Cartridge seals are self-contained units consisting of a shaft
sleeve, seal, and gland plate. The unit is fitted onto the pump shaft as a built assembly,
and no further fitting is required.
Cartridge seals are an attempt to over-come the fitting problems
of conventional seals. They are supplied in single and double form, and the gland plate is
often fitted with various tappings to provide for flushing, cooling water injection, and
disaster control. The cartridge can overcome several limitations of the standard pump
gland. Tangential tappings for introducing fluids into the gland cause less disturbance to
the seal face than the radial tappings often found on pump units. Look again at the
tapping provided for flush water to the
packed gland, a tangential
tapping would provide a less turbulent water source. For a packing gland this may not be
important but to a mechanical seal it can be another adverse condition with which to cope.
The setting position of the seal is set by the cartridge
design, but the seal unit should not be screwed to the shaft until the gland plate is
secured to the pump and any adjustments made to the shaft position. Once this has been
completed the set screws can be tightened and the spacers removed.
So, what can go wrong. The design works to eliminate many of the
common causes of seal failure on installation and because the seal is presented on its own
shaft sleeve any damage that might be caused by a conventional seal to the pump unit is
also eliminated.
The problem is that the seal has to be very compact to fit into
the available space. Seals require mass to produce as high a co-efficient of inertia as
possible. The higher the mass the less affected the rotation parts are to transient
vibrations. Such vibrations will be absorbed lessening the tendency to vibrate the seal
faces apart. Remember if the seal faces open then they are immediately vulnerable to
contamination from the myriad dirt particles swirling around your plant. When the seal
faces close those particles are trapped, in a carbon ceramic combination the particles
become embedded in the carbon and begin to score away at the mating face. I must make it
clear that all seals fitted into gland boxes pre-owned by a packing gland are subject to
these problems. All seals are a compromise on dimensions, they are no good if they do not
fit! The specific problem of the cartridge seal is that the components further reduce the
available space into which a large enough mass can be inserted to cope with vibrations.
If you are going to use this type of seal then it is important
that the unit you fit it to is in good condition. Bearings within tolerance, shaft unbent,
impeller in balanced condition, system not experiencing the effects of cavitation.
You will pay a lot of money for a first class seal unit, so make
sure you achieve all of the benefits by checking out the pump unit for defects.
I scheduled an upgrade for a series
of boiler water circulating pumps working on a central heating
system for a Naval base complex. The refurbishment of the
system was carried out by engineering contractors. To ensure
that as few problems occurred as possible Chesterton 123 cartridge
seals were specified. Fit and forget. One of the four
seals failed with 14 days of fitting. The boiler house
operator reported a strange "buzzing" sound coming from the pump
unit. It was a moments work to identify the possible cause,
the noise was coming from the rubber shaft coupling. In
addition the seal emitted an intermittent high pitched scream and
steam could be seen puffing out of the gland area.
The motor shaft axis was found to be a full 0.032"
higher than the pump shaft axis. The rubber coupling compensated for the extreme
misalignment but in so doing a vibration was set up in the shaft which bounced the seal
off its seating. The hot water in the system flashed off to steam across the seal
face so causing the seal face to run dry. The emerging steam could be seen at the
gland and the screaming noise indicated that the seal had re-seated.
The fault lay with the re-installation of the pump
unit. There was nothing wrong with the seal installation and the other three seals
ran for many years without any trouble. The fourth seal had to be re-built and once
the motor feet had been machined, it too ran for many years trouble-free.
It only takes a few minutes to carry out the installation
checks, so why waste time rebuilding pump units a second time when you can get it right
first time around?
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