I. Outline of Article
This article is written with the purpose of explaining by means of definitions and specific examples the elements which, taken together, constitute a complete analysis of the engineering performance of destroyers ; the object being to give simple and practical explanations of these elements, in order that they may be used daily aboard ship toward the more efficient and economical operation of the engineering plant.
The following outline will be pursued: The units and terms employed will be defined as they apply to the subjectmatter, rather than the absolute scientific definition. Practical values of the foregoing defined characteristics will be given for each machinery unit by means of the simplest and most useful curves that can be employed. A concrete example of performance will be given and worked out. This will be explained, the explanation serving to show the method of handling the data, and the detection of faults in the operation by means of this analysis. Finally, suggestions for the practical application of this analysis aboard ship will be given. The installation aboard the U. S. S. Blakeley (150) is the basis of the analysis.
II. Units and Terms Employed
The units and terms as defined below are in the simplest and most useful form for this analysis. It is understood that most of these terms and units should be corrected for the various conditions other than the standard conditions to which all test results are reduced for comparison. For the actual use of this method aboard ship these corrections are omitted, but in case the information is desired it can be found in the standard engineering handbooks and especially in the Power Test Code of the A. S.*M. E.
I. UNITS EMPLOYED
Notation for Power Formula
P = Mean effective pressure from indicator in pounds per square inch.
P_{1} = Mean effective pressure of the air end of compressor.
L = Length of stroke in feet.
A = Area of piston less onehalf area of piston rod in square inches (net area).
N1 = Number of single strokes per minute.
N2 = R. P. M. or double strokes per minute.
W'— Net weight or pressure on brake arm in pounds.
L1 = Effective length of brake arm in feet.
H = Total head in pounds per square inch. ^{1}
H1 = Head as above, in feet.
Q — Quantity of water in pounds per hour.
Q1 = Quantity of water in gallons per minute.
Q = 720 Q1 (fresh water).
Q = 7.02 Q1 (salt water).
One horsepower = 33,000 foot pounds per minute.
One horsepower = 550 foot pounds per second.
Note. The above formula can be used for reciprocating compressors. The air horsepower of the fireroom blowers is found from formula that contain elements which require special measuring apparatus (not supplied to ships).
The pound of steam is taken as the unit of energy input The real energy is the number of B. T. U.’s of heat contained in the pound of steam at the existing pressure; this value can be found in any steam tables or handbook; but as the steam is the actual medium that is measured and which the boiler gives off even though it is just the “carriage” for the actual energy, it is the one used to represent the input.
The “water rate” is the number of pounds of steam that a machinery unit uses in a unit of time, usually an hour. As an
example, if a pump has a water rate of 50 pounds per horsepower hour at a certain pressure and rate of output, it means that 5° pounds of steam at a pressure of, say 200 pounds, is being supplied to the pump each hour for each I. H. P. The heat which actually does the work is carried by the steam and from the steam tables amounts to 1198 B. T. U.’s per pound. Therefore Sox 1198 = 59,900 B. T. U.’s are supplied to do the work of each I. H. P. Although much of this heat is wasted owing to the unavoidable inefficiency of the machine, nevertheless, it is the energy supplied or the input.
The criterion of the final performance should always be the “gallons per knot” ; that is, the number of gallons of oil that are required to drive the ship a distance of 1 knot at the various speeds as defined above. This states, in other words, that so many gallons of oil burned under the boiler supplied just enough steam for the main engines and all necessary auxiliary machinery (including generators for lighting, etc.) to drive the ship a distance of 1 knot at a speed of so many knots per hour. Naturally, the faster the ship goes, the more oil will be required to go the same distance. This is shown on Curve 2. For other than standard trial displacements the relation between the horsepowers required are shown on Curve 3.
2. terms and their interrelations
The Willan’s line is a curve plotted with the arguments, “ input,” or total steam used per hour, and “ output,” or the work done by the machine, expressed in appropriate units. The output of a generator is in kilowatts; of a pump, in gallons per minute; and of an engine, in horsepower, etc. The load or output (abscissa) should be given in terms of revolutions or strokes of the machine as these are easily determined. This can be done since all of the machines work under practically constant conditions. The feedpump works against the boiler pressure of 260 pounds. A fire and bilge pump when on the flushing system runs steadily at a pressure of 10 or 15 pounds, and when on the fire main at 70 pounds, and so on. The main turbines at a specified R. P. M. are pushing against a steady load. In other words, the strokes or R. P. M. of a machinery unit can be taken as the direct measure of the power developed. (See capacity curve.)
