All About Engines

ii8 All About Engines placed by the D valve illustrated in Chapter I. This grew out of the original invention by Murdock, one of the ablest of Watt’s assistants, in 1799, and is shown in Fig. 69. It consisted of a piece of semi- Fig. 69.—Murdock’s D slide valve circular tubing, and plates at the ends of the flat side to form a passage between the steam and exhaust ports. It was gradually replaced in large engines by the long D valve. This is very similar to the short D valve, but flatter, and its mode of operation was exactly the same as that with which we are already familiar. The chief objections were the long travel, and the large amount of friction, which became more serious as the pressures em- ployed increased. A simple example of the pressure needed to move a valve will suffice to show how serious this was. Suppose the valve to have a rubbing surface of 12 inches by 10 inches, or 120 square inches, the steam pressure to be 150 lb. on the square inch, the stroke to be 4 inches, and the number of revolutions 150 per minute. The total pressure on the valve face would be 120 x 150 ~ 18,000 lb. The force re- quired to slide the valve backwards and forwards would be about one-tenth of this. The distance