HOW COMPRESSION PACKINGS WORK
Proper function of compression packings used in rotating or reciprocating pump service normally depends on a fluid film between the surface of the moving component of the equipment and the packing for lubrication. Sources of this fluid film are either the self-lubricating properties of an advanced packing material, the leakage of the fluid pumped, the built-in lubricants in liquid, solid, or combination form, or an external lubricant supply.
On equipment start-up, built-in lubricants may be released from the packings by gland pressure to provide initial lubrication and sealing. During service, continuous fluid film lubrication is provided by ongoing adjustment of the gland pressure and any combination of these sources: the built-in lubricants, the packing material, the pumped service fluid, or the external lubricant. If the pumped fluid must not flow through the packing rings, the advanced packing material or built-in lubricants must block its passage. When pump suction is under vacuum, an external means of lubrication such as a lantern ring (seal cage) may be used. For a positive pressure configuration, see the corresponding setup.
Gland pressure is regulated to provide optimum lubrication to seal and prevent overheating and consequent damage to the shaft or rod. The built-in lubricants that are lost gradually during the operation of the equipment are compensated by further gland adjustment. In a live loaded packing system, the gland follower will continue to push against the packing even when packing volume is lost by friction, extrusion, consolidation, etc. The effective service life of the packing is reached when adjustments to the stuffing box gland do not result in consistent control of the leakage from the packing seal. When the packing density increases due to consolidation and/or extrusion of the packing, this process often results in volume loss that can no longer be compensated by gland adjustments, either because the gland bottoms out against the stuffing box, or leakage level does not respond to the adjustments.
Compression packings used in valves with slow or infrequent motion or static operations are required to seal without leakage. Various impregnants may be used to assist in this function. Some packings are designed with non-migrating stable lubricants for operation under extreme pressure and temperature. Refer to Types of Packing Materials and Lubricants for descriptions of lubricants and impregnants.
LUBRICANTS BUILT INTO THE COMPRESSION PACKINGS, LEAKAGE OF THE PUMPED FLUID, AND/OR EXTERNAL LUBRICANTS SUPPLY THE FLUID FILM LUBRICATION BETWEEN THE PACKING AND MOVING COMPONENT.