The economy curve is plotted with the arguments, pounds of steam per unit of output per hour, against the load. This is the “waterrate” curve. It gives the steam used per unit of output at the different loads.
The efficiency curve gives the ratio of useful work delivered by the machine to the whole work supplied or to the energy received by the machine. For each type of machine, there are a number of different efficiencies, each one having a special use, but for this analysis the “overall efficiency” is the one used.
The capacity curve gives the rate of output in terms of the rate of working of the machine. The rating of the machine is usually the capacity of the machine at its most economical load The capacity varies greatly with wear and faulty adjustment, but as all naval machinery should be kept in the best condition, it should not vary much.
The rate of evaporation or boiler output is the number of pounds of water evaporated from and at 212^{0} F. into steam for each pound or gallon of oil burned. In testing, the oil is stated in pounds, but in the practical work the gallon is the more useful unit, since this is the unit used in the engineering log and in the ' everyday work with oil. Now, different oils have different thermal values, and the oil having the greatest number of B. T. U.’s per gallon will naturally give the greatest evaporation per gallon but the variation is small.
B. T. U.’s per gallon=144,670 average value.
B. T. U.’s per pounds 18,500 average value.
The higher the rate of evaporation, or, in other words, the higher the capacity (output), the lower is the efficiency of a boiler, although for a certain range (from 100 to 200 per cent) above the rated capacity, the efficiency remains practically constant.
The oil consumption or boiler input is stated in terms of the number of gallons of oil burned per knot for a specified speed. When the ship is not under way the oil burned is that required to give the steam for the necessities of that condition. In each case, whether under way or at anchor, the oil burned can be stated in terms of pounds of water evaporated and this apportioned to the various machines which take steam.
III. Actual Values of Interrelations
Actual values of the foregoing interrelations are given by means °f the following curves for each machinery unit: (a) Willan’s lines; (b) economy curve; (c) capacity curve; (d) special curves for boilers. These values are mainly from the data supplied by the builders, from the results of tests at the Naval Experimental Station where possible, and from the standard textbooks and handbooks. In all cases the curves are plotted with the load or output as the abscissa and the opposite argument as the ordinate. In actual operation these values are rarely obtained, but the effort should be made to approach them.
The following considerations will give some idea of the assumed conditions that also govern these values. The best possible vacuum is maintained for the main turbines; curves of correction for other vacuums are given. All the auxiliary machinery operates against a back pressure, usually of 10 pounds gauge, which increases the steam consumption of each unit about 8 to 15 per cent. The given curves are for the operating conditions:
Boiler Data.—The expenditure of oil for the 24 hours is obtained from the oil meters or by soundings; in the former, it should be corrected for the error of the meter. 1 he hourly expenditure is obtained from the total oil burned. Using the hourly rate, find from curve C of the boiler curve sheet (Curve 4) the water evaporated per hour from and at 212^{0} per gallon of oil burned. This quantity times the total number of gallons of oil burned will give the total equivalent evaporation for the 24 hours. Find the factor of evaporation from Curve 5, entering with the feedwater temperature and applying the tabulated correction for of evaporation = (H – h)/970.4 where H is the total heat in a pound of dry steam at boiler pressure and h is the heat in a pound of feed water above 32° is for dry and saturated steam.) The total equivalent evaporation divided by the factor of evaporation will give the pounds of water actually evaporated under the existing conditions. Example:
Oil for 24 hours...................................................................... 7,800.. gallons
Oil for 1 hour............................................................................. 325.. gallons
Equivalent evaporation per gallon from curve 120 pounds
Total equivalent evaporation.............................................. 936,000.. pounds
Steam pressure........................................................................ 250.. pounds
Feed temperature.................................................................... 230
Factor of evaporation............................................................ 1.034
Actual evaporation per hour.................................................. 905.200 pounds
Main Turbines (Parsons).—The waterrate curve of the main turbines is given, as its use is more accurate than the use of the Willan's line plotted to the scale of these curves. The power is the total shaft power and the water rate is for the two shafts ( four turbines). To secure a certain speed of the vessel through the water a definite number of R. P. M.'s are necessary for a given displacement. To give these revolutions the turbines must develop a definite shaft horsepower. With different displacements different speeds will result from the same R. P. M. or shaft horsepower.
