. The Locomotive . Boiler No. i. Boiler No. Fig. I. condensation, so that it is indeed hard to foretell what the actualpressure will be at any given point. When boiler No. i is fired harderthan No. 2 a greater amount of steam will flow from the No. i boilerthan from the other. This increased flow of steam will result in in-creased friction in the steam pipe leading from boiler No. i and con-sequently the pressure in this boiler will rise somewhat above thatin boiler No. 2. The pressure difference need not be very much to cause a con-siderable, variation in water level. With the water in the bo
. The Locomotive . Boiler No. i. Boiler No. Fig. I. condensation, so that it is indeed hard to foretell what the actualpressure will be at any given point. When boiler No. i is fired harderthan No. 2 a greater amount of steam will flow from the No. i boilerthan from the other. This increased flow of steam will result in in-creased friction in the steam pipe leading from boiler No. i and con-sequently the pressure in this boiler will rise somewhat above thatin boiler No. 2. The pressure difference need not be very much to cause a con-siderable, variation in water level. With the water in the boiler at a 70 THE LOCOMOTIVE [July,. 1920.] THE LOCOMOTIVE. 71 temperature of 220 degrees Fahrenheit a pressure difference of Y^of a pound will cause the water level to differ approximately 734inches, so that with the water only a few inches above the highest partof the heating surface it will be seen how a slight unbalance of pres-sure between two boilers may cause dangerously low water in one ofthem. With the return water entering the lowest part of the boilers, as iscustomary, the use of check valves is also necessary to prevent thewater from backing out of the boilers into the heating system throughthe return pipe when, for instance, the stop valve on the boilers (ifpresent) or else the steam-valve on every radiator of the system mightbe closed. The use of check-valves on the return pipes, however, isfar from being an ideal means to guard against these irregularitiesfor the reason that to lift a check-valve a certain amount of headis necessary so that the operation of the check-valve itself involves acertain amount o
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