“Suppose the load continues light, with good fires, steam rises to 110 pounds, the water will rise to 10 per cent. more pressure or 121 pounds, thus automatically giving more pressure to ‘feed up’ on high steam, and store away the heat that would be wasted by radiation, absorption, or perhaps blowing off.”
The water pressure will vary only as the steam pressure varies, always keeping the same per cent. of excess. The results are directly opposite to what would or does occur where feed water is delivered at a stated pressure. On a battery of boilers, during the cleaning of fires, the closing of feed valves on one, two or more boilers, does not affect the feed of those already set in the least, the pump will simply make less strokes necessary to properly feed the others.
The regulating is done by the feed valves at the boilers; if it is desired, all feeds may be closed, and the pressure will not rise, the pump will stop; if its plungers need packing it will be detected by the fact that the pump will creep, to keep up the required pressure. When feed valves are once regulated to admit the required amount of water, to replace the evaporation, they may be marked, and when in this position, they, with an even steam pressure, will always admit the same amount of water to the boilers.
It is understood that this valve is placed between the ordinary throttle valve and the pump.
Fig. 638.
Fig. 639.
The Bordo blow-off valve is shown in Figs. [638] and [639]; it consists of a brass or iron body which resembles the shell of a plug-cock, but with this difference, it has a sharper taper than the regular plug-cock; in this device the plug is usually made of brass—tinned on the outside. In process of making and while hot a sheath of babbitt metal or its equivalent is cast upon the plug; the metals amalgamate and practically become one casting.