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M AT E R I A L S S E L E C T I O N & D E S I G N Erosion of Control Valves in Gas Transmission Lines Containing Black Powder FIguRE 3 Measured (points) and curve ft (lines) dimensionless erosion rates for v = 90 m/s and d = 6.9 μm. Legend refers to materials abbreviations in Table 1. FIguRE 4 inversely proportional to the as-received hardness but instead exhibited no clear relationship with the as-received hardness values of the material. The former observation might be attributed to the fact that larger particles have led to a larger degree of surface hardening than the smaller ones.5 The hardened surface layer not only led to a lower intrinsic erosion rate when using larger particles, but also led to an increase in particle fragmentation, which served to further reduce the effective erosion rate. The latter behavior might be attributed to superfcial changes in hardness (i.e., softening or hardening) of the surface layer as a result of particle impact. In either case (softening or hardening), the erosion rate is predominately controlled by the hardness of the surface layer and not by as-received bulk hardness of the material.5 For example, in the case of CS A53 Gr.B with an as-received bulk hardness of 157 VHN, one would have expected this material to be the least erosion resistant (i.e., highest erosion rate), but instead it exhibited the lowest erosion rate (Figure 6), indicative of a hardened surface layer due to impact. Conclusions Measured (points) and curve ft (lines) dimensionless erosion rates for v = 130 m/s and d = 6.9 μm. Legend refers to materials abbreviations in Table 1. 72 MATERIALS PERFORMANCE May 2013 • Black powder generated inside sales gas transmission pipelines due to internal corrosion of those lines is erosive to many engineering materials used in pressure control valves. • The erosion behavior of pressure control valve candidate materials cannot be predicted solely by the as-received bulk hardness of the material. Instead, hardening or softening of the surface layer of the material due to particle impingement will determine the erosion rate of the material. This makes erosion testing using feld conditions an essential step in materials selection. NACE International, Vol. 52, No. 5