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Following the selection of a centrifugal pump that is properly “performance” matched to the systems requirements, it is necessary to select a driver that is properly “power” matched to the pump requirements.
Drivers are machines that take energy in one form (electrical, chemical, etc.) and convert it to a different form resulting in power, which can be used to drive other machines. Any rotating shaft driver can be used to power a centrifugal pump, provided that the output shaft is rotating the correct direction and the speed of the shaft is suitable for the pump. The drivers most commonly used to power centrifugal pumps are electric motors and internal combustion engines. Each kind has its own particular output and shaft speed characteristics and limitations which must be taken into consideration when matching a pump and driver.
Basically speaking, the power load against the driver must be within the continuous duty rating load capability of the driver at the operating conditions for the pumping system. When the best estimation of power load (including all power losses) has been made then a driver with suitable power and speed characteristics can then be power matched to the pumping system requirements. As discussed earlier, the formula to determine the pump power requirements (BHP) is as follows:
BHP = (GPM X TDH X SG) / (3960 X PUMP EFFICIENCY)
Depending upon the pump type, there may be power transmission losses, which must be added to the basic hydraulic load of the pump to obtain the total power input requirement. Some of these types include belt-drive or chain-drive power transmission systems, vertical hollow shaft motor, and right angle gear drives. Typically, losses will be between 5% - 10% for belt or chain drive units, variable for VHS motors depending on the diameter of the shaft and it’s length, and approximately 3% for right angle gear drives.
Proper power matching can make the difference between a “good” pump installation or a “bad” pump installation.
Where satisfactory electrical power is economically available at the pump site, it is common practice to drive the pump with an electric motor (squirrel cage induction type).
When electric current (amperes) flows through the wire coils (stator windings) inside the motor, very strong magnetic fields are produced. The magnetic forces attract the rotor of the motor, producing a twisting effect (called torque), which is combined with the rotational speed of the motor to produce horsepower (BHP).
Squirrel cage induction motors are essentially constant speed machines. The RPM of the motor shaft is closely related to the rotational speed of the magnetic field inside the motor. The rotational speed is determined by the number of poles (coils of wire in the stator of the motor) and the frequency of the alternating current (AC) electrical power. This is called the synchronous speed of the magnetic field and is shown in the following formula.
RPM = (120) X (f) / (p)
RPM – Revolutions per minute. This is the synchronous speed of the magnetic field.
(f) - Frequency of the alternating current (AC) power expressed in cycles or hertz per
(p) - The number of poles in the motor stator.
The squirrel cage rotor spins at a speed slightly less than the field synchronous speed. This speed difference is called “slip”, and it will range between 3% - 5% at full-rated power output for the kinds of motors used to drive centrifugal pumps. The motor will produce only the amount of power demanded by the load. When spinning without any load (no power demand), there is very little slip and the electric power consumption is very low. When the load power demand is increased the slip increases, which increases the motor power output and the electric power input.
The wire coils inside the motor, which generate the magnetic field that causes the motor to turn, also act like electric heaters, converting some of the electrical energy coming in from the power system into heat. The amount of heat produced is related to the amperes passing through the wire coils, which is proportional to the load placed against the motor. Electric motors have a thermal limit (a maximum internal operating temperature), that is determined by the class of insulation used. Modern insulation systems will tolerate very high continuous operating temperatures. If the temperature limit is exceeded, however, the insulation will fail leading to the burning out of the motor windings. That is why every motor has a maximum continuous current (ampere) limit. In a three-phase motor, this is the maximum current through any leg, and is not the average current.
It is standard practice in motor manufacturing to show a “full load amps” (FLA) rating on the motor nameplate. FLA is the amperage rating at the motor nameplate horsepower rating. When operating at this load, the motor efficiency and power factor are usually at their maximum. It is at this point that the motor prefers to operate. If the load is other than full load, both the efficiency and power factor will change. This can substantially change the over-all efficiency of the pumping system.
Squirrel cage induction motors are usually designed to have extra thermal capacity. This permits continuous operation, without thermal failure, at a higher rating than “full load”. This extra capacity is called “service factor” (SF) and is stamped on the motor nameplate. The amperage rating on the motor nameplate for this extra capacity is called “service factor amps” (SFA) or “max amps”.
