A double cartridge type mechanical seal is a pre-assembled package of seal components (including 4 seal faces) making installation much easier with fewer points for potential installation errors to occur.
A Cartridge Seal assembly is "pre-set" so that no installed length calculations must be performed for determining where to set the seal. Only the external seal in a double cartridge have "set tabs" that are removed once the seal is installed and the pump assembled.
Some products are not compatible with a single cartridge mechanical seal. Examples include toxic liquids whose leakage into the environment would be hazardous, liquids whose suspended abrasives would rapidly wear the faces, or corrosive liquids requiring seals made of costly materials. In recent years most states are requiring double mechanical seals to maintain fugitive emissions limitation requirements. In an effort to limit water usage, many users are converting to double seals. There are two ways to design for such products.
One method is to incorporate environmental controls. This method is the more economical solution than the second option of incorporation high priced metals that may be compatible with the corrosive liquids being pumped. The use of a double seal allows the containment of clean fluid between the sets of faces. This allows the seal faces to only see clean lubricating fluid, greatly extending the life of the seal.
There are two basic double seal arrangements: tandem and face-to-face. A double cartridge mechanical seal can be configured in either configuration. It is necessary to understand the differences between these arrangements in order to select the best design for a specific application.
The double face-to-face seal is usually cartridge mounted with one seal inside the stuffing box and one seal on the outside. This arrangement is ideal for pumps with shallow stuffing boxes. Both seals rotate against common or separate stationary inserts.
The face-to-face arrangement may be used as a double seal. If the liquid between the seals is at a higher pressure than the product in the stuffing box, the inner seal is lubricated by a sealing liquid. This creation of an artificial environment for the mechanical seal is the role of a true double seal, if the liquid is circulated between the seals at a lower pressure than the equipment stuffing box pressure, the purpose of the inner seal is the same as any single seal, and the outer seal simply serves as a back-up.
The double face cartridge, assembly, is compact easy to install and can be bench tested with using air as a barrier fluid.
The weakness of a face to face arrangement is the exposure of the inner seal to abrasive or corrosive materials.
A tandem seal increases on line reliability and is considered the safest environmental design. If a primary seal failure occurs on a critical piece of equipment, the outboard seal can take over and function until maintenance of the equipment can be scheduled. Alarm systems can be installed with censors to determine an increase in the pressure between the two sets of seal faces. This is easily established by adding a pressure gauge on the line coming from the seal pot to the double cartridge seal.
Tandem seal systems are used for products such as vinyl chloride, carbon monoxide, light hydro carbons, and a wide range of other volatile, toxic, carcinogenic, or hazardous fluids. Tandem seals eliminate icing and freezing of light hydrocarbons and other fluids which could fall below the atmospheric freezing point of water in air (32°F or 00C) because both seal faces are inside the stuffing box.. Typical buffer fluids (contained in the seal pots ) in these applications are ethylene glycol, methanol, and propanol.
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