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. 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.
DOUBLE FACE-TO-FACE SEALS
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.
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
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
to the double cartridge 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
) in these applications are ethylene glycol, methanol, and