10-15 lbs. the possibility of starving and hence overheating one of the loops is reduced. While the presence of scale cannot be condoned the more even steam distribution resulting from a higher pressure drop minimises its effect.
To improve steam purity, cyclones can be designed to fit into the drum.
I agree that unless coal is presented correctly to the boiler it will not be able to handle it.
I have had very little experience of chemical additives but that which I have had has been con- trary to what is described in this paper. I am pleased to note that a measure of success has been achieved.
On an economiser three different types of gas side fouling can be expected. A phosphatic deposit at the high temperature end, a sulphatic deposit at the low temperature end and general fouling due to dust. Since the main fouling problem reported was due to dust by removing this the economiser was kept clean. Water washing appears to be the only way to remove phosphatic deposits. Sulphatic deposits are highly corrosive and unless high metal temperatures are maintained they will cause a lot
th African Sugar Technologists' Association — June 1969
of damage. Cast iron economises are better suited to withstand sulphatic deposit attack. If metal temperatures, however, can be kept above the dew point, mild steel economises can be safely used without danger of corrosion.
As regards birdnesting, if the temperature of the gases leaving the combustion chamber can be re- duced to about 500°F below the ash fusion tem- perature, this will largely be eliminated. To achieve this a fully water cooled combustion chamber of fairly large proportions must be incorporated in the design.
Mr. Shum: The steam drier referred to in the paper is not to improve the quality of the steam, but to cope with priming and/or foaming caused by contamination of the feed water by unwanted impurities such as sugar.
Plant layout and initial design determines the pressure drop across the superheater headers. Our factory has 220 psi boilers and the turbine was designed to run at 200 psi. We try to keep the pressure drop across the superheater headers as high as possible, but frictional and other normal resist- ances in the steam line limit the amount of pressure drop across the superheater headers since the maxi- mum allowable pressure drop between the drums and the turbine is 20 psi by design, and consequently the pressure drop through the superheater tubes is only 1 or 2 psi.
As regards clinker build up, the boilers in use at the factory where I am now are having a clinker build-up despite the application of an additive which is not Blitox. However, I am giving it a longer trial before changing the additive to Blitox.
Water washing an economiser causes corrosion of the tubes and fins and this irregular surface permits dust and fly ash to adhere and cause a build-up and consequent choking of the tubes as can be seen
—photograph 1. Our experience has been that, without water washing, the tubes have remained clean and the gas flow remains unimpeded.
It should be borne in mind that this paper deals only with the operation of existing plant and not with the initial design of the boilers.
Mr. Magasiner: I did not make myself clear about the superheater pressure drop.
The boiler drum should be designed for about 10 to 15 pounds higher pressure than one anticipates and this should be dropped across the superheater by using high velocity of steam through the super- heater tubes.