Pumps in series switched to meet demand
Rotodynamic pumps may also be installed in series and switched to meet the demand. This arrangement can be more appropriate to systems with a high friction head loss and low static head. Figure 4.9 illustrates the operating points for one, two and three pumps running. It should be verified that all the operating points remain in the preferred operating region around pump best efficiency.
The second and third pumps in series have a progressively increasing suction pressure and working pressure and they must be selected appro priately for this operating condition. It is also difficult to configure the pipe system to vary the ‘lead’ pump.
In this method of control, the flow is varied by switching the pump on or off. It is necessary to have a storage capacity in the system e.g. a wetwell, an elevated tank or an accumulator type pressure vessel. The storage can provide a steady flow to the system with an intermittently operating pump.
When the pump runs, it does so at the chosen (presumably optimum) duty point and when it is off, there is no energy consumption. If intermittent flow, stop/start operation and the storage facility are acceptable, then this is an effective approach to minimize energy consumption. It may also be used to benefit from “off peak” energy tariffs by arranging the run times during the low tariff periods.
To minimize energy consumption with stop/start control, it is better to pump at as low flow rate as the process permits. This minimizes friction losses in the pipe and an appropriately small pump can be installed. For example, in a system dominated by friction, pumping at half the flow rate for twice as long can reduce energy consumption to a quarter.
The stop/start operation causes additional loads on the power transmis sion components and increased heating in the motor. The frequency of the stop/start cycle must be within the motor and pump capability and will be limited by the inertia of the load, the power and the speed of the pump, and the quality of the power supply.
Flow control valve
With this control method, the pump runs continuously and a valve in the pump discharge line is opened or closed to adjust the flow to the required value.
To understand how the flow rate is controlled see Figure 4.10. With the valve fully open, the pump operates at Flow 1. When the valve is in the half open position it introduces an additional friction loss in the system. The new system curve cuts the pump curve at Flow 2, which is the new operating point. The head difference between the two curves, at Flow 2, is the pressure drop across the valve.
It is usual practice with valve control to have the valve 10% shut even at maximum flow. Energy is therefore wasted overcoming the resistance through the valve at all flow conditions.
There is some reduction in pump power absorbed at the lower flow rate (see Figure 4.2), but the flow multiplied by the head drop across the valve, is wasted energy. It should also be noted that, whilst the pump will accommodate changes in its operating point as far as it is able within its performance range, it can be forced to operate high on the curve where its efficiency is low (see again Figure 4.2), and where its reliability is impaired.
Maintenance cost of control valves can be high, particularly on corrosive and solids-containing liquids, and so the life cycle cost of this widely used method of control can be surprisingly high.
In this approach, the pump runs continuously at the maximum process demand duty, with a permanent by-pass line attached to the outlet. When a lower flow is required the surplus liquid is bypassed and returned to the supply source. An alternative configuration may have a tank supplying a varying process demand, which is kept full by a fixed duty pump running at the peak flow rate. Most of the time the tank overflows and recycles back to the pump suction. This is even less energy efficient than a control valve because there is no reduction in power consumption with reduced process demand.
The small by-pass line sometimes installed to prevent a pump running at zero flow is not a means of flow control, but required for the safe operation of the pump.