What is wrong with the modern centrifugal
Ask for a pump recommendation from your favorite supplier
and chances are he will recommend one of the standard pump
designs that conform to either the A.N.S.I., I.S.O. or D.I.N.
specifications. On the surface that might seem like a good
recommendation, but the fact is that all of these designs
will cause you maintenance problems.
Please refer to the following illustration. I have pictured
some of the more obvious problems that we find with these
THE IMPELLER IS TOO FAR AWAY FROM THE BEARINGS
The pump was designed for packing and that is where the problem
starts. To produce enough axial space to accommodate at least
five rings of packing (any less would cause sealing problems),
a lantern or lubricating ring, a gland to tighten the packing
and enough room to get your hands in there, the manufacturer
had to move the pump impeller too far away from the bearings.
He is depending upon the packing to act as part of the bearing
system, especially at start up when the shaft is subjected
to its maximum radial deflection.
Impeller imbalance, vibration, misalignment, pipe strain,
cavitation, critical speeds, and other forms of shaft deflection
add to the existing problem, causing excessive movement of
the mechanical seal components.
If the pump had been designed for a mechanical seal the impeller
would have been positioned closer to the bearings saving considerable
initial investment cost (short shafts cost less money). This
was not done, however, and so the seal is jammed into the
small radial space provided for the packing.
IN MOST CASES A SLEEVE WAS INSTALLED ON THE
The shaft diameter was reduced to accommodate the sleeve
and this compounded the problem. Sleeves are installed for
- To provide corrosion resistance when building the shaft
of corrosion resistant material would be too costly.
- To provide a wear surface for packing and those seals
that frett or damage shafts.
- To position an impeller
- Some seal manufacturers use the sleeve as a convenient
method of attaching a metal bellows seal to the shaft.
In ninety percent of the cases the second reason is why
most manufacturers use shaft sleeves. To evaluate the relationship
between shaft diameter and length, familiarize yourself with
the concept of L3/D4 that was explained in several of the
earlier papers published in this series.
THE STUFFING BOX INSIDE DIAMETER IS TOO SMALL
The stuffing box cross section was narrowed to about 3/8
inch (10 millimeter) to accommodate small cross section packing.
In the smaller shaft sizes the cross section is 5/16"
(8 mm.). This narrow space does not give the seal enough room
to utilize centrifugal force to throw solids away from the
lapped seal faces, or provide enough clearance for adequate
cooling of the components and sealing fluid. This has caused
many customers to provide expensive and unreliable flushing
that could be eliminated in many instances, if there was adequate
room between the seal and the inside wall of the stuffing
THE STUFFING BOX IS TOO LONG.
The length was added to accommodate all the rings of packing
and the lantern ring. Recognizing this length as a problem
the manufacturer did not leave enough room between the face
of the stuffing box and the first obstruction, to accommodate
some of the modern cartridge double seals or the newest split
THERE IS NO DOUBLE VOLUTE TO PREVENT RADIAL
LOADING OF THE IMPELLER
This has caused customers to install inefficient "by
pass" lines to prevent shaft breakage and seal damage
at start up or when running too far off of the best efficiency
point of the pump.
MOST OF THESE PUMPS ARE OF THE BACK PULL
This means that the wet end is left on the piping and the
power end along with the adapter are brought back to the shop
for seal replacement and repair. Unless you have a seal cartridge
mounted, or you are using a split seal design, you will have
trouble making an initial impeller adjustment with most of
the open impeller designs in use today. The direction of adjustment
varies with manufacturers.
THE IMPELLER ADJUSTMENT IS ALMOST ALWAYS
MADE FROM THE BEARING END OF THE PUMP.
This means that to compensate for wear (a very common problem
when pumping abrasives) the shaft has to be moved either towards
the front of the pump volute or, as in the case of the Durco
pump, back towards the back plate. This movement can be as
much as a total of .250 inches (6 millimeter). In either case
the seal setting is disturbed and short seal life follows.
Most plants have both types of designs that causes confusion
with the mechanics.
THE WRONG MECHANICAL SEALS ARE BEING SUPPLIED
BY THE PUMP MANUFACTURER.
Unless you have specified a particular seal brand and model
number the seals are always unbalanced designs with unknown
grades of materials, having very limited application and causing
a profusion of spare parts.
