strips riveted to the outside of the plates, of the same thickness as the plates, and of a width equal to the distance between the rows of stays, C may be taken as 175, if P is taken to be the distance between the rows, and 190 when Pis taken to be the pitch between the stays in the rows. For steel plates, other than those for combustion chambers, the values of C may be increased as follows: If flat plates are strengthened with doubling plates securely riveted to them, having a thickness of not less than of that of the plates, the strength to be taken from Cx(T+t/2)2. = working pressure in lbs. per square inch; where t = thickness of doubling plates in sixteenths, and C, T and Pare as above. NOTE. In the case of front plates of boilers in the steam space these numbers should be reduced 20 %, unless the plates are guarded from the direct action of the heat. For steel tube plates in the nest of tubes the strength to be taken from 140 x T2 = = working pressure in lbs. per square inch : where the thickness of the plates in sixteenths of an inch, P = the mean pitch of stay tubes from centre to centre, For the wide water spaces between the nests of tubes the strength to be taken from Cx T2 = where P the horizontal distance from centre to centre of the bounding rows of tubes, and C= = 120 where the stay tubes are pitched with two plain C =130 if they are fitted with nuts outside the plates, C C C = = 150 if they are fitted with nuts outside the plates, = C = 170 if every tube in these rows is a stay tube, and each alternate stay tube is fitted with nuts outside the plates. The thickness of tube plates of combustion chambers in cases where the pressure on the top of the chambers is borne by these plates is not to be less than that given by the following rule : where P working pressure in lbs. per square inch, = W = width of combustion chamber over plates in inches, d inside diameter of plain tubes in inches, = T= thickness of tube plates in sixteenths of an inch. Girders. The strength of girders supporting the tops of combustion chambers and other flat surfaces to be taken from the following formula: Cxd2xT (L-P) ×D×L working pressure in lbs. per square inch; where = width between tube plates, or tube plate and back plate of chamber, P = pitch of stays in girders, D= distance from centre to centre of girder, d = depth of girder at centre, thickness of girder at centre. All these dimensions to be taken in inches. Wrought Iron. 6,000 if there is one stay to each girder, 9,000 if there are two or three stays to each girder, C= 10,000 if there are four or five stays to each girder, 10,500 if there are six or seven stays to each girder, 10,800 if there are eight stays or above to each girder. Wrought Steel. 6,600 if there is one stay to each girder, 9,900 if there are two or three stays to each girder, C=11,000 if there are four or five stays to each girder, 11,550 if there are six or seven stays to each girder, 11,880 if there are eight stays or above to each girder. Circular Furnaces.-The strength of plain furnaces to resist collapsing to be calculated as follows: Where the length of the plain cylindrical part of the furnace exceeds 120 times the thickness of the plate, the working pressure is to be calculated by the following formula : 1,075,200 × T2 = LXD working pressure in lbs. per square inch; where the length of the plain cylindrical part of the furnace is less than 120 times the thickness of the plate, the working pressure is to be calculated by the following formula : 50x (300 T-L) working pressure in lbs. per square inch; D = where D outside diameter of furnace in inches, L= length of plain cylindrical part in inches, measured from the centres of the rivets connecting the furnace Ꮓ to the flanges of the end and tube plates, or from the commencement of the curvature of the flanges of the furnace where it is flanged or fitted with Adamson's rings. The strength of corrugated furnaces made of steel, having a less tensile strength than 26 tons per sq. in., the corrugations being 6 ins. apart and 1 in. deep, to be calculated from 1,000 × (T-2)_ D = working pressure in lbs. per square inch. The strength of furnaces made of steel, having a tensile strength between 26 and 30 tons per sq. in., and corrugated on Fox's or Morison's plans, to be calculated from 1,259 × (T-2) D = working pressure in lbs. per square inch. The strength of ribbed furnaces (with ribs 9 ins. apart) to be calculated from the following formula : 1,160 × (T-2) = D working pressure in lbs. per square inch. The strength of spirally corrugated furnaces to be calculated from the following formula:-- 912 × (T-2) D where T = = = working pressure in lbs. per square inch; thickness of plate in sixteenths of an inch, diameter of the ribbed furnaces, in inches. The strength of Holmes' Patent Furnaces, in which the corrugations are not more than 16 ins. apart from centre to centre, and not less than 2 ins. high, to be calculated from the following formula: 945 x (T-2). D Working pressure in lbs. per square inch = ; where T = thickness of plain portions of furnace in sixteenths of an inch, D = outside diameter of plain parts of the furnace in inches. Donkey Boilers. The iron used in the construction of the fire boxes, uptakes, and water tubes of donkey boilers shall be of good quality, and to the satisfaction of the surveyors, who may in any cases where they deem it advisable apply the following tests: The material to stand bending hot to an angle of 90°, over a radius not greater than 15 time the thickness of the plates. General Remarks about Boilers under Construction.