These outlets were suitable for use only with hose having screwed connections. When an attempt is made to remove the cap with the vacuum producer in operation, there is a tendency for the vacuum to cause the cap to hug the last thread and render its removal difficult. Also, when the suction is finally broken it is accomplished with considerable hissing noise.
FIG. 51. INLET COCK TO PREVENT AIR LEAKAGE WHEN NOT IN USE.
In order to permit the use of the slip type of hose coupling, a hinged flap valve was substituted for the screwed cap, a rubber gasket being placed under the cap. This was held firmly in place by the vacuum in the pipe line. The interior of the casting inside of the flap was turned to a slip fit for the end of the hose coupling. With this type of valve and the slip hose coupling, described in [Chapter VI], it is possible to reverse the hose to equalize wear and remove obstructions.
These inlets have been made with valves that are closed only by gravity when there is no vacuum on the system and many are so constructed that when opened wide they will remain open with the vacuum on the piping. This type of valve will often be opened by the inquisitive person when no vacuum exists in the system and as there are no immediate results, they may be left open with the result that there will be a very large leakage of air on starting the vacuum producer. This makes it necessary for some one to make a tour of the building in order to close the valve which is open before the system can be efficiently operated. If the vacuum producer is designed to operate several renovators simultaneously, it may not be discovered that there are any valves open and a considerable amount of power will be wasted.
In order to overcome this difficulty it is necessary to provide a spring on the hinge of the flap valve that will automatically close the valve whenever the hose is withdrawn. When the inlets are located in public places they should be fitted with a lock attachment to prevent them from being opened by unauthorized persons.
A valve of this type is illustrated in [Fig. 52]. This valve has a projection on its inner face which engages with a ridge on the hose couplings, preventing the removal of the hose without slightly raising the cap and making it impossible to accidentally pull the hose out of the inlet.
FIG. 52. TYPE OF AUTOMATIC SELF-CLOSING INLET COCK.
The particular valve here shown is suitable for use only with the all-rubber hose connection described in [Chapter VI].