Most original equipment manufacturer (O.E.M.) seals will
damage shafts (fretting) causing the use of shaft sleeves
that will weaken the shaft and raise the L3/D4 number above
60 (2 mm. the metric system)
"C" OR "D" FRAME ADAPTERS
ARE NOT BEING PROVIDED AS STANDARD EQUIPMENT.
Although not available for every pump, these adapters can
be used to eliminate the need for time consuming and costly
alignment procedures. None of the popular pumps are equipped
with jack bolts to facilitate the manual alignment and this
just compounds the problem. The result is that we find alignment
not being done at all in some cases, and done poorly in others,
The excuse is always the same, "There is no time to do
it correctly". The result is poor seal and bearing performance.
LIP OR GREASE SEALS ARE BEING PROVIDED TO PROTECT
THE BEARINGS FROM WATER GETTING IN AND DESTROYING THE LUBRICATION.
These lip seals have a design life of less than two thousand
hours (three months) and will damage the expensive shaft,
as they remove the protective oxide layer. All pump manufacturers
recognize the short life problem and they install a small
rubber ring outboard of the lip seal to try to deflect the
water or chemical away from the bearings.
Water ingestion is a major cause of bearing failure. Liquid
enters the bearing through the lip seals from three different
- Packing leakage
- From the water hose that is used to wash away packing
- From the atmosphere (aspiration) when the pump stops
and the bearing case cools down. As much as 16 ounces (0,5
liters) of air is expelled from the pump as its' temperature
increases from ambient to operating . This moisture laden
air returns through the vent or lip seals as the bearing
case cools down at pump shut off.
The problem with water ingestion can easily be solve by
replacing the lip seals with mechanical face seals and providing
an expansion chamber on the bearing case. Labyrinth seals
are another solution although they are not as totally effective
as face seals. Neither the labyrinth seals nor the face seals
should cause fretting problems at the bearing location.
THE BEARING LUBRICATION SYSTEM IS POORLY DESIGNED.
- The oil level must be located half way through the lower
ball of the bearings when the pump is shut off. You need
a good sight glass to see this location. Most pumps do not
have a proper sight glass and an oiler doesn't make any
sense since there is no place for the oil to go, and it
cannot wear out.
- Greased bearings applications have no protection to prevent
over greasing. The recommended greasing procedures generally
are not followed
- An oil mist system would be the best if you could solve
the problem of leakage of the mist to atmosphere and the
resultant fugitive emissions problems.
- If you open the bearing case of your spare power ends
you will find that the inside of the case is often badly
rusted. The manufacturer should have provided some type
of a protective coating to prevent this problem. If you
elect to provide your own coating (and you should) be careful
about using synthetic oils for your bearing lubrication.
They contain strong detergents and can remove many of these
A RECIRCULATION LINE HAS BEEN INSTALLED FROM
THE DISCHARGE SIDE OF THE PUMP BACK TO THE STUFFING BOX..
Many liquids contain solids. Centrifugal force will throw
these solids against the inner wall of the volute and out
this recirculation line. They will then enter into the stuffing
box at high velocity, causing premature seal failure.
In most cases the problem can be solved by eliminating this
line and connecting a new line from the bottom of the stuffing
box to the suction side of the pump. This will recirculate
fluid from behind the impeller, (where it is much cleaner)
through the stuffing box, and back to the suction side.
- CAUTION do not connect to the suction side if you are
pumping the fluid at or near its vapor point. It could flash
in the stuffing box.
This system is not as effective if you are using an open
impeller design that adjust towards the back plate (Durco
as an example)
THE THRUST BEARING IS BEING RETAINED BY A SIMPLE
Up to 65% of its efficiency most centrifugal pumps thrust
towards the thrust bearing, but between 65% and 100% of the
pumps efficiency (the normal running mode) the thrust is towards
the pump volute and this means that the simple snap ring is
carrying the whole load. This is the reason we see so many
bent and broken snap rings. A more positive retaining system
THE WET END IS NOT A CENTER LINE DESIGN
The above illustration explains the centerline concept. This
design will compensate for metal expansion at the wet end
of the pump. It should be specified every time the pumping
temperature exceeds 200° F (100° C).
Note that the volute is being supported on its sides. This
will allow thermal growth to take place both up and down eliminating
a great deal of suction pipe strain, wear ring damage and
subsequent seal misalignment at the stuffing box face.