There is a remarkable similarity between camels, the motive power of the desert, and boilers, the motive power of the ocean, at least as far as the light cruisers are concerned. And the story told of the camel—how he faithfully bears his load, uncomplaining as long as he is given food and occasional water, only to crumple absolutely and completely when the last straw is laid on his over-burdened back, when physical examination shows nothing wrong with legs, heart, or stomach—finds its counterpart in the boilers which carry on cheerfully, enduring plenty of abuse and rough treatment as long as they are fed fuel oil and water, until some last straw causes a general breakdown. Then an investigation shows the metal of the tubes still in good condition, the drums and brick walls still durable and fit for service; only the boiler has sprung leaks galore, tube sheets, and the tubes themselves are warped and twisted until they must be renewed before the boiler can be cleaned and made ready for use again. And this straw is believed to be the result of many years of rapid acceleration and deceleration at fairly low or moderate speeds.
It has always been generally assumed that in going from 5 to 15 knots under three boilers or from 20 to 25 under six boilers that little or no care need be taken. The officer of the deck may ring up “full speed” as nonchalantly as he would step on the accelerator of an automobile making about 20 miles and get the same almost instantaneous response. Down below the water tender on watch may curse vehemently as he rushes on all burners, speeds his blowers to their maximum, and opens full wide the fuel-oil heater. The officer of the watch in the engine-room may groan as he sees his “score” for the watch ruined. He helplessly resents the imperative “stop smoking” signal from the bridge, but no one pauses to think or realize that all of a sudden, without any preparation, a full-power-run performance has been demanded of the boilers in use. And finally after years of this, the boiler just gets tired. Some little additional demand for steam during this critical speeding-up period upsets its equilibrium—the tubes warp or pull loose from the drums and “low water” or “split tube” is blamed for the damage found when the boiler can be examined. This damage is, however, the effect and not the cause of the failure.
No engineer officer in his right senses is going to try to jump from 20 or 25 knots even with all twelve boilers lit off, right up to 31 or 32 knots. Instead he takes hours to make that last step, “cooking” the boilers to insure thorough and slow heating of all the elements. Yet the increase of load on each boiler in doing this is no greater than the increase of load on each of three boilers in going from 15 to 20 knots or on each of six boilers in going from 20 to 25 knots, if those are all the boilers in use in the last two cases. A time allowance running into hours is not advocated for these “moderate speed” changes (in fact it is believed that too much time is usually taken in working up to full-power speeds) but it is contended that a reasonable time should be given each boiler in use to adjust itself to changed steaming conditions, regardless of the actual speed of the ship. In other words it is contended that a definite maximum rate for increasing or decreasing the rate of burning fuel oil in a boiler should be prescribed and that that rate should not be exceeded except in emergency.
In Fig. 1 the gallons of fuel oil per hour, per boiler, is plotted against speed in knots for four steaming conditions, viz., with three, six, nine, and twelve boilers in use. Data for these curves were taken from the records of the U.S.S. Raleigh and represent actual, clean bottom, best performances at all speeds. Other types of cruisers may differ as to exact figures, but the form of the curves will be the same.
From a consideration of the curves, it can be readily seen that at 20 knots for three boilers, 25 knots for six boilers, 28 knots for nine boilers and 31 knots for twelve boilers, approximately the same rate of burning oil is encountered per boiler, i.e., around 800 gallons per hour. (Under adverse circumstances, such as foul bottom, these curves shift bodily upward and at those speeds the maximum that can be burned per boiler will be reached, slightly over 1,000 gallons per hour, limited by the blower speed allowed.) These rates per boiler being the same, then it is apparent that the same precautions from the standpoint of boiler welfare should be observed whether the case under consideration is in carrying three boilers from 15 to 20 knots speed, six boilers from 21 to 25, nine boilers from 24 to 28, or twelve boilers from 28 to 31. And these precautions are that the increase in boiler load should be made gradually and steadily, giving the boiler time to adjust itself, and not in a sudden burst when each boiler is forced to the limit until the desired speed is reached.
