182 SUPERHEATED STEAM IN MARINE PRACTICE. 



42. Steam Pipes, stop valves and fittings should contain no copper or brass. 

 Many governmental regulations also prohibit cast iron and malleable iron. The 

 rules of the United' States Steamboat Inspection Service require steel, where "pres- 

 sures of more than 300 pounds and a temperature of more than 425° F." are used. 

 This requirement is adhered to in the best practice, both here and abroad. Steam 

 pipes and flanges should be well insulated. This is, of course, good practice 

 whether the steam is saturated or superheated, but lack of such insulation, when 

 an investment has been made in the superheater, is a cause of wastefulness that 

 ought not to be tolerated. The main stop- valve should have a cast-steel body, and 

 the valve spindle should be of steel. 



43. Steam Chest Valves on the High-Pressure Cylinder should be of the piston 

 type with rings, wherever the degree of superheat is sufficient to eliminate all 

 condensation through the first cylinder. Balanced slide valves on high-pressure 

 chests have been, in some instances, satisfactorily operated, but, as a rule, if the 

 temperature of the steam is 500° or more, piston valves are generally recommended. 

 For the intermediate chest of triple and quadruple engines, or the low pressure of 

 compounds, piston valves are desirable, but well-balanced slide valves, if properly 

 provided with grooves for oil lubrication, have been successfully used. For the 

 low-pressure chest of triple and quadruple engines, it would appear safe to use 

 slide valves. The steam temperature, even with high initial superheat, to which 

 the low-pressure valves will be exposed is but little, if any, above saturated steam 

 operation. 



44. Engine Cylinders and Liners must be of first-class, close-grained cast iron, 

 as hard as can be worked. Iron, as a material, for these details has been proven to be 

 entirely suitable for high superheat. It is well known that cast iron does not lose 

 strength,* but on the contrary the strength increases slightly when exposed to tem- 

 peratures up to about 800° F. It is probable that under exceptionally high tempera- 

 tures cast iron may "grow," but, within the range of temperatures to which steam 

 chests and cylinders are subjected with superheated steam, no appreciable change 

 will be found. If this were really a serious matter the number of superheated steam 

 marine plants would not be increasing at the present rate. No fear need be enter- 

 tained, therefore, that there will be deterioration of these parts, due to steam tem- 

 peratures up to 650° or 700°. That this is borne out by facts will be indicated by 

 the satisfactory operation of steamships and locomotives using superheat in excess 

 of 200°, as referred to above. In these instances, steam chests, cylinder bushings 

 and cylinders are made of cast iron. Furthermore, in American locomotives, cast 

 iron is almost exclusively used for the steam pipes leading from the superheater 

 headers to the steam chest. This is standard practice on railroads, and no troubles 

 of any consequence are found. Still more remarkable is the fact that the super- 

 heater headers in practically all locomotives, where the fire-tube superheater is used, 

 are of cast iron. A great many of these castings have been in continual service 

 for over five years, and the percentage of failures is very small. 



♦See Kent, p. 383 (5th edition). 