Example: A 100HP motor, SF = 1.15, can operate up to 115HP
It is normal practice in the pump industry to use, at least, some of the service factor to get the most power possible from a given motor. It should be understood that the amperage demand at 1.15 service factor load is 15% greater than the full load amps, and the resulting higher internal temperature will reduce the life of the motor insulation. In some cases, this is an acceptable loss. The insulation life reduction will vary based on the actual operating temperature and the insulation materials used in the motor. A rule of thumb in the industry is for every 100C increase, the insulation life will be cut in half.
Water can conduct electricity and is also somewhat corrosive. Therefore, the electrical windings and internal parts of an electric motor must be protected from water or other liquids.
The most common enclosure for motors used on centrifugal pumps is “open drip proof” (ODP). It is suitable for indoor use and normal outdoor use where there is little danger of water directly entering the motor. These motors are cooled by air blown through the inside of the motor by integral fans. The air enters the motor through ventilation openings that are protected from downward falling water. In vertical motors, used on vertical pumps, this type of enclosure is called “weather protected – Type 1” (WP-1). Normally, these types of motors will have a 1.15 service factor.
If the operating environment is wet and there is substantial risk of water entering the motor, all openings into the motor are closed, and the risk of water entering the motor under normal conditions is eliminated. For very small motors this enclosure system is called “totally enclosed non-ventilated” (TENV). Larger motors have an external fan to blow air over the outside of the motor frame and are called “totally enclosed fan cooled” (TEFC). Normally, these types of motors will have a 1.0 service factor. They can be ordered with a 1.15 service factor. Caution must be advised on these motors because catalog performance curves for centrifugal pumps are frequently based on impeller diameters that will load the motor to the 1.15 service factor. When using the motors with a 1.0 service factor, the impeller diameter must be reduced to prevent serious damage to the motor.
Another motor enclosure, called “explosion proof”, will sometimes be required for operation in explosive atmospheres. The explosion proof enclosure is like the TENV and TEFC enclosures with additional provisions to prevent the entry of explosive gas into the motor or the escape of sparks into the surrounding atmosphere. These motors are usually made with a 1.0 service factor and can be very expensive, as well as difficult to obtain. The same caution applies to explosion proof motors as with TENV and TEFC motors regarding motor loading and impeller diameters.
The following information is needed to properly select a motor. The speed (RPM) of the motor shaft should match the rated operating speed of the pump. The motor must be compatible with the power system voltage, phase, and frequency. The motor enclosure selected should provide proper protection when operating in the pump installation site environment. The greatest load that the pump can demand under any possible operating condition must not exceed the service factor rated output of the motor. On close-coupled centrifugal pumps the motor frame size is needed as well. If it is economically feasible, the pump and motor set should be selected to load the motor as close as possible to the nameplate rated horsepower. This will allow long insulation life, the best motor efficiency, and the best power factor.
Electric motors commonly used as pump drivers are usually cooled by air, which is circulated by an integral fan, through the windings or over the motor frame. If the air density (pounds of air per cubic foot) is decreased, the amount of air flowing through the motor will have reduced ability to remove heat from the windings. This could lead to the winding temperature becoming too high. If the operating altitude will be above 3,300 feet (1000 meters), the ambient temperature could exceed 104°F (40°C). The motor manufacturer should advise you whether the insulation system of the motor tolerates the higher operating temperature or if the motor needs to be de-rated.
After installation, with the pump supplying the system demands satisfactorily, measurements and calculations can be made to verify motor loading for the installation. The preferred method of measurement uses a Wattmeter, which shows the input electrical power (HPIN) in Watts (or Kilowatts). The following formula can be used for the calculations.
HPIN = Watts / 746 or Kilowatts / 0.746
The power input to the pump (which is the power output from the motor) can be determined by multiplying the electrical input horsepower times the motor efficiency. The motor efficiency can be obtained from the motor nameplate or from the motor manufacturer. In the absence of better information, the approximate motor efficiencies listed in many engineering manuals will allow you to come up with a good estimate of the power output for a motor operating near the normal, full-load rating.
On three-phase motors, it is important to verify that the electrical load is being shared properly between the windings of the motor. The maximum allowable deviation of the current (amps) in any leg, from the average of the three legs, is 5%. Also, the current in the highest leg must not exceed the Service Factor amperage rating listed on the motor nameplate.
Internal Combustion Engines
Where satisfactory electrical power is not available, or when a pump unit is moved to various locations, it is common practice to drive the pump with an internal combustion engine (diesel, gasoline, LPG, natural gas, etc.).
