SUBJECT: Some more things you should know
about centrifugal pumps 11-4
The limitations of a
magnetic drive pump
- They are less efficient than conventional centrifugal
- They operate in a narrow window. You cannot pump too far off
the best efficiency point (B.E.P.)
- They use sleeve bearings instead of precision bearings with
correspondingly more radial movement.
- The product you are pumping must be a lubricant for the
- The product you are pumping must be clean because of the very
narrow clearances in the bearings and between the housing and the
magnets. This means you are almost always limited to the pumping
of a finished product.
- Be careful of products that are sensitive to an increase in
temperature. The product will get warmer in the close clearances
you find in magnetic drive pumps.
- Do not run the pump dry, you will trash it
you switch from anti-friction ball and roller bearings to
hydrodynamic (sleeve) bearings in a centrifugal pump?
- Any time the DN number exceeds 300,000 (Bearing bore times
- If the standard bearings fail to meet an L10 life of 25.000
hours in continuous operation or 16,000 hours at maximum axial and
radial load and rated speed.
- If the product of the pump horsepower and speed in rpm, is 2.7
million or greater.
Increasing the impeller speed increases the efficiency of
- About 15% for an increase from 1500 to 3600 rpm.
- Less dramatic at lower speeds.
- Maximum efficiency is obtained in the specific speed range of
2000 to <3000
If the wear ring
clearance is too large:
- The pump will take on excessive vibration caused by internal
recirculation. This can cause seal and bearing component
- The pump will not meet its designed capacity because of
- Wear rings should be replaced when their clearance doubles.
This additional clearance will increase the pump power
requirements with the amount varying according to the specific
speed( NS ) of the impeller
- NS 200 14% increase
- NS 500 7% increase
- NS 2500 Insignificant increase
normally throttled with a discharge valve, but in rare cases it can
be done with a suction valve.
- You must have sufficient NPSH to prevent cavitation.
- Suction throttling prevents the over heating caused by
discharge regulation. This can be important with fluids like jet
fuel where the additional heat could vaporize the fluid.
Because an overhung impeller does not require the extension of a
shaft into the impeller suction eye, single stage impellers are
preferred for pumps handling suspended matter such as sewage.
Electric motors are sized considering the specific gravity of the
liquid being pumped. If a low specific gravity pump is tested with
water, or any higher specific gravity fluid, the increase in motor
amperage could burn out the motor.
Do not hydrostatically test a high temperature pump with water.
Water trapped in small recesses and gaskets will flash to steam in
high temperature applications, expand and then break something.
There are several ways to
prime a centrifugal pump with a suction lift:
- Fill it full of liquid prior to starting.
- Install a foot valve in the suction piping to prevent the
fluid from draining back to the sump. Be careful of these valves,
many of them leak and defeat the purpose of installing them in the
- Install a vacuum pump in the discharge line to pump out any
- Install a priming tank in either the suction line, the
discharge line or both.
- Purchase a self priming pump.
Pumps with variable
speed drives have several potential problems:
- The fluid viscosity can change with speed if it is a non
- The shaft can hit a critical speed.
- You can get too much capacity that can burn out the
- Operating off the BEP can cause shaft deflection.
- Explosion proof motors must be approved to operate over the
entire operating range. At the lower rpms the cooling fan is not
rotating fast enough.
- Variable speed demands may affect the electrical power
distribution system by reducing electrical demand.
- The mechanical seal has to be designed to operate over the
entire speed range. At higher speeds the design has to be of the
stationary type with the spring face load reduced.
- At higher shaft speeds the NPSH requirement is higher to
prevent cavitation problems.
You cannot vent a running pump. Centrifugal force throws the
liquid to the outside of the volute leaving the air at the eye of the
off the BEP can break the pump shaft because the force is always in
the same direction while the shaft is turning. This has the affect of
flexing the shaft twice per revolution. In many cases you can easily
exceed the endurance limit of the shaft material.
- The stresses imposed in reverse bending are cumulative.
- Most fatigue failure occurs in one million cycles or less. At
1750 rpm you get 2,520,000 cycles per day.
