Polished Rod

What Makes a Polished Rod Different

A polished rod is the topmost rod in a sucker rod string and is the only component that passes through the wellhead stuffing box. Because it reciprocates through the packing, its surface must be smooth, straight and hard enough to resist wear and to seal without damaging the packing. The critical surface is precision ground and polished, typically to a surface roughness of about 0.4 micrometres or better, and is frequently hard chrome plated to extend service life. The other difference is the ends: the top end has a polished rod thread sized for the polished rod clamp or a polished rod coupling, and it is longer than a standard sucker rod thread. The bottom end carries a standard sucker rod pin so it connects directly to the top sucker rod. A polished rod has an unpolished section at each end where the clamp or coupling sits, and it is important not to grip or damage the polished section during handling.

Chrome Plating and Surface Treatment

The polished surface is subject to abrasive wear from the packing and to corrosion from produced fluids and from the atmosphere at the wellhead. Hard chrome plating, typically 0.05 to 0.15 mm thick with a hardness of 800 to 1000 HV, markedly improves wear resistance and reduces friction against the packing, extending both rod and packing life. Where the well is sour, the plating and the base material must both be suitable for the service, because an improperly applied coating can contain residual stress and even small defects through which hydrogen can enter the base metal. An alternative is a stainless steel or nickel based overlay applied by welding, which gives better corrosion resistance than chrome and is used in high chloride or CO2 environments. When replacing a polished rod, note whether the wellhead packing is designed for a chrome plated or a bare surface, since the packing type and the surface finish must be compatible.