To get the steam used by the turbines enter Curve 1 with the R. P. M. and find the S. H. P.; entering Curve 6 find the water rate for this developed S. H. P. The total S. H. P. multiplied by the water rate will give the total pounds of steam used per hour by the turbines. There are losses of steam through the glands, but with the "feather of steam" called for, instead of the "Channel Fog Bank" sometimes seen, this loss will be relatively small. The increase of the steam rate with vacuums below 29 inches can be found from the insert of Curve 6.
Condensing Plant.—The operation of the main condensers requires the services of one or two main air pumps and two circulating pumps if the ship is not making enough way through the water for the scoops to operate. The air pumps run continuously while under way and daily for a short period, perhaps an hour, while at anchor. Under the first condition they develop 8 to 9 H. P., while in the second case the horsepower amounts to 1 or 2. 1 he circulating pumps run while warming up and are necessary up to a speed of about 15 knots, when the scoops take charge. Even then it is sometimes advisable to keep the circulators turning over. While turning over at about 400 R. P. M. the turbine develops about 5 to 6 B. H. P. and when actually working, at around 150° R. P M., about 15 to 16 B. H. P. are developed when the water has free flow through the condensers. In both cases the actual strokes or revolutions should be used in entering the curves to get the steam consumption.
Since the circulator is a centrifugal pump, the power developed cannot be taken directly from the R. P. M. The pumps can be driven at a certain number of R. P. M. by a low “ ring pressure ” (or the pressure of the steam to the pump turbines) when lightly loaded, but the same number of R. I’. M. can be obtained with a higher ring pressure when the pump is carrying a heavier load; in other words, the greater the load (the greater the volume of water being pumped), the higher the steam pressure that will be required to maintain the same R. P. M. This shows that there are three arguments, ring pressure, R. P. M. and horsepower developed. Using Curve 7 to get the horsepower, enter the curves with the R. P. M. found with a tachometer and go to the curve of ring pressure. This will give the horsepower being developed. Entering Curve 8, and going to the Willan’s line, the total steam flow per hour is found.
Pumps—As the curves for all reciprocating pumps are similar, the explanation of one set will suffice for all. The legend of each set of curves states the use and dimensions of the pump. As stated before, the steam quantities are for a 10pound gauge back pressure, this being the usual operating condition. The horsepower is given for the discharge pressure at which the pump works. The capacity of the pump can be found from the dimensions as follows: The piston displacement in cubic inches divided by 231 will give the theoretical discharge in gallons per single stroke; this must be corrected for leakage by the piston and through the valves, etc. The actual discharge equals the theoretical discharge found above multiplied by 100 minus the percentage of slip. This slip varies with the wear and condition of the pump, the temperature of the fluid being handled, the piston speed, fluid density, etc. Average figures for leakage are 5 to 8 per cent for hot water and 12 to 14 per cent for cold water.
Evaporating Plant.—Only the maximum rate of operation will be considered, since, when fresh water is needed, it is made as fast as possible and the plant then shut down. The maximum rating is taken as the 100 per cent rated capacity plus the 40 per cent overload capacity for clean coils. With the orifice valve installed in the steam line to the evaporator, the apparatus runs at a constant capacity, so the output is regular and is stated as so many gallons of water per hour. The input will consist of the steam for the following pumps: Feed pump (Curve 19), distiller circulating pump (Curve 13), freshwater pump (Curve 18), and if installed, the distiller vacuum pump (Curve 18); also, if running on live steam, the steam required for the actual evaporation. The amount of this steam used can be found from the formula, W = pa/70 where W = the weight in pounds of steam flowing per second, p = the pressure per square inch absolute on the inlet side of the orifice, and a = the area of the orifice in square inches. If running on exhaust steam, the steam used in the evaporators will already have been charged to the machinery running and so, in the overall summary of the plant analysis, the input of the evaporating plant will be the pump steam only. Aboard any ship these items could be combined into two sets of curves, one set for evaporating with live steam and the other set for evaporating with exhaust steam, each set containing the total steam input of the plant and this plotted against the gallons of water made per hour.
