Sinker Bar

Why Sinker Bars Are Used

A sinker bar is a heavy rod, either solid or of heavy wall construction, installed immediately above the pump. Its purpose is to add weight to the lower end of the rod string. This weight performs two functions. First, it keeps the rod string in tension during the downstroke. Without sufficient weight, the lower rods go into compression on the downstroke, and a rod in compression buckles, rubbing against the tubing and eventually failing in fatigue. Second, it provides the force needed to overcome the friction and the buoyancy of the pump plunger, so the plunger follows the rod stroke rather than lagging, which improves pump efficiency and reduces rod loading. The required sinker bar weight depends on the pump size, the fluid viscosity and the pumping speed, and it should be calculated as part of the rod string design rather than estimated.

Solid versus Heavy Wall Sinker Bars

Sinker bars are made in two forms. The solid bar is machined from solid round bar to the same outside diameter as the rod string section it is installed in, so it has a much smaller internal bore or none at all. The heavy wall bar is made from thick wall tubing, giving a larger bore through the center while still providing additional weight. The solid bar gives the greatest weight per foot, which is useful where space in the string is limited, but it is heavier to handle and has no internal passage. The heavy wall bar is lighter per foot but is easier to handle and allows the string to be run with a wireline or with a through bore if required. Both use standard sucker rod threads so they can be inserted directly into the string with a coupling. In sour service, the grade must meet NACE MR0175, since the sinker bar is under full tensile load.