Page 233 - Valve Selection Handbook
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220                  Valve  Selection Handbook

              The  graphite  commonly used  for  rupture discs  is made  from  low  ash
            petroleum  cokes,  calcined  at  high  temperatures.  It  is  then  mixed  with
            pitch,  formed  into  blocks,  and  then  heat  treated.  The  result  is  porous,
            brittle material that requires  sealing for  use in rupture discs. This is com-
            monly done by impregnating the graphite under vacuum with  either phe-
            nolic or furane  resins.
              Less frequently  used is pure graphite. This is exfoliated graphite,  origi-
            nally in powder form. When suitably compressed, the graphite forms into
            an impervious flexible sheet. Restricted to some instances, however, pure
            graphite  will  absorb  some  liquid.  This  problem  can  be  overcome  by
            applying  a  suitable coating to  the process  side  only. The maximum per-
            missible  operating  temperature  of  the  disc  is  limited  in  this  case  by  the
            temperature resistance  of the coating.
            Temperature and    Burst Pressure  Relationships


              Temperature  influences  the  strength  of the  disc  materials  so that  there
            is  a  relationship  between  temperature  and burst pressure.  The  relation-
            ship  varies  between  rupture  discs of identical  material  but  different  con-
            struction.
              The temperature  and burst pressure  relationships  shown in Figure  6-1
            apply to solid-metal forward-domed rupture discs as made by one manu-


























            Figure 6-1. Temperature/Burst  Pressure Relationship of Solid-Metal  Forward-Domed
            Rupture Discs Made of a Variety  of Materials. (Courtesy of Continental Disc
            Corporation.)
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