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April, 1909 'TAE Marine REVIEW safety Valves for Marine Boilers. The recent exper ments carried out at Barberton, O., for the ~urpose ot determining the actual lifts and dis- charge capacities of standard spring- loaded safety valves, and forming the subject of a paper presented at the February meeting of the Ameri- can society of Mechanical <Buei- neers, are likely to result in rad cal modifications of the existing rules. The rule of 1904, still in force, deter- mines the diametral area of safety valves by dividing the weight in pounds of water evaporated per hour by the absolute pressure in pounds and multiplying the quotient by the constant .2074. The derivation of this constant is not material to this discussion, but it is based on Napier's well known form- ula for the flow of steam; the difficulty lies in the fact that the experiments demonstrated conclusively that: scarce- ly any of the standard makes of valve satisfy the conditions as to lift. The rule assumes, arbitrarily, a lift of one thirty-second of the nominal diameter, so that a 4-inch valve for instance is assumed to lift % in. Only one of the seven standard makes tested sat- isfied this condition, the others rang- ing down to about one-fourth of that, yet under the rule all have the same rating as to relieving capacity. The tests demonstrated further that the average lifts were fairly alike for all diameters and less than half that assumed by the rule, and the capacity tests showed that the rate of flow on which the rule is based is practically correct. For comparison let us as- sume a Scotch type boiler with two 44-in.. furnaces and 5 ft. 3 in. bars, equal «6 38 sq. ait:.. arate area, and working under mechanical draft. A very ordinary rate of combustion wn- der such conditions is 30 Ibs. per sq. ft. of grate per hour, and with an evaporation rate of say 10 lbs. the total evaporation is 11,400 Ibs. hour. For comparison with one of the valves under test we will assume that the working pressure is the same, 135 lbs., or 150 lbs. absolute. 38 X 39 X 10 X .2074 -- -- We have -then : 150 15.76 sq. in. area for valve, or say 4%-in. diameter. The lift of this valve the rule assumes to be 9-64 in. Cor- rected for comparison with a 4-in. valve under test which actually dis- . per. charged 11,020 lbs. per hour with just under 3-16 in. lift, at the same ab- solute pressure, and it is seen that if the assumed lifts actually obtained the rule would be extremely satisfactory. The rule defective in other ways. It leaves the dominant factor, the evapora'ion, wide open, and the inspectors are in no position to de- termine its value in the equation. The rate of combustion and consequent evaporation is entirely too flexible for use as a determining factor; it may easily be largely increased beyond the rate assumed in the calculation and entirely outside of the knowledge or even the control of the inspectors. The substitute rule proposed as a re- however is sult of these experiments, while still - retaining the evaporation as the dom- inant factor, takes account of the lifts as actually found, and, for the case illustrated, would give a valve 8 in, diaméter, or,since circumferen- tial area is the end sought, two 4 in. valves. This is based on an actual average lift of 3-32 in. and the usual 45 degree seats. An interesting point 'brought out in the discussion has reference to the fact tthat in naval work the area of escape pipes from safety valves is usually not over half the nominal area of the valve or valves, whereas United States inspectcrs of steam vessels have always insisted that it be at least equal, although the rules are silent on 'the subject. It is also instructive to consider that whereas in 'the case assumed for illustration, if the working pressure had been 175 lbs. the valve would, under the rule, be 4 in. diameter, but if for any reason the steam pressure was cut down to say 125 Ibs. and the rate of evaporation maintained as before, the valve would have to be increased about 50 per cent. So that while se- vere penalties are provided for carry- ing an excess of pressure there is nothing to prevent a reduction, which under some circumstances might be equally dangerous. HYDROSTATICS OF SHIP BUILDING. Mr. W. Boyd recently brought before the members of the Liverpool Engineer- ing Society "a few of the problems and difficulties with which shipbuild- ers have to contend in the earlier 35 stages of their efforis to construct for the very exacting and critical ship- owners of the present day; a ship. which will meet with all their mani- - fold requirements and will prove . worthy of the best tradi'iois of the © building yard." Th: various prob- lems relating to the.cesi-n of vessels of the present day were considered by the author in outl ne as follows: Suppose a shipowner rcquires a ves- sel of 7,000 tons deadweight and to | carry the: Same on a draught of 23 {t. mean in salt water. Speed at sea to. "be 12. Knots per hour loaded other particulars may bz supplied as to the type of vessel required, wheth- et it is to 'be classed at Lloyd's Gr British Corporation; shelter deck type, or to have poop, bridge and forecastle decks; the number of steel decks and if sheathed with wood, as well as particulars of any passen- ger accommodation and special fit- tings, etc. The builder first of all makes an estimate of the dimen:ions,. weight of hull and machinery, dis- placement at the specified load draught and block co-efficient, and then proceeds to estimate the horse- power to drive the vessel at the speci- fied speed. The general practice is to base this latter approximation upon the actual performances of preceding ships, making use of recorded co-ef- ficients of performance. The block co-efficient of fineress should be such that it will give the most economic results for speed taken in conjunction with the dimensions and draught of water, it being obvious that a fast vessel requires a finer co-efficient than a slow vessel. For the above ship, suppose the required dimensions to be 400 ft. by 50 ft. and the block co- efficient 0.74 and the loaded displace- ment 9,700 tons, made un of 7,000 tons of cargo, coal and stores: the remainder being the weight of hull and machinery. . Other Problems to be Solved. The lines of the vessel are then designed to give this displacement at 23 ft. draught on the dimensions fixed upon. In deadweight carriers, the lines cannot vary very much with different bulders, as the mid- ship area is usually a maximum and extends for fully half the length of the vessel, so as to obtain the finest entrance and run possible; but in the case of fast passenger vessels great care must be taken to obtain the best under-water form of least re- sistance and at the same time satisfy conditions. of trim and stability. &