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.
Figure 5. A system with two control valves
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:
- Prepare system head curves. In case the destination pressure is variable, prepare curves similar to Figure 2 for each destination pressure. Then prepare an overall system curve (see Figure 6) which should connect the extremes—such as maximum flow, maximum destination pressure on one side and minimum flow, minimum destination pressure on the other side.
Figure 6. Overall system curve
- Specify the maximum and minimum flow requirements. Determine the flow required by the process through the control valve. Add positive and negative flow margins, respectively, depending on the control duty and determine the extreme maximum and minimum flows for which the valve should be selected.
- Fix about 10 to 15 psi (7 to 10 meters of WC) for the control valve loss at maximum flow. Using Equation 3, compute the control valve head required at minimum flow and at normal (100 percent) flow. Add the system head and control valve head requirement at minimum, normal and maximum flows and prepare plot similar to Figure 3.
- Based on Figure 3, specify the pump flow and head requirement at all three points. Pump vendors should also be asked to supply family of curves in the range of 110 percent to 90 percent of the head, which will help optimize the pump selection.
- Maximum flow—The pump is to provide no less than the specified head 10 to 15 psi (7 to 10 meters WC) at this flow condition (or the control valve will have to open more than the maximum recommended opening, which is not desirable).
- Normal (100 percent) flow—The pump should have the highest efficiency at this flow and head condition.
- Minimum flow—The excess above the minimum flow head should be as low as available from the pump (or the control will have to throttle below the minimum recommended opening, which is not desirable).
- The selected pump curve (see Figure 7) must touch all the three duty points. If this does not occur in the first operation, the two options using the vendor’s family of curves are:
Figure 7. Pump curves
- Increase the control valve’s minimum flow. Re-compute the required pressure drop at the minimum flow and at 100 percent of flow using Equation 3, and recreate Figure 3. Then superimpose a curve from the vendor’s family of curves. In this case, a second control valve in parallel will be needed for the low-flow-rate operation.
- Match the duty points with the vendor’s curve by increasing the control valve pressure drop (instead of 10 or 15 psi) at the maximum flow. This will avoid a second control valve but will have higher energy losses. Evaluate the two options and decide which provides optimum life cycle cost.
- ANSI/ISA-75.01.01 Standard, Flow Equations for Sizing Control Valves.
- ANSI/ISA–75.11.01 Standard, Inherent Flow Characteristic and Rangeability of Control Valves.
- A Rational Approach to CV sizing, Ind. Eng. Chem. Res., 1990, 29, 700-703. William L.