The Electrical Plant.—The curves for the generator are in terms of the rate of output in kilowatts, which is found directly from the switchboard instruments. The electrical power is used to run numerous auxiliary machinery, such as the ice machine, ventilating motors, for lighting, etc. The approximate number of amperes that each of these circuits takes is listed below, but the only item needed in this analysis is the total output in kilowatts per hour.
Entering Curve 20 with the kilowatts per hour, the number of pounds of steam per kilowatt hour is found as the ordinate. This figure multiplied by the load in kilowatts will give the steam used per hour.
ForcedDraft Blowers.—Since fans are of the centrifugal type, there can be different numbers of R. P. M. for a constantinlet steam pressure according to the load on the fan; that is, the inlet pressure must increase in a certain relation as the load on the fan is increased if the revolutions are to be kept constant. In practical operation the arguments at hand are the R. P. M., steaminlet pressure, and static pressure of the air,—the first two being sufficient to work with. Entering Curve 21 with the R. P. M. and the steaminlet pressure, the S. H. P. being developed by the turbine is found, and from Curve 22 the steam consumption per S. H. P. per hour is found. The product of these two quantities gives the total steam used per hour by one blower set.
Steering Engine.—Owing to the intermittent working of the steering engine and the various powers developed when working, due to the various speeds of rotation, the best way to handle the steam consumption of this unit is to allow a number of pounds of steam per hour of operation. This number will vary over very wide limits, but a fair estimate would be about 200 pounds per hour during steady steaming. This figure is only approximate at the best; a practical and much better figure could be found aboard ship by running a simple test on the engine itself.
Anchor Engine.—As this unit is used for short periods at different times a set figure should be adopted until the approximate rate of steam consumption can be found on the ship. The power and steam consumption varies widely, but a suggested figure is 30 pounds of steam for a 15minute period of operation at the rated speed of hoisting the anchor and chain.
Torpedo Air Compressor.—These machines when used should run only at the rated capacity; under these conditions the water rate is 30 pounds per I. H. P. per hour, at a speed of 350 R. P. M. and 3000 pounds air pressure. The machine develops 42 I. H. P., the mechanical efficiency being 65 per cent. It follows that one compressor running for one hour uses 1260 pounds of steam.
Westinghouse Air Compressor.—This compressor is rated for air delivered at 100 pounds pressure, but in practice this pressure is seldom used. A compressor runs from 12 to 13 hours a day to supply air to the galley oil range at a pressure of 15 pounds. It is run periodically at an air pressure of about 50 pounds for blowing the boiler tubes and occasionally at a pressure of 75 to 80 pounds for pneumatic tools. Under the above conditions the following steam consumptions obtain:
Air pressure  Total steam per hour 
Pounds 

IS  400 
50  1200 
80  1600 
IOO  2500 
FuelOil Heaters.—As the fueloil heaters use live steam as the heating element, it must be taken into account in the final balance. The steam consumption will vary with the oilinlet temperature and with the amount of oil being used. Taking the final oil temperature at 150° F. and the specific heat of oil at 0.45, the pounds of steam required per hour to heat the oil used at the different speeds is given approximately by Curve 23.
Heating Systems.—The three heating systems have a total radiator surface as follows:
Circuit No. 1............................................. 126.02 square feet
Circuit No. 2............................................... 99.22 square feet
Circuit No. 3............................................... 118.44 square feet
Total.......................................................... 343.68 square feet
The steam used depends upon the initial pressure (the reducing valve is set for 25 pounds gauge, but this pressure is seldom used), the outside temperature, and the desired temperature of the compartment. As a rule, only a few of the radiators on a circuit are used, these heating the whole of the compartment. Due to the various elements that affect the amount of steam used, it is impossible to Say how much steam will be condensed in a circuit, but a working rule is that 0.5 pound of steam will be condensed per square foot of heating surface per hour at a steam pressure of 25 pounds gauge and a temperature of 75^{0} F. in the compartments. Therefore, for a total heating surface of 344 square feet, 172 pounds of steam would he used per hour under the above conditions.
