5.9　Pistons，Piston Rods，Piston Rings

5.9.1　The connection of piston-to-piston rod shall be in accordance with 6.10.1 of API Std 618-2007.

5.9.2　The connection of piston rod to crosshead may be a direct connection，an indirect connection，or indirect clamped rod connection（refer to API Std 618-2007 Appendix G Figures G6，G7 and G8），and in accordance with 6.10.2 of API Std 618-2007.

5.9.3　When the hollow piston is used，a single hole 3 mm in diameter shall be located in the head-end face of the hollow piston or at the bottom of the piston ring groove.The hollow piston shall be single piece or multi-piece type.Other depressurizing methods，if applied，shall be approved by the purchaser.

5.9.4　Piston rings should be arranged between the wear bands.

5.9.5　The piston shall be in accordance with 6.10.3.2 of API Std 618-2007.

5.9.6　If hydrogen sulfide，hydrogen chloride or other stress corrosion promoting agents present，unless the piston rod with surface coating，the materials for piston rods should be semi-austenitic precipitation hardening stainless steel，such as CC 450，17-4PH or 0Cr17Ni4Cu4Nb.

5.9.7　The technology and provisions of coated from the piston rod packing to the oil wiper packing travel areas with a wear-resistant material and other surface-hardness provisions shall be provided by the vender，and approved by the purchaser.

5.9.8　High-velocity and high-impact thermal coating processes are acceptable for the coating of piston rods.Metal spray techniques requiring roughening of the surface of the base metal should not be recommended，because of the potentially destructive stress risers left in the surface.Use of subcoating under the main coating shall not be used.

5.9.9　The coating material and piston rod base material shall be mutually agreed upon between the purchaser and the vendor.The areas to be coated shall be strictly sealed from the non-coated areas to prevent corrosion of the base material at the interface of the coating.Fusion techniques that require temperatures high enough to affect the mechanical characteristics of the piston rod base material are unacceptable.

5.9.10　The piston rod surfaces that have been subject to previously induction-hardened shall not to be coated with a wear resistant material.

5.9.11　Piston-rod base material and coating material for use in corrosive environments shall be suitable for the operating conditions specified on the inquiry technical requirements or data sheets.

5.9.12　The piston rod shall be in accordance with 6.10.4 of API Std 618-2007.