About Mechanical Packings

The problem of containing fluids in pumps has existed since the invention of the lift pump many centuries ago. Initially the rods on these early reciprocating pumps were “seated” with lubricated ropes. Since that time, many types of fluid handling equipment have been developed which require increasingly sophisticated sealing devices.

The oldest of these devices is still the most common – Mechanical Compression Packing – so named because of the way it performs its function. Mechanical Packings are made from various compliant fibers and appropriate lubricants that are selected for their suitability in specific uses. These materials are usually braided on specialized machines designed to generate a square cross section. This ‘square rope” is then cut is then cut into a number of rings that are “fitted’ to the equipment shaft and inserted into the “stuffing box” (an annular space about the shaft in the equipment housing) which retains the packing. See fig 1.

The installed packing is “compressed” by tightening a gland against the packing, which is then forced to expand radially between the shaft and housing. This creates a “seal”.

Mechanical packings are the easiest sealing device to install, require little maintenance after installation, and when properly selected and installed, provide reliable service.

Mechanical compression packings used in both reciprocation and rotating equipment, depend upon the existence of a thin fluid film between the packing surface adjacent to the moving shaft for lubrication and cooling. Upon start-up, this film is initially provided by lubricants melted out of the packing itself (break-in lubricant). Shortly after start-up, the fluid being sealed (or a lubricating barrier fluid) provides this cooling lubrication on a continuing basis. If a clean barrier fluid (often called seal water) is used, there is usually a “lantern ring” inserted in the packing set at the axial location of the barrier fluid inlet.

During the break-in period, the gland is adjusted to gain control of leakage to reduce it to a minimum while maintaining the cooling and lubrication film on the shaft. If this process is too rapid or if the lubrication is eliminated by over-tightening the packing set on the shaft, the packing can “burn out”. When this occurs, the inner surface of the ring set “glazes” (becomes hard and non-compliant) and the equipment must be repacked.

The following steps are important to achieve the greatest value in service (success) with mechanical packings:

1. Proper selection of the packing material. The packing style must be carefully selected for a given application (equipment) and service (the fluid being sealed)
2. Proper environment for the packing material. The controls to maintain the lubricating film must be provided.
3. Proper equipment maintenance. The equipment must be in good operating condition – a true running, balanced shaft with a smooth polished sleeve ensures the longest service.
4. Proper installation. Attention to fit and cleanliness during installation is essential.
5. Proper break-in procedures must be followed. A little tender loving care at start-up can extend the service life of a packing set many times.

Properly installed, mechanical packing can provide the lowest cost and the highest value in service of any sealing device.

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