- If a 300 series, stainless steel shaft is running in a fluid
containing chlorides, the shaft is subject to chloride stress
corrosion problems that can be another cause of shaft cracking and
Slurry pumps have some features
that make them different than chemical pumps.
- The pumps are more massive
- Looser tolerances.
- The clearances are more open.
- Parts have blunt rather than tapered edges.
- The metal parts are harder.
- They utilize "through bolt construction" because it is
difficult to drill and tap the harder metal.
- Some designs are rubber lined to absorb the impact of abrasive
- They are less efficient than chemical pumps.
- Many slurries are dilatants. Their viscosity increases with
agitation. You may have to convert to a positive displacement
- Kaoline or china clay is a good example. Some sugar syrups
fall into this category also.
If you need a
pump with high head, low capacity features:
- High speed centrifugal pumps are the most popular.
- Multistage vertical and horizontal pumps are another
- Regenerative turbine pumps work well, but the necessary close
clearances dictate only clean fluids.
- Gear or rotary positive displacement pumps work well, but they
have slippage problems in low viscosity service and their very low
capacities may not be sufficient for the application.
- Metering pumps are good for very low flow, but the inherent
pulsations can damage some instrumentation.
- You can connect single stage centrifugal pumps in series if a
single pump cannot meet the head requirements.
- Partial emission pumps can operate at a specific speed as
little as two (2). They utilize a "Baske" straight vane impeller
with a diffuser that allows flow from a small section of the
impeller channels to pass to the pump discharge at any time (hence
partial emission). This pump was developed during world war II to
handle the high head low flow rate requirements of the German ram
jet fuel pump.
- Throttling a centrifugal pump to get a high head will cause
- The resultant shaft deflection can damage the seal or break
- Internal recirculation can overheat the volute and cause
- A high differential pressure across the pump can damage
close internal clearances.
- The power loss can be expensive.
- The increase in stuffing box temperature can cause a
premature seal failure.
The optimum control valve location is within five feet (1,5
meters) of the pump discharge to prevent too much surging of fluid in
the system when the discharge is throttled.
- The optimum pipe size will consider the installed cost of the
pipe (the cost increases with size) and the pump power
requirements (the power required increases with pipe
- Try to limit the friction loss at design flow to 2-5 feet for
each 100 feet (1-2 meters for each 30 meters) of pipe).
- To prevent the settling of solids you need a minimum velocity
of about 4 to 7 feet per second (1.5 to 2.5 meters per
- Velocities of no more than 10 feet (3 meters) per second are
recommended in the suction side piping to prevent abrasive
Here is the proper way to
vent a centrifugal pump after it has been installed, or the system
has been opened. I am assuming the pump is empty of liquid and both
the suction and discharge valves are shut.
- Open the suction valve. The pump fills part way.
- Close the suction valve.
- Open the discharge valve part way. Once the pressure equalizes
the air will rise in the discharge piping.
- Open the suction valve.
- Start the pump.
If you are using a high speed pump
(greater than electric motor speeds) there are some additional things
- You must go to a stationary seal design if the seal face
surface speed exceeds 5000 fpm. (25 meters/sec). These designs use
a hydraulic balance ratio of about 60/40 instead of the
conventional 70/30, and the spring load on the seal faces drops
from 10-30 psi.( 0,7 to 2 kg/cm2) to 8-15 psi. (0,5 to
1 kg/cm2 ).
- You will probably have to install an inducer if the suction
specific speed of the pump is greater than 12,000. Be sure to
remember that although a high speed inducer can generate an
additional 25-100 feet (10-30 meters) of head, you cannot use this
additional head when sizing the pump because of inlet losses at
- At higher shaft speeds the bearing oil level is critical to
- Be aware that ball bearings have speed limits:
- The bearing bore, in millimeters, times the rpm must not
- The pump horse power times the rpm must not exceed 2.7
- Cavitation is always a problem when you have the combination
of a high speed pump and low specific gravity fluid.
- If you double the speed of a pump, abrasives will cause eight
times the wear you would experience in the slower speed pump
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