There are several ways of determining the amount of water pumped into the boiler. The best method is to weigh it in tanks or barrels set upon standard scales. There should be two or more barrels of sufficient size, so that the filling and emptying may not be hurried. They should be. set high enough to discharge readily into the tank or hot well from which the feed-water is drawn. The valves should be large and should open quickly, so that the emptying may not be delayed. If barrels are used, they should be numbered, and the .weight of each accurately noted, so that there may be no mistake in deducting the weight of a barrel from the total weight of barrel and water. When one barrel is. being emptied, the other may be filled. The weigher must use care and intelligence; otherwise he may become confused in his records, as in a boiler of considerable size the barrels fill and empty rapidly. At the beginning of the test, the level of the water in the hot well should be recorded, and at the end of the test should be brought to the same mark. If inconvenient to weigh the water, it may be measured bya meter; but if a meter is used, itshould be tested and its error determined under like conditions of temperature and pressure. The feed-water should be free from air, as otherwise too large a meter reading will be recorded.
The level in the water-glass of the boiler should be carefully noted at the beginning and end of the test. If possible, the level should be constant throughout the test; and if there is any difference between the beginning and the end, due allowance should be made for it.
The temperature of the feed-water can be taken best by means of a thermometer in a cup filled with oil screwed into the feed-pipe near the check-valve. If the temperature is nearly constant, readings at 15minute intervals will suffice; otherwise readings should be taken every five minutes.
The steam pressure shown by the gauge should be as nearly constant as possible throughout the test, and should be practically the same both at the beginning and at the end. Gauge readings should be recorded every 15 minutes, and the fireman should see that the pressure is constant. The gauge should be tested, and corrected if necessary.
Barometric readings should also be taken, two or three being sufficient for a ten-hour run. These readings, in inches, may be made to indicate pounds pressure by multiplying by .491, this being the weight of one cubic inch of mercury. If the trial is on a vertical boiler which furnishes superheated steam because of the heat being in contact with the tubes above the water-level, both the pressure-gauge and the thermometer should be used, so that the amount of superheating can readily be found by subtracting the temperature due to pressure (obtained from the steam tables) from the temperature readings.
The quality of steam can readily be determined by a calorimeter.
If there is sufficient steam space within the boiler, from 1 to 2 per cent priming will generally result. If the steam space is inadequate, there will be more priming. If more than 2 per cent priming is present, the steam will blow white from the gauge-cocks when opened; if less than 2 per cent, it will appear blue.
The above observations are of the more important class, and must be taken. In addition to these, it is well to take samples of the flue gas at intervals and from various places in the furnace or chimney, the object being to determine whether there is a sufficient supply of air admitted, or whether there is too much. The draft of the chimney may be measured by means of a U-tube partially filled with water, or by a draft-gauge.
It is well to bear in mind that in making the boiler test the utmost care must be used, both in taking observations and in recording them, and in working up the results of the trial. A committee of the American Society of Mechanical Engineers has recommended a code of rules for boiler trials, and the following standard form for recording results. These are too voluminous for complete reproduction, and they can be found in full in Vol. XXI of the Proceedings of the above Society for the year 1900. The following code of rules is practically an abstract of the above-mentioned code: Preliminaries to a Test 1. In preparing for and conducting trials of steam boilers, the specific object of the proposed trial should be clearly defined and steadily kept in view.
2. Measure and record the dimensions, position, etc., of grate and heating surfaces, flues, and chimneys; proportion of air-space in the grate-surface; kind of draught, natural or forced.
3. Put the boiler in good condition. Have heating surface clean inside and out; grate-bars and sides of furnace free from clinkers; (lust and ashes removed from back connections; leaks in masonry stopped; and all obstructions to draught removed. See that the damper will open to full extent, and that it may be closed when desired.
Test for leaks in masonry by firing a little smoky fuel and immediately closing damper. The smoke will escape through the leaks if there be such.
4. Have an understanding with the parties in whose interest the test is to be made, as to the character of the coal to be used. The coal must be dry; or, if wet, a sample must be dried carefully, and a determination of the amount of moisture in the coal must be made, the calculation of the results of the test being corrected accordingly. Wherever possible, the test should be made with standard coal of a known quality. For that portion of the country east of the Alleghany mountains, good anthracite egg coal or Cumberland semi-bituminous coal may be taken as the standard for making tests. West of the Alleghany mountains and east of the Missouri river, Pittsburg lump coal may be used.