The number of tons of steam coal that will be burnt per day with a pair of average surface condensing engines having cylinders 50 inches in diameter will be, under average conditions, 16 tons per day, the calculations being based upon the common assumption that the diameter of one cylinder squared and divided by 100 gives the consumption of fuel in tons per day for condensing engines not compounded; thus, 40 × 40 = 1600 ÷ 100 = 16 tons of coal burned per day.
Here again, the ratio of expansion not being specified, the calculation has no real practical value.
If at sea and short of coal, bear in mind that the consumption of fuel per mile run is greater for fast than for slow speeds; hence the following points should be attended to:
Reduce the speed of the ship to say half the usual. Regulate the fire so as to keep up full boiler pressure without blowing off. This will allow the expansion or cut off valve to be set to cut off early in the stroke, and thus save steam. If, under these conditions, the steam should sometimes blow off at the safety valve, cover up part of the fire grate area.
Use a thin, rather than a thick, fire, but be careful that it is not so thin as to let currents of cold air pass through.
To Relieve the Boiler in Case of Emergency.—Suppose an engine breaks down at a time when the fires are heavy and going full, that the steam gauge shows blowing off pressure, but that the safety valve is stuck, or from some cause or other is prevented from blowing off, and cannot be eased or lifted, and the following is the course to be pursued:
1st. Close the ash pit dampers and open the smoke box door and fire door. If there are no ash pit doors, close the damper in the up take and open the fire and smoke box doors.
2d. Start the donkey engine to feed cold water into the boilers.
3d. Start the steam winches, and any other small engines that take steam from the main boilers.
4th. Slacken the escape valves, and open the drain cocks of the cylinders and receivers, and steam will blow through the H.P. cylinder escape valve and drain cock at once. The H. P. slide valve may then be worked by hand, back and forth, to let steam pass into the receiver and blow through its escape valves and drain cock.