Page 260 - Valve Selection Handbook
P. 260

Rupture  Discs                     247

          Figure 6-33.  Double Disc Assembly
          with Product Leakage Warning
          Devices Consisting of  Pressure
          Gauge and  Excess Flow Valve.
          (Courtesy of Sempell  A.G.)








            The ASME Code does not deal with double disc  assemblies.

          Quick-opening  device.  By  lowering the  rating of  the  rupture discs in a
          double disc  assembly to below the operating pressure,  the assembly may
          be used as a quick-opening device.  One method of initiating quick open-
          ing  is  to  pressurize  the  space  between  the  discs  until the  outlet  disc
          bursts. The  operating  pressure  will then burst the  first  disc  within milli-
          seconds.  The  second  method  requires  the  maintenance  of  a  pressure  of
          approximately  half  the  normal  system  pressure.  On  dumping this  pres-
          sure, both discs open simultaneously within milliseconds.
            When employing the second method, the following limitations apply:

          • The pressure  between  the rupture  discs  cannot  exceed  the  operating
           pressure  unless the  rupture  disc  facing  the  operating  pressure  is  fitted
           with a back-pressure support.
          • The burst pressure of either rupture disc,  including burst tolerance, can-
           not be greater than the operating  pressure.

            For  conditions  of  both  rupture discs  being  identical,  the  following
          pressure relationships  exist:













            where  Pj  = normal gauge operating pressure
                  P 2 = gauge fluid pressure in space between rupture discs
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