Galley, Baths and Pantry.—The steam used in the galley, in heating wash water, in the pantry, and for heating the baths amounts to an appreciable figure in 24 hours. As an estimated figure to work with, 25 to 75 pounds of steam should be allowed per day.
IV. Examples of Analysis
Two examples of the application of the analysis are given below, the first for the vessel under way and the second for the vessel at anchor. In each example the first part consists of extracts of data that are available daily aboard ship, that is, without special data being taken. The second part consists of these data transferred to a form of analysis.
Example 1.—Under way.
Extracts of data: Speed =18 knots; R. P. M. = 2o6; displacements 1400 tons; (standard R. P. M.S187 for standard displacement of 1160 tons); feedwater temperature = 220° F.; oil for 24 hours = 15,000 gallons; water distilled = 10,500 gallons.
EXPLANATION AND DISCUSSION
The total steam used by the various machinery units is found as explained in Section III. In these examples the steam used by the evaporators had to be taken from the figures given by Mr. Stuart of the Naval Experimental Station. He stated in his article, “An Improved Method of Operating Evaporators” (Journal A. S. N. E. of February, 1919), that in doubleeffect evaporation 0.6 to 0.75 pounds of steam are required per pound of vapor, and for singleeffect evaporation 1.2 to 1.35 pounds of steam per pound of vapor. As a gallon of fresh water weighs 8.34 pounds, the figure used, that is, 6.27, is the weight of steam required to produce a gallon of condensed vapor working double effect.
The kilowatts per hour generated is taken as the mean for the 24 hours. The generator water rate for this mean rate of output is used.
The total actual evaporation is found as shown in the examples and the final factor E is the ratio of the steam used to the steam made.
The value of E for the first example may be too high, but it illustrates what should be approached.
It must be understood that the values of the water rates as given are not final. There are errors and false assumptions, no doubt, but the figures used are the best that could be obtained. In some cases they may lie too high and then again too low, but in any case they serve as a working basis and a guide to the securing of better data.
In each example given above, the steam used in the galley, pantry, for washing, etc., is left out, owing to the known inaccuracy of the figures given.
V. Faulty Operation
To secure definite knowledge of existing faults, it is necessary to compare the efficiency of the machinery unit found from standard tests to the efficiency found under the operating conditions. This requires the running of short tests aboard ship. Block tests run about 10 per cent higher than the results found in actual operation. If, after allowing for this correction, a discrepancy remains, it means that the unit is giving results that are below its best possible results by the amount of the difference between these two efficiencies.
VI. Suggestions
It is suggested that whenever possible such trials of the machinery units as can be handled on the ship should be carried out for the information of the operating personnel. As stated before in this article, the given values of the steam consumption for many of the machinery units are at the best only approximate. Much better values can be secured on board ship by some simple tests, using the formulae of Section II. For instance, the steam condensed in the heating system can be found by collecting and weighing the discharged water from the traps. This applies to all other circuits that discharge through a trap, such as the fueloil heaters. Tests of a pump could be run by installing a special exhaust line through the auxiliary condenser from the pump being tested and running the remainder of the exhaust steam to a main condenser for the duration of the test, then by weighing or measuring the water from the auxiliary airpump discharge the actual water rate can be found.
The regular application of this method of analysis to the results of the daily performance of the engineering plant will tend toward the better understanding of the plant by the personnel concerned. It will reveal the faults and points that require first attention when time is available for overhauling. Finally it will tend to result in increased economy; that is, the personnel will see how and where to get more out of the plant for the same amount of oil burned than formerly or, what amounts to the same thing, the desired performance will be secured for less oil burned.
REFERENCES USED
Mark’s Handbook.
Power Test Code, A. S. M. E.
“ The Economy Factor in Steam Power Plants,” G. W. Hawkins.
“Power Plant Testing,” Moyer.
Reports of Tests of Pumps by the United States Naval Experimental Station.
Data for Curves Nos. 3, 4 and 6 were secured from Lieutenant Fineman (C. C.), U. S. Navy; from reports of boiler tests held at the Philadelphia Fuel Oil Testing Plant; and from Admiral Dyson’s article on “The Passing of the Direct Connected Turbine for the Propulsion of Ships” (Journal A. S. N. E., August, 1919).
The actual pump data were furnished by the pump builders. The steaming data were checked with data secured from the U. S. S. McCalla.