[Illustration: FIG. 13.]
_a._ An air vent with a brass gate valve and brass pipe for connecting
up, must be provided and connected with delivery elbow and discharge
cone.
_b._ The size of this air vent should be one inch for 500-gallon and
750-gallon pumps, and one and one-fourth inches for the 1000-gallon and
1500-gallon sizes.
_c._ The hand wheel of this valve must be marked as per Fig. 13. The
lettering must be very open, clear and distinct, not liable to be
obstructed by grease and dirt, and of a permanent character.
The object of this valve is to reduce the pressure above force valves
and secure a prompt riddance of all air that may come through the
water cylinders when first starting up.
This valve, of course, should be closed when once pump is under way,
to prevent waste of water.