According to the U.S. Department of Energy, misapplications of control valves in pumping systems offer a significant opportunity to save energy.(5) This article explains how the pump and control valve should be selected to save pumping energy and reduce lifecycle cost.
The amount of flow through a control valve depends on the size of the valve, the pressure drop over the valve, the stem position and the flow properties. A simplified design equation for non-flashing liquids, ignoring the effect of the connected fittings, can be derived from References 1, 2 and 3 as
where F = fluid flow through control valve, Cv = valve flow coefficient (at full open condition); x = valve stem position, f(x) = fraction of valve size coefficient at any given valve stem, opening indicating the inherent flow characteristics of the valve; ∆Pv = pressure drop across the control valve and sp.gr. = specific gravity of the fluid. (More detailed equations are available in the standards (References 1 and 2) and publications of the control valve manufacturers (Reference 4), which also address flashing liquid services.)
See also: Does valve size matter?
The most common valve characteristics are equal percentage, linear, and quick opening. Due to distortion in valve characteristics at the extreme conditions of full open and full shut, as well as the performance of the valve actuator and spring, the usable valve opening range is less than 0 – 100%. Typically, it is about 10% to 90%. Distortion is also due to system losses, which vary the pressure drop across the valve and result in a valve characteristic different from the valve’s theoretical characteristics 6.
The present analysis is based on the linear characteristic, which is often the choice where constant control valve gain (the magnitude ratio of the change in flow through the valve to the change in valve travel under conditions of constant pressure drop) is required for accurate control of parameters such as level or flow.
Develop the system head curve
To specify the pump and control valve, we first need to develop the system head curve, which is the head required (m) to be developed by the pump at different flows (m3/hr), ignoring the losses across the control valve.
For systems comprising of static and dynamic losses, such as shown in Figure 1, the system head curve can be represented as follows:
Where, HCstatic = a constant, sum of the static head and pressure difference between source and destination, and q is a multiplying factor between dynamic head and square of flow, which is a constant in turbulent flow condition. Figure 2 shows a system head curve developed using Hstatic = 50 m and q = 0.009 [m head/(m3/hr flow)].
Define the flows
The next step is to define the required operating flow range. The maximum and minimum flows should be defined at a very early stage, as they not only affect the selection of the control valve, but also the minimum required pump head, as will become clear later in this article.
Maximum flow: Generally, operation at the design capacity is considered 100% flow. Increase above 100% is expected to be temporary for purposes such as making up a drum level that has depleted due to a process upset. Such margins will depend on the nature of disturbance, but for level and flow control they are generally about 10-15%. For restoring normal conditions in a distillation column, the margin for reflux control may be even higher at 20%. The maximum for the control valve will therefore be 110-120% of the design, depending on nature of corrective action required.
Minimum flow: Processes are expected to generally operate at 100% (design capacity) for the best overall efficiency, however they may have to be designed for operating at less than 100% for significant periods, for example, due to limitations of feed availability or product offtake. Generally, the turndown condition is defined at the design stage so all components of the process can be suitably specified. In this case, from the control point of view, the turndown condition should be treated like an alternate operating case.
Margin below the turndown is required for reasons similar to those for maximum flow. A temporary high level in a drum may have to be restored to normal by reducing intake flow into the drum during turndown operation of the process. Therefore, the control valve minimum has to be computed considering about -10-20% below the steady-state turndown, with the margin depending on the nature of corrective action.