There is another very vital factor entering into this -- the old “force equals mass times acceleration” of our physics days. It actually takes more power, a lot more, while a ship is increasing speed to any desired point than it does to maintain that speed once it is attained. Especially is this true if the propellers are forced to take up the r.p.m. for the new speed long before the ship can accelerate up to that speed over the ground. The reason for this is obvious. While the increase in speed is going on the slip of the propellers is much higher than normally and the shaft horsepower is increased accordingly. A common instance of this presents itself when one ship tows another. The r.p.m. for speed made good are far greater than under ordinary conditions while the fuel per mile is much above that commonly used. All this is because the slip is so much greater than under normal conditions and hence the shaft horsepower is much greater, too. It is harder to move the ocean than the ship; and that is what you are trying to do when the slip increases. Dock trial when the ship is immovable and the slip is 100 per cent is the ultimate analogy of this condition for, unless cavitation occurs, it is found impossible to make more than a small fraction of the r.p.m. that the boiler power in use would give in the open sea.
For this reason, the greatest demand for steam, the highest rate of burning oil, and the heaviest strain on the boilers all come just when the boilers are trying to accommodate themselves to new and more arduous conditions And the next few paragraphs will endeavor to picture what those arduous conditions are like.
Suppose a light cruiser, Raleigh type, to be steaming at 5 knots (one-third speed) on three boilers. The officer of the deck rings up “standard speed” which means 15 knots. What happens? According to standard practice at present, in 1.25 minutes the propellers must be making turns for 15 knots. The fireroom force are obliged to cut in all burners under all three boilers and speed the blowers up to their maximum to hold the steam pressure. Prior to this signal the boilers had been coasting along burning oil at the rate of 133 gallons per hour per boiler, developing around 1,000 boiler horsepower per boiler. In default of better figures, let us assume a furnace temperature of around 1,000° F.
Now, with all burners cut in, the furnace temperature must rise to very near the maximum, probably around 2,800° F., and the fireside rows of tubes feel this tremendous increase in temperature, but this increase does not immediately penetrate throughout the boiler. An appreciable time is required for it to be felt in the outermost rows of tubes and even then it is nowhere near as intense.
Consider Fig. 2 in which possible temperature gradients through the walls of a fireside tube are shown for the “one-third speed” and the “standard speed” conditions. The difference between the furnace temperatures and the water (steam) temperature must give a temperature gradient in each case, which passes through three mediums: The film of gas on the fireside of the tube, the metal of the tube, and the film of water (steam) on the waterside of the tube. The study of film resistances leaves much to imagination, but a reasonable assumption is that the drop through the gas film is twice that through the hot water or steam film. With this assumption the absolute values may very well be shown as in Fig. 2. In assuming these arbitrary values, effort is made to stay on the conservative side.
In “case one” at 5 knots, one burner in operation, the mean tube temperature will be about 640° F. In “case two,” 15 knots or accelerating up to it, six burners in operation, the mean tube temperature must be at least 1,370° F. The mean tube temperature has then increased 730° F. in the short space of time that it takes to cut in five additional burners. But, according to the formula in Mark’s Handbook, page 300, for expansion of steel due to heat, the increase in the length of the tube must be, for a mean temperature change from 640° F. to 1,370° F., at least 0.94 inch. And, as the fireside is departed from, the difference in mean temperatures will become less and the increase in length will become less until at the outside rows it is insignificant, probably less than 0.1 inch.
Then what happens if the fireside tubes increase in length almost an inch and the outside rows increase in length less than 0.1 inch? The mud drums are fixed in the boiler saddles, the steam drum is held by all the tubes, so either the steam drum must be caved in where the fireside tubes enter it, the outside tubes must tend to draw out of the tube sheets, or the fireside tubes must tend to walk into the tube sheets or else warp and buckle to permit this inch of expansion to take place. And any one of these occurrences is bad for express-type boilers.
Given sufficient time the increase in length is not as radically different between inside and outside rows, and the boiler can adjust itself to the new conditions gradually; but when the rate of burning oil is as suddenly and markedly increased as in this case something in the boiler is strained and eventually the weakest member gives way.
There is still another condition which may operate, given any unfavorable set of circumstances. The Bureau of Engineering Manual considers it in chap. 2, par. 2-302 (2):
Extreme care should be exercised never to force a boiler to such extent that the rate of evaporation is too high, as this procedure is apt to break up the circulation of water in the boiler and cause sagging of the lower rows of tubes.
