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8 MARINE REVIEW. Safety-Valve Area for Water Tube Boilers. It has been known for some time that in the matter of safety-valve area for the Belleyille tubulous generators, with which the new passenger steamers of the Northern Steamship Company will be equipped, the builders have met with an obstacle in the rules of the United States steamboat inspection service, although it was not intended to direct public attention to the subject until it would ba brought up at a regular meeting of the board of supervising inspec- tors. But in a paper relative to the steamboat inspection service, read at the inaugural meeting of the new Society of Naval Architects and Marine Engineers in New York several days ago, Mr. James T. Boyd of the George I'. Blake Manufacturing Company referred to the correspondence between the inspectors and the builders of the Belleville generators, and the following letter will accordingly prove interesting to all manufacturers of tubulous boilers : Lo the United States Inspectors of Steam Vessels: GENTLEMEN: The Globe Iron Works Company, Cleveland. O., of which Mr. John F. Pankhurst is the vice-president and general manager, has under construction what are known as Belleville boilers ; these are to furnish about 7,000 horse power for each of the two steamships also under construction at these works. The boiler power in each vessel consists of twenty-eight genera- tors in three groups (two of ten and one of eight generators) with athwartship bulkheads between and at the ends of the groups. Three smoke-pipes are used and the natural draught is supplemented by fan blowers delivering into the open fire rooms. Hach generator or boiler is independent of all the others in its group having its own check- valve, water-guage column, stop-valye and safety-valve. We desire to call your attention to the latter attachment, whose area is proportioned by a rule never expected to deal with an internal pressure be- yond 120 pounds per square inch. The rule prescribes the safety-valve area to be, in square inches, equal to the grate area in square feet, divided by three, thus equipping each of these boilers of 29 square feet grate area with safety- valves of 9.66 square inches area or at least two and one-half times the area of the steam pipe, which is proportioned to carry away the maximum evaporation. Taking into consideration the comparatively small amount of water contained in water tube boilers, ani to which system this design belongs, such an area opened suddenly would in seventeen seconds completely empty the generator, and expose the heating surface to damage from the furnace heat. Our complaint arises from the fact of the present rule prescribing the safety-valve area from the grate area, taking no account of the steam pressure which is the prime factor of discharge volume. It is plain that under forced combustion the evaporative power is increased to more than four fold that obtained by natural draught, showing that the evaporation is prefer- able to the grate area in a formula of accuracy. The prime function of a safety-valve is well expressed by its descriptive title of "safety,' meaning | security against damage or explosion by over-pressure from the action of the fire upon its heating surface. This security is provided for by giving such an area to the valve that it will discharge a volume of steam which shall equal in weight the maximum evaporative power of the boiler, As the amount of that discha: ge is directly as the internal pressure of the boiler, it needs no argu- ment to establish that pressure, as one factor in a formula to decide the proper valve area, It is also evident that another principal factor is the amount of steam that can be made nnder the maximum furnace heat. must next be given to the time element, for the area must be so proportioned that the discharge shall exceed in a degree the evaporative power, but this excess must not be so great as to sudddenly empty the boiler, else the safety- valve action intended to prevent explosion might create the very disaster it should prevent. As we haye stated above, the area by the rule would, at 250 pounds pres- sure, empty these boilers in seventeen seconds. his is an explosive effect ; besides the dry heating surface that might be exposed would surely be dam- aged by the furnace heat, as no fire could. be hauled inside of three minutes. This fact presents a time factor seemingly desirable, Assuming the time limit tobe three minutes--and further on we show this to be the time limit for Scotch boilers using 170 pounds pressure under your rule at the present time-- and recognizing the fact that the discharge of steam in pounds per square inch per minute, at any pressure, is about three times the guage pressure in pounds per square inch then let E = maximum eyaporation per minute in pounds, W = total water in boiler in pounds, P = guage pressure in pounds per square inch, A== area of safety-valve in square inches, hon A ak + 4 W ___ (evaporation=feed ) + } water in boiler 38P Discharge per square inch. First Example.