Sharpe Stuffing Glands
A stuffing gland, or stuffing box, is the oldest shaft seal design known to man. Although many refinements have been made over the years to improve the performance, the basic seal is not much different from when the first man packed rags around a shaft to minimize leakage on the water wheel. A modern stuffing gland utilizes ropes of specialized "packing", square in cross section, which wrap around the shaft and are compressed with a "follower' ring. The rings are split for ease of installation, and the splits must be staggered for proper sealing. The number of packing rings varies from two to eight, depending on the application. Glands with two or three rings are generally known as low pressure glands, those with more rings are referred to as high pressure glands. The compression of the packing creates friction against the shaft and will generate heat, so the packing must be self-lubricating or be lubricated from an external source to reduce the friction. The source may be through a grease fitting in the gland, steam purging, or may even be the product in the tank on a side entry unit (high-pressure glands only). Even with lubrication, over-tightening a stuffing gland will increase the friction and the heat generated, and can cause burning of the packing and damage to the shaft if neglected.
All stuffing glands leak a small amount during operation. This is often not noticed on top entry units since the product seeping through the packing is usually vapor. Side entry units are more noticeable, usually leaking a few drops per minute out the end of the gland. If the product in the
tank is toxic, or is otherwise environmentally undesirable, a mechanical seal may be a better choice.
LOW PRESSURE GLANDS
The two ring low pressure stuffing gland is a very simple and dependable seal for those top entry applications which have no more than 15 P.S.I. pressure in the tank. It is relatively inexpensive and easy to maintain, and is therefore very popular in all industries. The majority are mounted in a housing which is registered to the mixer drive to guarantee alignment. It is possible to mount the seal remotely on the tank roof, but it is critical that the seal is installed concentric with the shaft (+/- 0.01 5") of the shaft will
rub on the flange and cause damage to the equipment. Low pressure glands have no provisions for external greasing so the packing must be self lubricating.
HIGH PRESSURE GLAND
Tank pressures as high as 150 P.S.I. or more can be handled with a high pressure stuffing gland, although as the pressure increases, the life of the packing decreases. The life of the packing also depends on the type of packing used, the way the gland is lubricated and the product being sealed. If the product in the tank on a side entry mixer is abrasive, it will attack the packing during operation. A tougher packing, such as Kevlar reinforced packing, may stand up better to the abrasion from the product, but will wear the shaft faster. It is therefore recommended that tough jobs have the seal area on the shaft surfaced with hard chrome, Chrome-Nickel-Boron or ceramic hardening to protect it from abrasion. Some industries prefer to have a replaceable wear sleeve on the shaft so repairs are more easily accomplished. Packing glands with grease fittings allow lubrication of the seal using a grease gun, but the intermittent injection of grease does not provide the small continuous flow of lubrication which would be optimum. A spring loaded or weight loaded lubricator will improve the performance of a high pressure packing gland because a more continuous flow of grease will be provided to the packing. The weight loaded lubricator is the better solution of the two (and the more expensive) because the flow can be infinitely adjusted and the pressure is more consistent than with the spring loaded lubricator.
The standard high pressure gland uses seven rings of packing separated by a "lantern ring", which is essentially a spacer which allows grease from a zirc fitting on the outside of the gland to reach the shaft and be evenly distributed along the inside of the packing rings. Without a lantern ring (or if the ring is in the wrong position) the grease would pool on the outside of the packing and never reach the shaft. Heat would build in the gland and shaft scoring could occur (it is therefore very important to visually confirm the position of the lantern ring after re-packing a gland by removing the zirc fitting).
Another common high-pressure gland uses five rings of packing compressed on top of a "throttle bushing". This style gland is used mostly on side entry mixers in the pulp and paper industry. The throttle bushing acts as a bearing to minimize shaft run-out within the gland, and is machined to distribute flushing water around the shaft. The flushing cools and lubricates the shaft, and keeps the stuffing gland area clean of the fibrous product in the tank. The flow of water through the gland is best controlled with a rotometer.
All stuffing glands leak a small amount during operation. This is often not noticed on top entry units since the product seeping through the packing is usually vapor. Side entry units are more noticeable, usually leaking a few drops per minute out the end of the gland. If the product in the
tank is toxic, or is otherwise environmentally undesirable, a mechanical seal may be a better choice.
LOW PRESSURE GLANDS
The two ring low pressure stuffing gland is a very simple and dependable seal for those top entry applications which have no more than 15 P.S.I. pressure in the tank. It is relatively inexpensive and easy to maintain, and is therefore very popular in all industries. The majority are mounted in a housing which is registered to the mixer drive to guarantee alignment. It is possible to mount the seal remotely on the tank roof, but it is critical that the seal is installed concentric with the shaft (+/- 0.01 5") of the shaft will
rub on the flange and cause damage to the equipment. Low pressure glands have no provisions for external greasing so the packing must be self lubricating.
HIGH PRESSURE GLAND
Tank pressures as high as 150 P.S.I. or more can be handled with a high pressure stuffing gland, although as the pressure increases, the life of the packing decreases. The life of the packing also depends on the type of packing used, the way the gland is lubricated and the product being sealed. If the product in the tank on a side entry mixer is abrasive, it will attack the packing during operation. A tougher packing, such as Kevlar reinforced packing, may stand up better to the abrasion from the product, but will wear the shaft faster. It is therefore recommended that tough jobs have the seal area on the shaft surfaced with hard chrome, Chrome-Nickel-Boron or ceramic hardening to protect it from abrasion. Some industries prefer to have a replaceable wear sleeve on the shaft so repairs are more easily accomplished. Packing glands with grease fittings allow lubrication of the seal using a grease gun, but the intermittent injection of grease does not provide the small continuous flow of lubrication which would be optimum. A spring loaded or weight loaded lubricator will improve the performance of a high pressure packing gland because a more continuous flow of grease will be provided to the packing. The weight loaded lubricator is the better solution of the two (and the more expensive) because the flow can be infinitely adjusted and the pressure is more consistent than with the spring loaded lubricator.
The standard high pressure gland uses seven rings of packing separated by a "lantern ring", which is essentially a spacer which allows grease from a zirc fitting on the outside of the gland to reach the shaft and be evenly distributed along the inside of the packing rings. Without a lantern ring (or if the ring is in the wrong position) the grease would pool on the outside of the packing and never reach the shaft. Heat would build in the gland and shaft scoring could occur (it is therefore very important to visually confirm the position of the lantern ring after re-packing a gland by removing the zirc fitting).
Another common high-pressure gland uses five rings of packing compressed on top of a "throttle bushing". This style gland is used mostly on side entry mixers in the pulp and paper industry. The throttle bushing acts as a bearing to minimize shaft run-out within the gland, and is machined to distribute flushing water around the shaft. The flushing cools and lubricates the shaft, and keeps the stuffing gland area clean of the fibrous product in the tank. The flow of water through the gland is best controlled with a rotometer.