When five additional burners are suddenly thrown on a boiler which has been steaming on only one (six burners are all that Raleigh’s boilers are fitted with) that boiler is being forced unduly. This sudden burst of heat, felt chiefly by the fireside tubes, causes such a rapid rate of evaporation that the water circulation does not follow up the steam formation. The result is a pocket of superheated steam and a section or sections of these fireside tubes which are heated far above the remainder of the tubes. This follows from the fact that the specific heat of steam is only 0.6 as against 1.0 of water, plus the additional heat absorbed as heat of vaporization. Naturally the temperature of the tube containing the superheated steam will rise far above that of tubes containing water and exposed to the same or lesser temperatures. Then a local but extreme expansion of the tubes thus overheated occurs, and this increase in length must be absorbed by the tubes either pushing further (walking) into the drums, or by the tubes warping and deflecting. And when a straight tube becomes deflected more than 1.5 inches it should be renewed. (Bu. Eng. Manual, 2-475.)
It is true that the boilers in light cruisers have been undergoing this rough treatment fairly successfully for some seven years now, but these boilers are no longer young. Some have already failed in service or given indications of failure. In the Italian Navy this fact has been recognized (See Biriletin of Engineering Information No. 45 of Oct. 1, 1929.) and destroyers are forbidden to increase speed at a rate exceeding one mile per minute between 12 and 20 knots and one mile per minute and a half between 20 and 25 knots, with a further prohibition that variations of speed for maintaining formation must not exceed over twenty turns at one time.
It is suggested that a standard rate of acceleration be adopted, based on the number of boilers in use. This can be done by adopting a standard maximum rate of increasing the fuel oil supply to a boiler. It is suggested for consideration that the maximum rate of increasing the fuel oil supply be not over 100 gallons per hour in a minute, which is, on the Raleigh, approximately the increase in supply given by cutting in one more standard burner with a 7/64 tip and 250 pounds oil pressure. From such a basis a time-table can be worked out giving the number of minutes which should be allowed in changing from any given speed to any other speed, in accordance with the number of boilers in use.
The same curve before considered, Fig. 1, can be used for this purpose by considering the right-hand vertical scale marked “time.” To go from 5 to 15 knots under three boilers, take the intercepts of the 3-boiler curve at 5 and at 15 knots on the “time” scale, which are 1.5 and 4. Then the absolute time interval would be 4— 1.5 = 2.5 minutes. There must however be an arbitrary allowance for the extra load due to acceleration, which might be placed at one minute in default of better data. Then the total time before the engines would be making revolutions for 15 knots would be 3.5 minutes, during which time the boilers would have one burner per minute cut in under them. After the ship reached the new set speed of 15 knots, the number of burners could be reduced as necessary. In the same manner to go from 10 to 20 knots would take (7.25 — 2) + 1 for acceleration = 6.25 minutes if under three boilers, but if under six boilers would only require (3.75 — 1) + 1 = 3.75 minutes.
This would require careful instruction of the engineers on the throttle watch, to take all the extra steam made yet not to drop the steam pressure below a certain amount. But those details are easily worked out aboard ship and it is believed that the resulting gain in actual efficiency and longer life of boilers would be surprising if careful comparisons could be made.
Certainly if these steps are taken to prevent any undue forcing of boilers, the faithful “camels of the ocean” should have no cause for complaint of “overloading,” but should serve on until general decrepitude puts an end to their labors, and engineer officers will cease to tremble in their boots every time “full speed” is rung up on the annunciators.
It is realized that graphs of temperature drops can only be approximations. The purpose is to bring out in this graph that it does not matter how much actual temperature drop takes place through the tube wall; it may be 5 or it may be 500 degrees.
The important thing is the mean tube wall temperature. If a proportion can be correctly assumed between the fall of temperature in the gas film and that in the hot water (steam) film of about two or three to one, then the mean tube temperature will be the same and the difference between the mean tube temperature at easy steaming under one burner and hard steaming under all six would be just about the same, i.e., around 700° F. This is the only point open to question, and one on which there may be a divergence of opinion; but a careful consideration of heat transfer has led me to assume this proportion of two or three to one. Of course if scale exists on the waterside or soot on the fireside, the problem becomes much more complicated. However, the fact that scale on the waterside tends to make tubes heat up and burn through leads me to believe that there must be considerable water film resistance there already if a thin scale can upset the flow of heat to that extent.