Running standby pumps. Is it a good idea?
Vol 17 #03
If there were a simple answer to this question, it would never
come up. The more I look into the subject, the more I'm convinced
that there is no easy answer. Let's look at both sides. We'll begin
with reasons for alternating pumps:
- If the pump is not run, the oil will drain away from the
movable components, causing excessive wear and heat at
- Maybe the standby pump is frozen up. It's too late to learn
that when you need the pump.
- All standby equipment is subject to vibration. Bearings can be
affected by this vibration with a condition known as "false
brinneling," causing round, hard indentations in the bearing
People that do not believe in alternating pumps also have some
- Every time you switch pumps you are causing a system upset
than will probably affect the finished product. Temperatures and
pressures change and some products become viscous or solidify when
the system cools even a small amount. All of this translates to
"off product" that will end up in the alcohol plant, have to
refined all over again, or is sometimes shipped to the customer
who refuses the shipment and asks you to initiate a program that
will prevent this from ever happening again.
- Each time you let the bearing cavity cool down you are
producing moisture in the bearing case. The more often you start
the pump, the more frequently this occurs. Moisture is a major
cause of premature bearing failure
- Startup torque is five times running torque meaning that it
takes five times the power to start a pump than it takes to keep
it running. These power surges can trip breakers or, in some
cases, cause you to exceed your peak loads that will have an
adverse affect on next month's electric bill.
- Every time a pump starts the shaft thrusts towards the thrust
bearing and then somewhere close to its 65% efficiency point, the
shaft thrust in the opposite direction towards the pump volute.
This axial shifting can cause seal and bearing problems.
- Shutting down a pump will cause its internal temperature to
change and that can cause a problem with many fluids. In some
instances the shutdown pump has to be flushed out to prevent
product from solidifying in the seal or on the surfaces of the
impeller and volute. At startup, any of these solids that have not
returned to their liquid state can cause the rotating shaft to go
"out of balance."
Most knowledgeable people agree that pumps with long shafts should
be turned over on a regular basis to prevent "sag." Jacking gear is
often provided to do this.
If you decide that alternating pump makes sense to you, be careful
that you do not run the pumps for the same amount of time, or they
will both wear out with the same amount of hours.
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