Pump and Control Valve
As seen in Figure 4, about 45 meters less pump head is required at 100 percent of flow if the control valve minimum flow requirement is changed from 25.6 percent to 57.5 percent. Higher flow turndown results in higher control valve loss and, therefore, a higher pump head. This is also explained in Reference 3. The lower control valve loss at normal (100 percent) flow means continuously spending less energy since the system is expected to operate at 100 percent most of the time.
Deciding Flow Turndown
Extreme care should be taken when deciding the minimum flow required for control, and it is important not to aim for extremely low flow if it is unnecessary. If a very low flow condition is required on an infrequent basis, check to see if it can be handled in the field by using a small manual valve in the bypass of the control valve. Alternately, a smaller capacity control valve in the bypass of the main control valve can be provided, which will be sized specifically for the very low range of flow.
Split Range Control Valves
Figure 5 shows the two control valves, A and B, installed in parallel. Valve A is the original valve covering 57.5 percent to 115 percent of the flow range. Figure 4 shows (by simultaneous computation of Equations 2, 3 and 5) that a smaller control Valve B can be provided to cover 28.8 percent to 57.5 percent of the flow range with the same pump impeller diameter ratio, n, as was used for the 57.5 percent to 115 percent range larger control valve.
Overall, the two control valves can control through a range of 28.8 percent to 115 percent of the flow. Compared to having a single control valve to covers 28.8 percent to 115 percent of the flow, the two control valve arrangement consumes about 40 meters less pump head at 100 percent flow conditions. The energy saved by this alternate arrangement may justify the additional cost of the smaller control valve during the lifetime of the plant.
Selecting the Pump and Control Valve
The recommended steps for pump and control valve system design are summarized below: