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Butterfly  Check  Valves
Figure 28   Relief Valve

Mechanical Science Volume 2 of 2
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TYPES OF VALVES DOE-HDBK-1018/2-93 Valves Butterfly  check  valves  may  be Figure 27   Stop Check Valve installed horizontally or vertically with the vertical flow either   upward   or   downward. Care should be taken to ensure that  the  valve  is   installed  so that  the  entering  flow  comes from the hinge post end of the valve;  otherwise,  all  flow  will be stopped. Stop  Check  Valves A  stop  check  valve,  illustrated in  Figure  27,  is  a  combination of   a   lift   check   valve   and   a globe   valve. It   has   a   stem which,  when  closed,  prevents the  disk  from  coming  off  the seat  and  provides  a  tight  seal (similar    to    a    globe    valve). When  the  stem  is  operated  to the   open   position,   the   valve operates  as  a  lift  check.    The stem  is  not  connected  to  the disk and functions  to close the valve   tightly   or   to   limit   the travel of the valve disk in the                                                                           open direction. Relief  and  Safety  Valves Relief and safety valves prevent equipment damage by relieving accidental over-pressurization of fluid systems.   The main difference between a relief valve and a safety valve is the extent of opening at the setpoint pressure. A relief valve, illustrated in Figure 28, gradually opens as the inlet pressure increases above the setpoint.  A relief valve opens only as necessary to relieve the over-pressure condition.  A safety valve, illustrated in Figure 29, rapidly pops fully open as soon as the pressure setting is reached. A safety valve will stay fully open until the pressure drops below a reset pressure.   The reset pressure  is  lower  than  the  actuating  pressure  setpoint.    The  difference  between  the  actuating pressure  setpoint  and  the  pressure  at  which  the  safety  valve  resets   is   called   blowdown. Blowdown is expressed as a percentage of the actuating pressure setpoint. ME-04 Rev.0 Page 40







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