-The surveyors will be guided in fixing the working pressure by the tables and formulæ annexed. Any novelty in the construction of the machinery or boilers to be reported to the Committee. The boilers, together with the machinery, to be inspected at different stages of construction. The boilers to be tested by hydraulic pressure, in the presence of the engineer-surveyor, to twice the working pressure, and carefully gauged while under test. Two safety valves to be fitted to each boiler and loaded to the working pressure in the presence of the surveyor. In the case of boilers of greater working pressure than 60 lbs. per sq. in., the safety valves may be loaded to 5 lbs. above the working pressure. If common valves are used, their combined areas to be at least half a square inch to each square foot of grate surface. If improved valves are used, they are to be tested under steam in the presence of the surveyor; the accumulation in no case to exceed 10% of the working pressure. An approved safety valve also to be fitted to the superheater. In winch boilers one safety valve will be allowed, provided its area be not less than half a square inch per square foot of grate surface. Each valve to be arranged so that no extra load can be added when steam is up, and to be fitted with easing gear which must lift the valve itself. All safety-valve spindles to extend through the covers and to be fitted with sockets and cross handles, allowing them to be lifted and turned round in their seats, and their efficiency tested at any time. Stop valves to be fitted so that each boiler can be worked separately. Each boiler to be fitted with a separate steam-gauge, to accurately indicate the pressure. Each boiler to be fitted with a blow-off cock independent of that on the vessel's outside plating. The machinery and boilers are to be securely fixed to the vessel to the satisfaction of the surveyor. Steel Boilers. In cases where it is proposed to construct boilers of steel for classed vessels, or vessels intended for classification, the material is required to fulfil the following conditions: 1. The material of stays and of plates is to have an ultimate tensile strength of not less than 26 and not more than 30 tons per sq. in. of section. In all cases the ultimate elongation must not be less than 20% in a length of 8 ins." It is to be capable of being bent to a curve of which the inner radius is not greater than one and a half time the thickness of the plates or bars, after having been heated uniformly to a low cherry-red, and quenched in water of 82° F. 'Steel of a less tensile strength than 26 tons per sq. in., if satisfactory in other respects, may be allowed in any case where the scantlings are equal to those prescribed in the Rules for Iron Boilers. In such cases the surveyors should represent the facts for the Committee's consideration. 2. Steel rivets are to be considered as part of the material, and, in addition to being subjected to a shearing test, they must be capable of withstanding the same tests as the plates are required to undergo. 3. Samples for testing are to be selected from each batch of plates submitted for approval, care being taken in the selection that, as far as possible, each cast or furnace charge from which the material has been produced is represented. In addition to these tests, the temper test is to be applied to samples taken from every plate intended to be used in the construction of boilers. 4. All the holes in steel boilers should be drilled, but if they be punched the plates are to be afterwards annealed. 5. All plates that are dished or flanged, or in any way heated in the fire for working, except those that are subjected to a compressive stress only, are to be annealed after the operations are completed. 6. No steel stays are to be welded. 7. Unless otherwise specified, the rules for the construction of iron boilers will apply equally to boilers made of steel. SUMMARY. The preceding rules are summarised in the following short table, in which the method has been carried out of indicating all such dimensions as are measured in inches by capitals, such as are measured in sixteenths of an inch by small letters, and all coefficients, &c., by black letters : WP = permissible working pressure. = B and B' percentage of joint respectively of plate and of rivets. row. T and t = thicknesses of plates measured respectively in inches and in sixteenths of an inch. P = pitches in inches of rivets or stays in flat plates, or tubes in tube plates. = distance apart of girders or cross pitch of stays in inches. D= mean diameter of shells and diameter of furnaces in inches, measured as follows: for all plain furnaces, or made with ribs (Purves's), with flanges (Adamson's rings), or for Holmes's furnaces, the outside diameter of the plain cylindrical part is to be taken, and the thickness of the plates measured at these parts. For Fox's and Morison's corrugated furnaces the extreme external diameter is to be taken. D and D, effective external and internal diameters of plain or stay tubes, and effective diameter of rivets, or of stays, in inches. Lor L' length of plain cylindrical parts of furnaces measured = L = respectively in inches or feet. length of girders measured in inches H= depth of girders measured in inches. internal distance |