Dust seals and vapor seals are used primarily when the main objective is to reduce the possibility of product contamination from external sources, such as dust or rain water. Vapor seals can also be used to reduce evaporation from the tank, or to reduce the odor of a foul smelling product escaping from the tank. The most typical styles of dust and vapor seals are as follows:

DUST CUPS are essentially an inverted plastic cup mounted on the shaft with an 0-ring. The cup is designed to slip down over a small nozzle on the tank roof which the shaft passes through, acting like an umbrella to divert water and debris from entering the tank. Dust cups are not designed to hold any pressure, in fact vapors can pass around the space between the cup and the nozzle.

EXCLUDER SEALS are a type of lip seal, but rotate with the shaft. The body of the excluder is usually split and held onto the shaft with a spring or "zip strip". A skirt extends downward and rides directly on the tank roof, creating a simple seal. 

SPLIT RUBBER DUST SEALS are comprised of two sheets of 1/4" thick rubber bolted to the tank roof which cover the hole where the mixer shaft enters the tank. Each sheet is split and has a tight clearance hole in the center which rides on the shaft. The splits are staggered to minimize leakage.

VAPOR LIP SEALS normally consist of two lip seals mounted back to back and ride on the shaft to create a seal. The seals are usually mounted in a flange made of mild steel, mild steel faced with stainless or UHMWPE. The flange can be attached to the mixer drive and perform as a mounting base for the mixer, or remote mounted on the tank roof below the mixer drive. When attached to the drive, vapor seal flanges are registered to guarantee concentricity. If remote mounted, it is critical to adjust the flange so that it is centered around the shaft, or the flange may wear on the shaft and cause damage.

The family of shaft seals known as mechanical seals is the most advanced type of seal used in the mixing industry. They can handle the highest pressures, maintain nearly leak free operation, and require minimum maintenance if installed and operated properly. The down side is the higher initial cost (both for the seal and for the more complicated equipment required surrounding the seal) and the higher level or expertise needed to service the seals. Mechanical seals are increasing in popularity due to the growing environmental restrictions regarding any leakage from process tanks.

BASIC PRINCIPLES

There are hundreds of mechanical seal designs, but they all are variations of a basic layout consisting of a collar mounted on the shaft which uses springs to push a ring (which also rotates with the shaft) against another ring which is held stationary. The rings rotate against each other riding on a thin layer of lubricant, and the springs hold them so tightly together that leakage through the seal is reduced to an immeasurable amount. The mating surfaces of the rings must be perfectly flat to seal properly, and are manufactured to tolerances measured "light- bands". The rings must also be extremely hard to endure the pressure and wear, so they are usually made up of ceramic, carbon, silicon carbide, tungsten carbide or similar materials. The stationary "seat" is held in place and maintains a static seal with the mounting housing using gaskets or 0-rings. The rotating elements of the seal must attain a static seal with the shaft using 0-rings, wedges or packing.

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