Drifting. In operating a locomotive on the road the engine frequently runs with a closed throttle, as is the case in bringing the train to a stop or when "dropping" down grades. This condition is known and spoken of as drifting. Under such circumstances there may be little or no steam in the cylinders yet the effects of expansion and compression will be present. As a result, if the reverse lever is set near the central position the compression will be relatively high and expansion will be carried so far that a vacuum will result which will draw gases and cinders from the "front end" through the exhaust pipe into the cylinders. It is easily seen that the presence of smoke and cinders in the cylinders may prove to be a serious matter.
To prevent the conditions just described from arising, the reverse lever, when drifting, should be carried in the full position corresponding to the direction of travel, for in this position a vacuum will probably not be formed and no foreign matter will enter the cylinders. As a safeguard against damaged cylinders and valves both steam chests should be fitted with relief valves. Such valves are applied one to each steam chest and are arranged to open inwardly and admit atmospheric air whenever the pressure in the steam chest falls below that of the atmosphere and to close suddenly when the throttle is opened. They should be constructed to open by gravity so when once opened they will remain open and will not be worn out by being rapidly opened and closed during the drifting period. It is important that they be made of ample size to admit air freely, otherwise at high speeds a vacuum might be formed in the steam chests and smoke and cinders still be drawn into the cylinders.
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