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.

Stuffing boxes: the seal and the wellhead pressure limit

The stuffing box seals the annulus between the polished rod and the wellhead and is the part that most often leaks on a rod pumped well. The design follows from the wellhead pressure and from whether the well produces casing gas, and the box has to be matched to the polished rod size and to the pumping tee it sits on.

What the packing choice actually depends on

Packing material is not chosen by preference; it is chosen by what the well produces and by the polished rod surface it runs against. Three inputs decide it, and all three come from the well programme rather than from a catalogue:

Two failure modes drive most call-outs. Packing that is incompatible with the produced fluid swells or hardens within days instead of wearing gradually, and a box that is tightened to stop a leak on a damaged polished rod simply transfers the wear to the rod. Both are decided by the data above, not by tightening harder.

What to send with a stuffing box enquiry

• Polished rod diameter, and the wellhead connection the box has to make up to.
• Wellhead pressure, and whether the well produces casing gas or is subject to intermittent flow or injection.
• Whether a standard box is acceptable or a high pressure box is required, and how many packing rings the box has to carry.
• Packing requirement: whether packing is supplied fitted, and the fluid and temperature data the material has to suit.
• Whether a secondary or injection-type seal is needed, and whether the well has to stay on production during repacking.
• Certification, marking and whether a third party inspects before shipment.

Packing grade and box selection are confirmed against the well data supplied by the buyer. Where the fluid or temperature data is not available, the box is quoted to the stated pressure class and rod size and the packing is confirmed before dispatch.

Grades supplied and minimum properties

The grade fixes the minimum strength and the chemistry. The table below gives the requirements that separate the forged fitting grades ordered most often; the full composition and mechanical table for each grade is on its own page.

The complete chemistry and mechanical table for each grade, with the notes that qualify it, is on the individual grade page – for example ASTM A105 flange. The stainless grade has the higher tensile requirement but the lower minimum yield strength, so the two are not interchangeable on the strength table alone. The body material has to suit the produced fluid; the sealing element is chosen separately from the grade.

Source: specified minimum yield and tensile strength for each grade, per ASTM A105 / ASME SA105 and ASTM A182 / ASME SA182. The standard named against each grade is the governing document.

• A105
• A350 LF2
• F316

Download Stuffing Box data sheet (PDF) One page: supply range, grades and key sizes, what to state on the enquiry, and the inspection documents that come with the order. The standard is the governing document.