By Lauren DeLorenzo, Journalist, Pump Industry magazine
When it comes to pump drive selection, poor decision-making can result in overloaded motors, cost inefficiencies and a unit life that is drastically cut short. Here, Pump Industry uncovers the common mistakes and key factors to consider when choosing a pump drive that is right for your application.
Pump drive selection can be a confusing and complicated process, with many industry professionals leaving the decision-making in the hands of pump selection software.
While software can be a helpful tool, understanding the factors that influence pump drive selection will help to make a more informed choice, and will ultimately lead to a more durable, cost-effective and efficient solution.
Understanding pump drives
Pump drives provide the essential function of connecting a pump to an input rotational source, such as a motor or engine. The pump drive might entail a motor, an inverter or a gearbox that can moderate motor speed.
A pump drive can be selected after the pumps have been properly sized and chosen, and the type of engine has been decided upon.
Once these elements have been defined, the next step is to find which pump drive is most suitable for the application. It is helpful to choose the pump drive before plans and blueprints are drawn up.
If the selected pump is too big, money could be wasted in the initial cost of the unit. On the other hand, if the pump is too small, it will reduce the life of the motor and increase the chance that it will overload and fail the insulation system. To have the best chance of an efficient and long lasting pump drive, here are seven considerations which could impact your selection.
1. Number and type of pumps required
Each project will require a varying number and types of pump, depending on the needs of the project, the medium that is being pumped and the pump specifications. This will impact the amount of power that is needed to operate the application and ensure the pump will consistently meet the requirements for sustained operations.
The pad number determines how many pumps are mountable to a drive, which in turn will impact the type of drive that should be selected. The pump drive should be able to accommodate the necessary number of pumps for the application. These considerations are extremely important – without a suitable selection, pumps may be stressed past operating limits, eventually resulting in failure of the system.
2. Sizing the drive
The size of the drive will differ depending on whether a diesel engine or electric motor is being used. Sizing calculations are often determined by computer programs or graphs, as electric motors develop a very high starting torque, whereas the starting torque for diesel and gas engines remains relatively consistent.
The size of a hydraulic power unit should be based on torque, speed and power requirements. Since the cost of electricity needed to operate an electric motor over its lifespan is far greater than the cost of the motor itself, correct sizing of a power unit can save a significant amount of money in the long term.
3. Horsepower rating
Calculating the horsepower rating is important for determining the torque of the driver, and will influence a range of other factors when deciding on the appropriate pump drive. Horsepower can be calculated with specific computer programs or graphs. An excessive RPM could cause a gap between the expected and actual horsepower, meaning that the operational efficiency is reduced.
In addition to these factors, maximum input speed should be examined to make sure that the pump can accept the maximum input speed of the motor or engine.
A smaller engine, for example, might have a lower torque, but could have a higher RPM, meaning a pump should be chosen that is capable of accepting this high RPM. On the other hand, a larger motor or engine may have less rotational frequency, but a higher torque, which will mean the pump needs to fit these requirements.
Once the horsepower and RPM have been determined, the speed reducer service can be chosen. To determine the service factor rating required, factors such as the length of use throughout the day and the application itself will affect the outcome. Consulting the manufacturer for assistance with selecting the right reducer, or understanding the selection process will ensure the most optimal outcome.
Variable frequency inverters are becoming increasingly popular as a replacement for belt cases and traction drives, allowing for more customised programming where multiple speed operation is needed. Standardised speed reducers also make the device more compact than when using belt cases or traction drives.
5. Maximum torque
Torque refers to the twisting force, or the rotational force, of the engine or motor. Torque analysis can help avoid the possibility of incorrectly sizing a drive’s horsepower. Maximum torque output requirements should be below the maximum value of each of the pumps, to ensure uninterrupted and smooth operation of the pump.
Most pump drive manufacturers rate units by the amount of input torque they can handle. The maximum input torque should be at least 20 per cent below the maximum rated value for the prime mover (engine or motor) to ensure the pump is not over-stressed and that it can reach its full expected lifespan.
Torque measurements should be considered in conjunction with the type of rotational source that is being used. While electric motors run well near full load, air-cooled engines can sometimes overheat as they approach full load. Engines handle torque load requirements differently to most induction motors, and they must be oversized in order to run at the correct speed. Using an undersized unit for the application may reduce service life or result in failure.
Variable torque is required for functions which use different products with different rates of flow, while functions using the same pump need to have torque calculated individually. Here, the drive should be sized to fit the largest torque and greatest speed.
6. Gear ratio
Gear ratio measures the revolutions of small and large gears, which can modify the operating speed of the engine. The gear ratio creates the speed for the output shaft, and can include the input driver gear, the input ratio gear and the output ratio gear. Flow rate and displacement should be determined before looking at the gear ratio to ensure that the pump input shaft will work with the given speed.
Pump drivers can have multiple gear ratio options, or just a singular gear ratio. To select the right gear ratio, consider both the ideal RPM of the engine or motor, and the required operating speed of the pump.
7. Cooling capacity
The thermal characteristics of a pump drive’s gearbox should also be taken into account when selecting a pump drive. Consider the maximum thermal limit of the gearbox when the drive is static, when using a mobile application and when employing a mobile application, with the gearbox fitted with a standard low flow cooling pump.
Calculate the amount of cooling required for the drive (multiply the gearbox thermal limit by the inefficiency of the gearbox) to find how much heat the drive will need to dissipate. Environmental factors, such as temperatures around the installation area, can also influence the required cooling capacity. Pump drives surrounded by cooler temperatures may not require powerful cooling systems.