As described earlier in this guide, with rotodynamic pumps, relatively small speed changes can have a major effect on parameters such as power absorbed, NPSH required etc.
Generally VSDs will be used to reduce speed from the nominal. However if there is adequate motor power available the drive may be used to increase speed. In this case there are a number of precautions, which must be considered:
• There will be a change in the noise output from both the pump and the motor as the speed is changed. With higher speed, greater noise and potentially greater vibration will occur.
• If the pump is handling liquid containing abrasive particles, an increase in flow will result in a corresponding increase in abrasion and wear.
• With increasing speed, the user must ensure that the NPSH available at the pump is still at an adequate level to prevent cavitation. This must be checked, since the pressure drop along the suction pipe will increase as the flow/velocity increases, which will cause a reduction in the NPSH available to the pump. The situation is further exacerbated by the fact that the pump will also require more NPSH when running at higher speeds.
• Various mechanical constraints must also be checked. If the speed (and so the power) is increased then bearings, flexible coupling, magnetic coupling, motor, etc must be checked to make sure they can operate with the increased loading.
• Since a speed increase will also affect mechanical seals, the manufacturer should verify that they are still being operated within the allowable region.
• In some instances where shear sensitive liquids/chemicals are involved, it is necessary to limit maximum pump speed.
Lower speed generally benefits energy savings and maintenance costs but the following precautions must be considered:
• In the case of relatively high static head, care has to be taken that the pump is not operated in inappropriate regions. As shown in Figure 4.6, a relatively minor decrease in speed can move the operating point close to shut off head. If the pump is operated in such a region for a long period of time it will have severe consequences for the life of the equipment.
• If pump speed is reduced in some applications, particulates may come out of suspension and cause problems.
• A minimum speed limit applies if pumps have a balance disc or drum (generally multi-stage pumps) as unacceptable wear can occur.
• Self-priming and regenerative pumps will often not prime at low speeds.
• Gas seals have minimum peripheral speed and pump flow rate requirements, check with manufacturer.