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 pumps.
• 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 bearings.
• 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

When do 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 rpm)
• 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 centrifugal pumps.

• 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 damage.
• The pump will not meet its designed capacity because of internal recirculation.
• 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( N_S ) of the impeller
  • N_S 200 14% increase
  • N_S 500 7% increase
  • N_S 2500 Insignificant increase

Pumps are 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 first place.
• Install a vacuum pump in the discharge line to pump out any air.
• 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 Newtonian fluid.
• The shaft can hit a critical speed.
• You can get too much capacity that can burn out the motor.
• 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 impeller.

Operating 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 breakage.

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 fluids.
• 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 design.
  • 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 option.
• 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 some problems:
  • The resultant shaft deflection can damage the seal or break the shaft.
  • Internal recirculation can overheat the volute and cause cavitation problems.
  • 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 friction)
• 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 second)
• Velocities of no more than 10 feet (3 meters) per second are recommended in the suction side piping to prevent abrasive wear.

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 to consider:

• 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 the impeller.
• At higher shaft speeds the bearing oil level is critical to prevent overheating.
• Be aware that ball bearings have speed limits:
• The bearing bore, in millimeters, times the rpm must not exceed 300,000.
• The pump horse power times the rpm must not exceed 2.7 million.
• 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 design.

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