Fig. 636.
It is simply a balance valve and differential piston; it is in a class by itself, both as to its construction and operation in regulating feed water pressure in connection with steam boilers.
Fig. 637.
The initial steam pressure being on the ends of the valve, has access, through the neck, to the full area of the piston, and will force it into a position that gives the full valve opening, where it will remain until overcome by water pressure from the pump, acting against the opposite side, which is of reduced area.
It is necessary in operating this valve to have an excess of water pressure over the steam pressure in the boiler. The excess of the water pressure is obtained by the reduction of the area of the water side of the piston—thus to illustrate—if the total area were 10 square inches, and the reduction was one inch or 10 per cent., it would require that the water pressure should be 10 per cent. greater than steam pressure, to give the same thrust on the piston, then until the water has reached a pressure 10 per cent. in excess of the steam, the valve would be held open, but thereafter it is held open only wide enough to admit steam to the pump to keep up this 10 per cent. excess pressure. Should the excess pressure attempt to rise above this, it immediately forces the steam valve nearly shut, thus nipping the cause of the rise, namely, too great a piston speed.
Note.—“In starting the pump, ‘stand by’ until it has caught suction, and accumulated nearly the correct water pressure, now open the valve on the pulse, or pressure pipe to the controller and open pump throttle wide, thus giving the controller free action.
“Suppose the boiler pressure is 100 pounds, the water pressure will be 10 per cent. higher or 110 lbs. Carrying an even fire, with water at second gauge, feed valves properly set, the load suddenly increases, which pulls steam down to 99 pounds, the water does not remain at 110 lbs. as before, but is now 10 per cent. in excess of 90 lbs. or 99 lbs., thus in place of 10 lbs. excess water pressure there is but 9 lbs., which means there will be less water delivered through the feed valves, which will hardly ever have to be touched.
“Next the load will lighten—steam will rise, and the excess pressure will automatically increase, thus restoring the water used at a time when it was most necessary to lighten the feed to temporarily favor the fire.