. Twentieth century hand-book for steam engineers and electricians, with questions and answers .. . back head, extend down-wards to the floor of the combustion chamber, andthence horizontally out through the back wall, wherethe blow off cock can be located. The best blow off cocks are the asbestos packediron-body plug cocks, which are durable and globe valve should never be used in a blow off pipe,because the scale and dirt will lodge in it and preventits being closed tightly. A straight way or gate valveis not so bad, but an asbestos packed plug cock isundoubtedly the best and safest.
. Twentieth century hand-book for steam engineers and electricians, with questions and answers .. . back head, extend down-wards to the floor of the combustion chamber, andthence horizontally out through the back wall, wherethe blow off cock can be located. The best blow off cocks are the asbestos packediron-body plug cocks, which are durable and globe valve should never be used in a blow off pipe,because the scale and dirt will lodge in it and preventits being closed tightly. A straight way or gate valveis not so bad, but an asbestos packed plug cock isundoubtedly the best and safest. In order to protect the blow off pipe from theintense heat, a shield consisting of a piece of largerpipe can be slipped over the vertical part before it isconnected. 46 ENGINEERING Blow off cocks should be opened for a few secondsonce or twice a day, to allow the scale and mud to beblown out. If neglected too long they are liable tobecome filled with scale and burn out. A plan whichis said to give good results is to connect a tee in thehorizontal part of the pipe, and from this tee run a I FIGURE 9. pipe to a point in the back head at the water is claimed that this will cause a circulation of waterin the pipe and prevent the formation of scale. A surface blow off is a great advantage, especiallyif the water is muddy or liable to foam. By havingthe suiface blow off connected on a level with the BOILER SETTINGS AND APPURTENANCES 47 water line a large amount of mud and other matterwhich is kept on the surface by the constant ebullitioncan. be blown out. A combination surface blow off, bottom blow off,and circulating system can be arranged by a connec-tion such as illustrated in Fig. 9. By closing cock Aand opening cocks B and C the bottom blow off is putin operation; by closing B and opening A and C the sur-face blow off is started, and by closing C and leavingA and B open the device will act as a circulating sys-tem. The pipe should be of the same size
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