Rod wear rates are greatly influenced by whether a packer is lubricated or not, and the operating pressure and corrosivity of the gas. API 618 provides design standards for hardness and surface finish for various services. Without proper hardness, rod wear rates can be excessive. Surface hardness and finish become increasingly important as the amount of lubrication is reduced. In general, most rod and packing materials will perform well against each other if the surface finish and fitup are correct.
Manufacturers’ standard piston rods are normally made of case hardened, highly polished steels. Two common rod surface hardening techniques are induction and flame hardening. Both processes involve heating the surface of rods above the upper critical temperature followed by rapid quenching using water or other suitable coolants. Typical case thicknesses range from 1/16 to 1/8 inch, with surface hardnesses in the range of Rockwell C50 to 60.
In certain cases, the hardness must be limited due to potential embrittlement problems. Sour hydrocracking services are one such service. For these applications, rods are often fabricated from softer steels, then hardfaced for wear resistance in the packing and oil wiper ring areas.
Figure 800-13 provides general guidance on appropriate hardness and surface finishes for various applications. These hardnesses and finishes have normally resulted in acceptable packer sealing and life.
As metal is removed from the surface of case hardened rods, hardness decreases. Wear rates accelerate and susceptibility to galling and abrasion increases. The acceptable degree of packer leakage depends to a large extent on the nature and severity of the process application. In less severe applications such as lubricated, low-pressure service, it is possible to accept much greater wear before replacing or reconditioning a rod. In high-pressure hydrogen applications, however, leakage of hydrogen results in further heating of packing, lubricant and the rod (due to Joule-Thompson effect). An unacceptable operating condition quickly results.