Stuffing Box

Function and Sealing Principle

The stuffing box seals the small annular gap between the polished rod and the wellhead as the rod reciprocates. It is the primary barrier that stops well fluids and gas from escaping at the wellhead. Sealing is achieved by a stack of elastomeric packing rings compressed axially by a nut or a follow screw, which forces the rings into contact with the polished rod and the box bore. A lubricator, or oil saver, is often fitted above the packing so that oil can be introduced to lubricate and cool the packing, and a bleed valve allows the pressure to be released before the packing is adjusted or changed. Because the packing is a wearing item, it is expected to be tightened periodically and replaced at intervals; the key to long packing life is to keep the polished rod surface smooth, straight and free of burrs, since a rough rod will cut the packing quickly.

Standard and High Pressure Stuffing Boxes

Stuffing boxes are selected by the wellhead pressure and by the polished rod size. A standard box is used for wells with low wellhead pressure and ordinary service. A high pressure box has a heavier body, more packing rings and often a secondary seal or a hydraulic feature, and is used where the wellhead pressure is high, where the well produces gas, or where an unexpected pressure surge is possible. A related consideration is sour service: where H2S is present the body material and the packing must both be compatible, since standard packing compounds can degrade and lose sealing in sour conditions. Many operators also specify a stuffing box with an anti-rotation or a self-aligning feature to accommodate slight misalignment between the polished rod and the wellhead, which is a common cause of premature packing failure.