--A Belleville boiler of 30 square feet grate contains 2,100 pounds of water, and will evaporate 100 pounds of water per minute, pressure 250 pounds by guage. 'I'he present rule calls for 10 square inches valve aren und at 250 pounds pressure this atea will discharge 7,800 pounds of steam per minute, or at the rate of emptying the boiler in seventeen seconds. This is ubout the time required for an explosion to empty a Scotch boiler described in this Second Example.--A Scotch boiler recently built by the Globe IronWorks Consideration . Company fora lake steamer has 60 square feet of grates, contains 30,000 pounds water anu evaporates 200 pounds of water per minute ; steam pressure 170 pounds by guage, and safety-valve area by present rule 20 square inches. At 170 pounds pressure this area will discharge 10,200 pounds of steam per minute, or ata rate to empty the boiler in three minutes, It is therefore evident that the formula above given will place water tube boilers, in relation to their contained water, as the rule at present pruvides for the Scotch type. The safety-valve area under the above given formula for the Belleville boilers under consideration would be thus: From 29 square feet grate area we have say 100 pounds per minute normal evaporation; then let E=200 pounds per minute foreed combustion, P=250 pounds internal pressure to be allowed, W=2,100 pounds water in boiler, and we haye 200 + + W=700__900 3 2507 50r 750 In conclusion we ask your honorable board to take into consideration at an early period the matter of safety-valve area for the above mentioned Belle- ville boilers, which find themselyes today in an advanced position from an engineering view, through the designed use of 250 pounds pressure, presenting a strong case to you, in the public interest of safety, to provide a change or amendment to the present rule which shall embrace the newer designs of boilers, to which the Belleville type belongs. (Signed) Jonn F. PANKHURST, Vice-Pres. and Gen'! Manager. Miers CoRyYErt, ) Supt. of Construction. J = ],? area of valve. CLEVELAND, O., Oct, 27, 1893. The appended data gives details not included in the letter to the board of supervising inspectors: Subject: Safety valve areas to be used on the Belleville boilers build- ing by the Globe Iron Works Company, Cleveland, O. 1. Each generator is complete in itself, being fitted up with all attach- ments independently of the others in a group. 2, Each has 29 square feet grates, 850 square feet heating surface and the steam pressure is to be 250 pounds by gauge. 8. The maximum consumption may reach 20 pounds per square foot grates, the efficiency, say 10 pounds water per pound of coal, and evaporative power 96.667 pounds per minute. 4. The safety-valve area by inspection rule for this generator would he 9.66 square inches, about 33 inches diameter. 5. The designed area of steam pipe for each generator is 4 square inches, about 2} inches diameter; the safety-valve area by inspection rule excecds this area 2.415 times. 6. An opening of 1 square inch will discharge 780 pounds per minute at a pressure of 250 pounds by gauge, or eight times the weight of steam that can be generated. 7. The facts stated above Reese the following arguments : A.--A revision of the inspection rule governing safety valve area is desir- able, because in this case it exceeds the steam pipe area 2.4 times, instead of being equal to only one-fourth of that area. B.--As tubulous boilers contain but comparatively little water, it is desir- able that a disproportionate safety-valve area which presents a too large dis- charging capacity should be avoided. The objection applies not in the danger of explosion, but in the damage that would surely occur to the heating surface by the absence of water. In other words, the excessive area opened suddenly by the safety-valve would carry away. both steam and water. C.--To demonstrate this (B.) a 3} inches diameter of valve under a pres- sure of 250 pounds would in one-third of a minute discharge enough steam to equal in weight the total of water and steam in the generator; while : D.--A valve 14 inches diameter under 250 pounds pressurs, would require 3 minutes, which will give entire security against over pressure, under the control of a safety-valve of this dimension. Inventions of a Marine Nature. Specially reported from the patent office, Washington, D. C., for the MARINE REVIEW, 508,985--Ships windlass self register, by Seth S. Gerry of Thomaston, Me,; filed March 14, 1892. 508,9738--Signalling lantern for vessels, by Ernest H. L. Peterson of Ham- burg, Germany; filed May 11, 1893. 509,188--Propelling mechanism for boats, by John R. Mahoney of Sister- ville, W. Vu.} filed April 28, 1898. 509,204--Propeller, by Martin Davies of Jersey City, N. J.; 25, 18938. Copies of specifications accompanying these patents can be had at 15 cents each upon application to the MARINE Rryriew, 516 Perry-Payne building Cleveland, O. filed April If YOU SEND 50 CEN'ts To THE MARINE REVIEW, no. 516 PERRY-PAYNE BUILDING, CLEVELAND, O., AND YOU ARE NOT SATISFIED WITH THR BOUND VOLUME OF FIFTEEN PHOTOTYPES OF LAKE STEAMERS THE MONEY WILI, BE REFUNDED TO YOU.