152 



WATER CONDUCTORS AND ACCESSORIES 



hour, to weeks or even months. When the apparatus at the end 

 of long periods of idleness is called upon to operate it must per- 

 form its function without 

 fail, and must, therefore 

 be designed accordingly, 

 totally inclosed motors of 

 a moisture-proof design 

 being preferable. Metal- 

 line bearings are generally 

 used, as the motors may 

 be mounted in any posi- 

 tion from vertical to hori- 

 zontal. Due to the inter- 

 mittent nature of the ser- 

 vice, efficiency or power 

 factor need not be con- 

 sidered, the main consid- 

 eration being reliable 

 operation. 



The proper size of a 

 motor for driving a valve 

 will vary with the duty 

 and conditions under 

 which the valve operates. 

 A small valve may only 

 require a 1 -horse-power 

 motor, while very large 

 valves require up to 25- 

 H.P. motors. The size of 

 the valve is, however, not 

 the only factor determin- 

 ing the required motor 

 capacity, which also de- 

 pends to a very large ex- 

 tent on the pressure on 

 the valve and the time of 



FIG. 78. Heavy Pressure Motor-operated 

 Gate Valve Showing Motor Equipment 

 and Limit Switch. 



opening. 



The torque requirements vary greatly during the operating 

 cycle. It is maximum shortly after the time of unseating the 

 valve; that is, after the wedges have been released and the actual 



