Gate Valves

Figure 6-3.—Three-way ball valve. GATE  VALVES Gate  valves  are  used  when  a  straight-line  flow of  fluid  and  minimum  flow  restriction  are  needed. Gate valves are so-named because the part that either  stops  or  allows  flow  through  the  valve acts  somewhat  like  a  gate.  The  gate  is  usually wedge-shaped. When the valve is wide open the gate is fully drawn up into the valve bonnet. This leaves an opening for flow through the valve the same size as the pipe in which the valve is installed (fig. 6-4). Therefore, there is little pressure drop or  flow  restriction  through  the  valve. Gate  valves  are  not  suitable  for  throttling purposes. The control of flow is difficult because of  the  valve’s  design,  and  the  flow  of  fluid slapping   against   a   partially   open   gate   can cause  extensive  damage  to  the  valve.  Except  as specifically authorized, gate valves should not be used  for  throttling. Gate  valves  are  classified  as  either  rising-stem or   nonrising-stem   valves.   The   nonrising-stem valve is shown in figure 6-4. The stem is threaded into  the  gate.  As  the  handwheel  on  the  stem  is rotated, the gate travels up or down the stem on the  threads  while  the  stem  remains  vertically stationary. This type of valve will almost always have a pointer indicator threaded onto the upper end of the stem to indicate the position of the gate. Valves  with  rising  stems  (fig.  6-5)  are  used when   it   is   important   to   know   by   immediate inspection  whether  the  valve  is  open  or  closed  and when the threads (stem and gate) exposed to the fluid  could  become  damaged  by  fluid  contami- nants. In this valve, the stem rises out of the valve when  the  valve  is  opened. GLOBE  VALVES Globe valves are probably the most common valves in existence. The globe valve gets its name Figure 6-4.—Operation of a gate valve. 6-3 Figure 6-5.—Rising stem gate valve.


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