“…The derivation of these nomographs has been described by Lunn, Brookes and Nicol (1988 The nomographs were adopted in NFPA 68: 1988, are given in the 1998 edition of NFPA 68 and are shown in Figure 17.121.The nomographs are fora high energy(10,000J) ignition source, and there is a separate nomograph for each value of the vent opening pressure Pstat. The three values of Pstat are 0.1, 0.2 and 0.5 barg and that of Fre d is 0.2--2.0 barg.…”