By Béla Lipták, PE, Columnist
In this portion of this series, I will concentrate on the optimization of pumping stations and liquid distribution systems, which can save 25% or more energy. As was shown in the last segment, using multiple constant-speed pumps of different sizes is more energy-efficient than meeting the variable load with a single pump. It was also shown that this saving further increases if one of the multiple pumps is variable-speed (See Control, Jan. '10, Fig. 4, p. 22, or www.controlglobal.com/articles/2010/Optimization1001.html). Now I will show additional means of increasing energy efficiency, and describe methods to calculate the projected savings prior to their implementation.
One of the best methods of finding the optimum pump discharge pressure is based on valve position control (Figure 1). Here the optimum discharge pressure is selected to keep the most open user valve at a 90% opening. Thereby, the pumps will produce only as much discharge pressure as is needed to satisfy the highest load in the system, and, by so doing, will not only keep all other loads satisfied, but also will minimize the energy wasted in the form of valve pressure drops.
In this control system, as the pump discharge pressure controller (PC-01) raises the discharge pressure (PT-01), all valves will close somewhat, because they all will have more pressure drop available to them. Inversely, if PC-01 reduces their inlet pressure, they will open somewhat. Therefore, if the controls are configured so that the valve position controller (VPC-02) measurement is always the opening of the most open valve, and that valve is kept nearly open (say at 90%), then all other user valves will also open up. As the pump station's discharge pressure is minimized, this will minimize all the valve pressure drops in the plant, and, thereby, will also minimize the energy consumption of the pumping station.
In the cascade loop shown in Figure 1, VPC is the master and PC is the slave. Therefore, as VPC-02 lowers the setpoint of PC-01 (and thereby opens all user valves), it not only minimizes the valve pressure drops, but also can minimize valve cycling and, therefore, maintenance. In order to do that, the pressure controller (PC-01) setpoint must change slowly and in a stable manner. Consequently, the valve position controller (VPC-02) is provided with integral action only, and its integral time is set to be about 10 times that of PC-01.
In addition, reset windup must also be prevented. Such windup occurs when a controller output is unable to reach its control valve. Therefore, when the PC-01 controller is switched from cascade to local setpoint (and thereby the output of the VPC-02 is blocked), the cascade master's measurement must also be switched from detecting the valve position to the output signal of the discharge pressure transmitter as its external feedback signal. Similarly, if pressure controller is switched to manual, its external reset should be the pump speed.
If the pump station consists of two variable-speed pumps and only one pump is in operation, the second pump is started when the output signal from PC-01 approaches 100%, meaning that the single pump is approaching its full speed. At that point, PSH-03 is actuated, starting the second pump. When both pumps are in operation, and the flow drops to 90% of the capacity of a single pump, the second pump is stopped if this condition lasts longer than the setting of the time delay (TD-04). The time delay or a gap switch is used to make sure that the second pump is not cycled on and off too often.
It is also necessary to protect the system from discharge pressure spikes when the second pump is started. This strategy is illustrated in the upper part of Figure 1 and serves to only slowly change the pump speed when the second pump is started, so that the starting doesn't upset the discharge pressure. If this was not done, the discharge pressure of the pump station would jump from point A to C.
In order to eliminate this temporary pressure surge, a signal generator (PY-03) is introduced. This signal generator, upon actuation by interlock 1, drops the output being sent to the low-signal selector located on the output of PC-01 to a value, X%, which is lower than the output of PC-01. Therefore, the low-signal selector will select it. The value of X is set to match the speed required to generate the same flow by two pumps as the flow that was previously generated by a single pump (Point A). After the start of the second pump, the value X is slowly increased until it rises over the output of PC-01, which at that point takes over control. Simultaneously, the external reset is also switched back from pump speed to discharge pressure.
This same strategy can be used with any number of pumps, except that for each new pump another PY and FSL has to be added.
You should monitor the efficiency of all the pumps continuously. This will make it possible to meet any load with the most efficient pump combination. In addition, the continuous monitoring of pump efficiencies also serves maintenance scheduling by automatically requesting it whenever the efficiency drops. As we will see in the next section, on average, about 25% of the pumping energy can be saved by these strategies, which in large pumping stations (wastewater or agricultural water supply applications) can mean significant dollars.