SUBJECT : Pump selection
practices that cause high seal and bearing maintenance problems.
Purchasing well designed hardware does not bring
automatic trouble free performance with it. The very best equipment
will cause problems if it was not designed for your particular
application. Here are a few of the more common selection problems we
find with centrifugal pumps:
- Buying the same size pump as the one that came
out of the application. That's O.K. If the old pump was the
correct size, but the odds are that it was too big because of the
safety factors that were added at the time of purchase. This will
cause the pump to run off of its best efficiency point (bep) and
you will spend a lot of production money for the additional power
that is needed to run against a throttled discharge valve or
orifice installed in the discharge piping.
- Buying to a standard, or making a decision
based on efficiency, and believing that these two some how relate
to quality. Standards were written for packed pumps. When a
mechanical seal is being used the shaft L3/D4
number is almost always too large. Efficiency is always gained at
the expense of maintenance. Efficiency means tight tolerances and
smooth passages that will eliminate reliable double volute designs
and keep the maintenance department busy adjusting tight
tolerances to maintain the efficiency you paid for.
- Series and parallel installation problems. We
often find pumps installed in parallel, but no one knows it
because the second pump was installed at a much later date and no
one has bothered to trace the piping. Pumps in parallel require
that they have the same diameter impeller and that they run at the
same speed, or the larger pump will throttle the smaller one
causing it to run off the best efficiency point, deflecting the
shaft. The capacity should be looked at if the higher capacity
pump might exceed the N.P.S.H. available.
- When pumps are installed in series the
impellers must be the same width and they must run at the same
speed or the higher capacity pump will either cavitate because the
smaller capacity pump can not feed liquid at the proper capacity,
or it will run throttled if it is feeding the smaller pump. In
either case the larger of the two pumps will be adversely
- Purchasing a larger pump because it will be
needed in the future. Will raise the operating cost to
unacceptable levels (Power = head x capacity) as the pump is run
against a throttled discharge valve. This inefficient use of power
will translate to a higher heat environment for the seal along
with all of the problems associated with shaft
- Using a variable speed motor to compensate for
a pump curve that is not flat enough. Many boiler feed pumps
require a flat curve so that the pump can put out varying
capacities at a constant boiler pressure (head). We see this same
need if we are pumping a varying amount of liquid to a very high
- Varying the speed of a pump is similar to
changing the diameter of the impeller. If you look at a typical
pump curve you will observe that the best efficiency point (bep.)
comes down with impeller size to form an angle with the base line
(capacity line) of the graph. This means that if you vary the
speed of the impeller, the pump always runs off the bep. except in
the case where the system curve intersects the pump curve, or in
the case of an exponential system curve such as we find in a
typical hot or cold water circulating system.
- Double ended pumps installed in a vertical
position to save floor space makes seal replacement a nightmare
unless you are using split or cartridge designs.
- Specifying a desired capacity without knowing
the true system head. You can't guess with this one. Some one has
to make the calculations and "walk the system". The present pump
is not a reliable guide because we seldom know where it is pumping
on its' curve. Chart recorders installed on both the suction and
discharge side of the pump will give a more accurate reading of
the present head if they are left on long enough to record the
differences in flow. The trouble with this method is that it will
also record a false head caused by a throttled valve, an orifice,
or any other restriction that might be present in the
- Requesting too low a required N.P.S.H.
will cause you to end up with a different kind of cavitation
problem. See another paper in this series for information about
- Failure to request a "center line design" when
pumping temperature exceeds 200°F (100°C) it will cause
pipe strain that will translate to wear ring damage and excessive
mechanical seal movement.
- The use of "inline" pumps to save floor space.
Many of these designs are "close coupled" with the motor bearings
carrying the radial and thrust loads. Because of typical L3/D4
numbers being very high, the wear rings act as "steady bearings"
after the pump is converted to a mechanical seal. The pump should
have been designed with a separate bearing case and a "C" or "D"
frame adapter installed to connect a motor to the bearing
- Thrust bearings being retained by a simple
snap ring. Beyond 65% of its rated efficiency most centrifugal
pumps thrust towards the pump volute. The thin snap ring has to
absorb all of this axial thrust and most of them can not do it
very well .
- The mechanical seal has been installed in a
packing stuffing box that is too narrow to allow free seal
movement. If a mechanical seal was specified, the pump back plate
should have been manufactured with a large diameter seal chamber.
In most cases the stuffing box recirculation line should be
installed from the bottom of this large seal chamber to the
suction side of the pump or a low pressure point in the system.
There are some exceptions to this, however:
- If you are pumping at or close to vapor
- If the entrained solids have a low specific
- If you are using a Flowserve (Duriron pump)
that adjusts to the back plate.
- If you are using a double suction pump
where the stuffing boxes are at suction pressure.
- High temperature applications have several
- A jacketed stuffing box that isolates the
pumpage from the stuffing box contents by a carbon bushing to
retard heat transfer.
- A centerline design to compensate for
- A cartridge seal design that allows open
impeller adjustment after the pump has come up to operating
- A stainless steel shaft to retard heat
transfer to the bearings.
- A method of cooling the bearing oil, but
never the bearings.
- A coupling that will compensate for axial
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