SCALE DEPOSITED IN MARINE BOILERS.

The analysis given below may be looked upon as typical of the incrustation formed by fresh water, brackish water and sea water respectively in marine boilers:

Constituent.	River.	Brackish.	Sea.
Calcic carbonate	75.85	43.65	0.97
„sulphate	3.68	34.78	85.53
Magnesic hydrate	2.56	4.34	3.39
Sodic chloride	0.45	0.56	2.79
Silica	7.66	7.52	1.10
Oxides of iron and alumina	2.96	3.44	0.32
Organic matter	3.64	1.55	trace
Moisture	3.20	4.16	5.90
	100.00	100.00	100.00

From this it is evident we may look upon the incrustation from fresh water as consisting of impure calcic carbonate, whilst that from sea water is impure calcic sulphate, the brackish water from the mouths of rivers yielding, as might be expected, an incrustation in which both these compounds are present in nearly equal quantities.

The importance of these differences in the deposit formed is very great, as it enables the shipowner to arrive at the conclusion as to the treatment that the boilers have received during the voyage, by examination and analysis of the scale that those boilers contain. Taking, for instance, the case of a ship which uses fresh water both for filling and make up, it is manifest that on her return to port the scale should be very slight and should consist mainly of calcic carbonate, whilst if the scale exceeds ¹⁄₁₆ in., and shows a preponderance of calcic sulphate, it is manifest that such scale could only have been formed by sea water, either leaking in from faulty condensers or being deliberately fed into the boilers.

With the introduction of high pressure steam a new and dangerous form of deposit has added to the trouble of the marine engineer; having entered the boiler, the minute globules of oil, if in great quantity, coalesce to form an oily scum on the surface of the water, or if present in smaller quantities, remain as separate drops; but show no tendency to sink, as they are lighter than water.

Slowly, however, they come in contact with small particles of other solids separating from the water and sticking to them, they gradually coat the particles with a covering of oil, which in time enables the particles to cling together or to the surfaces which they come in contact with. These solid particles of calcic carbonate, calcic sulphate, etc., are heavier than the water, and, as the oil becomes more and more loaded with them, a point is reached at which they have the same specific gravity as the water, and then the particles rise and fall with the convection currents which are going on in the water, and stick to any surface with which they come in contact, in this way depositing themselves, not as in common boiler incrustation, where they are chiefly on the upper surfaces, but quite as much on the under sides of the tubes as on top.

The deposit so formed is a wonderful non-conductor of heat, and also from its oily surface tends to prevent intimate contact between itself and the water. On the crown of the furnaces this soon leads to overheating of the plates, and the deposit begins to decompose by heat, the lower layer in contact with the hot plates giving off various gases which blow the greasy layer, ordinarily only ¹⁄₆₄ inch in thickness, up to a spongy leathery mass often ¹⁄₃ inch thick, which, because of its porosity is an even better non-conductor of heat than before, and the plate becomes heated to redness.

When water attains a temperature, as it does under increasing pressure, ranging from 175° to about 420° Fahr., all carbonates, sulphates and chlorides are deposited in the following order:
First. Carbonate of lime at 176° and 248° Fahr.
Second. Sulphate of lime at 248° and 420°.
Third. Magnesia, or chlorides of magnesium, at 324° and 364°.

It is to take advantage of this fact that mechanically arranged jets, sprinklers and long perforated pipes are introduced into the interior of the boiler; these tend to scatter the depositing impurities and also to bring the feed water more quickly to the highest heat possible.

With regard to the oxide of iron or iron salts in solution, these can best be treated with small quantities of lime. By adding re-agents, they set up chemical changes, which result in precipitation, which give the water a milky appearance; they divide into particles, and ultimately settle, leaving the water pure and bright. The mechanical treatment on a limited scale would be easy, a settling tank sufficing; but this becomes a different matter when large quantities have to be dealt with.

ANALYSIS OF AVERAGE BOILER SCALE.


Silica	.042	parts.
Oxides of iron and aluminium	.044	„
Carbonate of lime	30.780	„
Carbonate of magnesia	51.733	„
Sulphate of soda	Trace	„
Chloride of sodium	Trace	„
Carbonate of soda	9.341	„
Organic matter	8.060	„
Total solids	100.	Parts

The percentage only of each ingredient the scale is composed of is given, as it cannot be told how much water was evaporated to leave this amount of solid matter.