Page 184 - Valve Selection Handbook
P. 184

Pressure Relief  Valves                171

          popping  pressure.  Raising  the  nozzle  ring  reduces  the  width  of  the  sec-
          ondary  orifice  that  leads  to  an  earlier  popping  of  the  valve but, in  turn,
          lengthens  the  blowdown  by  some  small  amount. Conversely,  lowering
          the nozzle ring increases  the difference  between set pressure  and popping
          pressure but, in turn, lengthens blowdown by some small amount.
            The lower blowdown ring is commonly factory set and should not nor-
          mally  need  readjustment  when  putting a  new  valve  into  service.  No
          attempt  should be made  to eliminate  the  difference  between set  pressure
          and popping pressure  altogether.
            The  valve  shown  in  Figure  5-5  has  done  away  with  the  nozzle  ring.
          The  width  of  the  secondary  orifice  is  fixed  in this case  by  the  geometry
          of seat and disc.
            Safety  valves  with  open  bonnets  are  partially  balanced  by  the  stem
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          guide.  This  may  allow  safety  valves  to  operate  against  a built-up  back
          pressure  of  about  20%,  but  the  manufacturer must  be  consulted.  They
          are,  however,  not  suitable  for  superimposed  back  pressure  because  of
          leakage  around  the  stem  to  the  bonnet.  For  this  reason,  safety  valves
          should not be manifolded.

          Safety  Relief  Valves

            Safety  relief  valves  are  general-purpose  pressure  relief  valves  for  use
          in  either  gas  or  liquid  service  as  encountered  in  the  process  industry.
          Because  the fluids  handled in these industries cannot normally be tolerat-
          ed to escape  around the valve, the bonnet of these valves is either  vented
          to the valve outlet  or  sealed  against  the  valve chamber  by means of  bel-
          lows. An exception  is safety  relief  valves for process  steam duty in which
          the bonnet is provided with windows as in safety  valves.
            When  used  on gas  or vapor  service,  the  valves pop  open.  When  used
          on  liquid  service,  the  valves  modulate  open  and  reach  the  fully  open
          position  at an overpressure  of about 25% or, in newer designs,  at an over-
          pressure  of  10%, as in the valves shown in Figure 5-7 and Figure  5-10.
            The  industry  distinguishes  between  conventional  safety  relief  valves
          and balanced  safety relief  valves.
          Conventional  safety  relief valves.  Conventional  safety  relief  valves are
          provided  with a closed  bonnet that is vented to the valve outlet, as in the
          valves shown in Figure  5-7 and Figure 5-8. By this design, back pressure
          is allowed to act on the entire back of the disc.
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