Variable Frequency Drives (VFD)
The VFDs have many of the advantages of soft starters but add the ability to control the motor speed by varying the drive output frequency to the motor. By varied the speed we can control the performance of the pump much like we saw when we vary the impeller diameter. The benefit here is that by pushing a button we can increase the speed and thus the performance again. It is very hard to put the metal back on the impeller after you have trimmed it.
Pumps can also be controlled through the use of multiple pump systems. Multiple pump systems can be arranged with the pumps in series or in parallel. Pump arranged in series take the output of the first pump and feed it into the inlet of the second pump. If the pumps are the same model number this will effectively double the head output at a given flow rate (see the curve below).
This type of system is often very useful if there are multiple loops or pathways that the pump can discharge to. An example of this might be discharging to a tank that is nearby or the tank that is far away. When pumping to the nearby take only one pump is required but when pumping to a tank far away the pressure from the two pumps may be required to reach the desired flow rate. This can also be setup as a booster system. When the second pump is placed downstream in the pumping system at a point where the pressure has fallen below some threshold value, the pressure can be increased in the second pump allowing it to drive the liquid further on down the line.
Another configuration is to plumb the two or more pumps in parallel. Multiple pumps can be set up in this manner potentially allowing the pumps flow output to significantly increase as each pump is turned on. With parallel pumps, the flow rate at a given head are added together. So for two pumps of the same model as shown below, turning on the second pump doubles the pumps available output at the same head. This does not mean that when you activate a second pump in a parallel system the system output doubles. Remember the intersection of the pump curves and the system curves is the operating point of the pump. Generally the system curve will increase in head-loss as the flow rate increases. Depending on the system curve the activation of the second pump may have little to no affect on the system output.
Deciding on a Control Scheme
Selection of the best control scheme for your application requires that you understand your applications performance parameters. The range of desired operating flow rates must be determined. Additionally suction and discharge minimum and maximum static head levels must be determined. This provides an envelope that the pump will be required to operate in. Now the task is to determine the most efficient and effective control methodology. The first question is does the system require a large amount of variability. If it does not, a simple pump running at a fixed speed may be sufficient. If the system has a large range of operation the situation becomes more complex. Fortunately the system curve can be your guide to the appropriate system control mechanism.
Shown below are the four extremes of the system control schemes. The left column represents systems that have system curves dominated by friction loss in the system. The right hand column is based on systems that have static head dominated loss curve. This represents a system with low velocity fluid in the pipes or a very short run of piping. The top row shows pumps being controlled using a variable frequency drive to alter the pumps operating speed. As the speed is reduced, the pump performance drops. The bottom row shows systems using multiple pumps in parallel to control the system.
As can be seen in the top left pane the friction based curve and the VFD controlled pump are a perfect fit. If the pump is selected to be operated at its best efficiency point (BEP) at full speed, it will continue to run at its BEP as it is slowed down.
The other ideal situation is the lower right pane. This is a static dominated loss curve with multiple parallel pumps being used to control the flow rate. As multiple pumps are activated, the flow rate essentially doubles. This allows operations to add pumps during peak demand and shut them down during lower demand. This is a simple and very effective method to control large variability in flow rate while allowing the pumps to operate near optimum efficiency.
The lower left and upper right panes are examples of using the inappropriate control technology for the system. In the upper right we see a very common error in system control. It stems from the fallacy that VFDs always save you money. In this case only a very small portion of the speed control can be used, because as the pump speed is reduced the system curve quickly moves away from the best efficiency point and to an unfavorable region on the pump curve.
The lower left pane shows another common problem. It seems logical that if I turn on a second pump my flow rate should double. As we can see this is not the case if we’re using parallel pumps on a friction dominated system curve. The friction loss increases with the square of the change in flow rate. This means every time that I double the flow rate through the system, I quadruple what the head the pump must produce. As I turn on the second and third pumps my flow rate only marginally increases and the pumps, sharing the load, move to the left on their respective curves to lower flow rates further from their BEP point.