. The Locomotive . return pipe. With such an installation the use of check-valveson the return connections to each boiler is necessary to prevent theexchange of water between the boilers. Referring to fig. i it will beseen that if the check valves a were not present, the slightest dif-ference in pressure on the water surfaces of the boilers Nos. i and 2will cause the water to flow from one boiler into the other with thepossibility of low water in one of them. It may seem a little paradox- 1920.] THE LOCOMOTIVE. 69 ical to some of the readers of this article that even a shght difiference inpres
. The Locomotive . return pipe. With such an installation the use of check-valveson the return connections to each boiler is necessary to prevent theexchange of water between the boilers. Referring to fig. i it will beseen that if the check valves a were not present, the slightest dif-ference in pressure on the water surfaces of the boilers Nos. i and 2will cause the water to flow from one boiler into the other with thepossibility of low water in one of them. It may seem a little paradox- 1920.] THE LOCOMOTIVE. 69 ical to some of the readers of this article that even a shght difiference inpressure can exist in two boilers connected to a common steam experience with low pressure boilers has shown this to be a fact,and the cause of this difference in pressure may be partly explainedas follows:— Within a low pressure steam system the balance of temperaturesand pressures is a very delicate one due to several causes, such aslocal excessive friction of the fluid passing through the pipes and local. 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,
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