1. Introduction
Sucker rods and polished rods are two integral components in a reciprocating piston pump installation for extracting oil or water from wells. Though they work together in tandem, there are some key differences between them in terms of their design, function, and maintenance.
2. Sucker Rods
Sucker rods are the main connecting components that transmit torque and up/down motion from the pumping unit on the surface to the piston assembly located downhole near the fluid reservoir. Made from high-strength steel alloys, sucker rods have an elongated solid bar construction with threaded ends. They are connected via couplings to form the rod string that extends into the wellbore.
(1)The key functions of sucker rods include:
- Transmitting pulling and lifting force to lift the oil/water to the surface
- Withstanding the dynamic cyclical loads during pumping action
- Providing the mechanical linkage between the surface pumping unit and the downhole pump
- Accommodating the transverse loads when the rod string follows the deviations of the wellbore
(2)Proper selection of sucker rod material, diameter, and grade ensures adequate structural integrity to perform these functions over long duty cycles. Periodic inspection and preventive maintenance of rods are required to detect corrosion and damage issues early.
3. Polished Rods
The polished rod forms the uppermost section of the rod string that connects via a coupling to the horsehead pivoting beam of the pumping unit. As the name suggests, polished rods have a smooth polished exterior surface. They are usually made from corrosion-resistant stainless steel or chrome-plated alloy steel.
(1)The key functions of a polished rod are:
- Providing a smooth surface to seal with the stuffing box and prevent leakage
- Minimizing wear and friction as it slides through the stuffing box
- Serving as a visual indicator of the downhole pump condition
- Protecting the rod string from corrosion damage
(2)Polished rods are machined to close diametrical and straightness tolerances. Their smooth surface finish allows slick lubricants to be applied to minimize friction. Regular inspection of the rod surface is needed to detect any pitting or damage.
4. Key Differences Between Sucker Rods and Polished Rods
Though sucker rods and polished rods work together during the pumping process, there are some notable differences between them:
- Sucker rods have coarse external surfaces as against the polished smooth exterior of polished rods
- The former make up the majority of the rod string length with the latter only at the top
- The former are threaded at both ends whereas the latter have threading only on one end
- The latter are made from specialized corrosion-resistant alloys while sucker rods use high-strength steel
- The former experience higher alternating stresses from lifting load whereas the latter see lower static bending
- The latter require protection from surface corrosion while sucker rods face internal corrosion from well fluids
- Failure point of rod string is more likely to occur in sucker rods than polished rods
- Inspection and maintenance of sucker rods involves greater time and cost
Sucker rods and polished rods work synergistically to convert the rotary motion of the pumping unit to a reciprocal movement that lifts oil/water to the surface. While sucker rods do the heavy lifting downhole, polished rods provide a smooth transitional interface at the surface. if you are looking for a trusted manufacturer, you can choose VIGOR, please contact us at info@vigorpetroleum.com.