When a mixture of fuel and air is burned inside an internal combustion engine, chemical energy is converted into power. The combustion process produces a twisting effect, called torque, at the engine shaft, causing it to rotate. The amount of torque being produced is combined with the rotating speed of the engine shaft to produce horsepower (BHP).
Internal combustion engines are variable speed machines. The actual operating speed of an engine will be the RPM where the BHP available from the engine exactly matches the BHP required by the pump at the same speed.
For a given operating point, if the BHP required by the pump is greater than the BHP that can be produced by the engine at that RPM, the pump and engine set will simply slow down. This reduces the power required by the pump until the BHP available from the engine and the BHP required by the pump match exactly. It is at this RPM, where power equilibrium is achieved, that the pump and engine “lock on”. Engines that are to drive a pump without an operator in attendance generally have a “governor”, which maintains a constant engine speed. If the pump power requirement increases the engine begins to slow down. The governor senses the RPM reduction and increases the flow of fuel to the engine. This increases the engine power output as it tries to match the increased power demand from the pump. If the pump power requirement decreases the engine RPM begins to increase. The governor senses this increase and reduces the flow of fuel to the engine, bringing the engine power output and speed back into an exact match with the pump power demand.
The conversion of chemical energy into power is a high temperature process. The metal parts of an engine can become quite hot. The engine will have a thermal limit, or maximum internal operating temperature, which is determined by the materials of construction used. Operating an engine above the thermal limit can result in reduced engine life or, in extreme cases, damage to moving parts. All engines should be fitted with a switch that will sense excessive temperature and stop the engine.
An engine manufacturer will determine, by tests, the permissible loads and the recommended operating range for each engine series. This information is made available by the engine manufacturer in the form of rating curves or tables. Since engines can be used for many different power applications (automotive, marine, generators, etc.) several rating procedures are used to list engine power output. Make sure you use the correct one when matching pump and engine performance. Engines are usually fitted with accessory devices and sub-systems that make them self-sustaining. These include the cooling system, cooling fan, fuel system, governor, battery-charging generator, exhaust system, and the air intake system. Many of these will consume power from the engine, some as much as 10%. Therefore, the only rating that should be considered, for engines to be used with pumps, is the “Net Continuous Horsepower Available at the Output Shaft” rating.
Internal combustion engines are “air breathing” machines. The power output of an engine is dependent on the amount of oxygen that is available to mix with the fuel. The amount of oxygen available to burn is determined by the density of the air (lbs./ft.3). The density changes with atmospheric pressure and ambient temperature changes. Published engine power ratings are based on engine industry atmospheric standards (usually 500 feet altitude above sea level and 85°F air temperature). At different altitudes or air temperatures the load rating for the engine will change.
Make sure your engine is de-rated for all accessories, as well as altitude, and air temperature.
The following information is needed to properly select an engine for use on a pumping system. The required pump operating speed must be within the engine manufacturer’s recommended speed range. The type of engine to be selected will be determined by the cost and local availability of fuel. An estimate of the number of anticipated operating hours is needed. The engine should produce the power, at the speed required, for the desired pump performance. You should not use all of the engine power available. A good rule of thumb is to use 70 – 80% of the power available. This allows for “wear and tear” on the pump and engine.
After installation, with the pump supplying the system demands satisfactorily, measurements and calculations can be made to verify engine loading for the installation. With the system operating, at the desired conditions, observe and record the engine RPM and the pump discharge pressure. Increase the engine speed 10%. If the engine power reserve is adequate, the engine should be able to increase the pump speed by 10%. The discharge pressure should show a 21% pressure increase (according to the Affinity Laws).
Note: When performing this test, the engine RPM at the 10% over-speed setting must be within the engine manufacturer’s ratings and limitations. Also, the 21% increase in pump discharge pressure should be within the operating ranges of the pump and piping system.
The exhaust gases leaving an engine can tell you how it is operating. The gases leaving an engine in good condition, when operating at normal temperature with an adequate air supply for combustion, should be practically invisible. Discoloration of exhaust gas can indicate a malfunction or an improper load condition. For example, black smoke indicates that the amount of fuel passing through the engine exceeds the amount of air that is available to burn. The combustion process is not complete. This can be caused by a plugged air cleaner or it can be an indication of excessive engine loading (the governor attempts to maintain the desired RPM by increasing the fuel flow). This condition will result in excessive fuel consumption, and could lead to damage or wear due to heat for the engine. For the pump this can mean broken parts (this will probably happen first). When the exhaust gases are not clear the cause should be determined and corrected to insure long pump and engine life.