Page 284 - Valve Selection Handbook
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Sizing Pressure Relief  Devices            271

          the  set  pressure  to  permit  stable  valve  operation  at  a  blowdown of  not
          lower than 5%. Consult manufacturer.


                           SIZING OF INLET       PIPING
                         TO  PRESSURE      RELIEF VALVES

            Codes  for  the  installation  of  pressure  relief  valves  require  that  the
          cumulative total  of non-recoverable  pressure  losses  in the inlet piping to
          the  pressure  relief  valves  at  rated  mass  flow,  including entry  loss  at  the
          pressure vessel connection, shall not exceed  3% of the set pressure.
            The following procedure  permits  the estimation of the resistance  coef-
          ficient  of the inlet piping for 3% inlet pressure loss:


            1. Initially  estimate  the  resistance  coefficient  of  the  proposed  inlet
              piping.
            2. Calculate  the  value of  (0.9A E /A NK).  Use this value to  obtain  from
              Figure  7-8  the  value  of  the  resistance  coefficient  for  an  inlet  pres-
              sure  loss  of  3%. The  isentropic  coefficient for  gases  allowed  for  in
              this diagram is  1.3. The dash line applies also to liquids, irrespective
              of pressure ratio P2/Pj.








          Figure 7-8. Diagram for
          fhe Estimation or
          Resistance Coefficient for
          3% Pressure Loss  in  Inlet
          Line to  Pressure Relief
          Valve, Applicable to
          Gases and Vapors with
          an Isentropic  Coefficient
          of k = 1.3. The Dash Line
          Applies also to  Liquids
          Regardless of Pressure
          Ratio P 2/Pi.(/tepr/ntec/
          from  AD-Merkblatt  A2,
          Courtesy of  Verband  cfer
          fecrin/scrien
          Uberwach ungs - Vereine
          e.V.,  Essen.)
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