It is common for process plants to contain many centrifugal pumps. In continuous processes, the conventional method for continuously controlling a pump is to throttle a control valve. The purpose is to defend the controlled variable against disturbances by adjusting the manipulated variable. In pump systems, the manipulated variable is usually the flow of liquid.
When designing a pump system, process engineers typically calculate the static and dynamic pressure drop of the pipework, fittings, in-line equipment and in-line instrumentation at a fixed design flow rate. They then add the control valve’s pressure loss, which often is derived from experience. This is traditionally a fixed value of between 0.2 and 0.5 bar. Another way is to allocate a certain percentage, for example, 25 to 50 percent of the dynamic pressure loss of the system-head curve’s to the control valve’s pressure drop.1
Any absolute or relative control valve pressure loss must refer to a certain opening, or travel, typically 60 to 80 percent of full travel. Instead of choosing a certain travel, a percentage of the valve sizing coefficient or rated Cv (Cv at full opening) may be fixed, for example, about 60 to 75 percent.2
Depending on a valve’s parameter, the flow rate will increase when the valve is opened to 100 percent travel. However, an operating controller will respond to disturbances by oscillating so the maximum flow rate at 100 percent travel cannot be reached in reality and constitute a limit. Nevertheless, a nearly fully opened valve allows increasing flow, hence higher production, and eliminating a bottleneck or pinch is possible.
Figure 1 shows a system in which a centrifugal pump (P) moves liquid from a suction side vessel (V1) to another vessel (V2). Immediately after the pressure nozzle of the pump, a bypass line forks off; this recycles part of the liquid through a resistance device to V1. The main flow passes an actuated control valve (RV) on its path to V2.
The control valve’s opening, or travel, is determined by a controller, in this case, the level controller (LC) in vessel V1. The controller could also be located somewhere else in the process, for example, a temperature controller in the bottom of a distillation column.
The pipework with all of its members exerts a dynamic pressure loss; so does the control valve RV. In addition, there is a total static pressure difference that consists of the difference in pressure between V1 and V2, plus pressure from the geodetic height.
A generalized example can demonstrate the influence of the control valve parameter on the maximum flow rate. Pump characteristics are described using the best efficiency point (BEP) as reference. Most radial centrifugal pumps, usually small pumps featuring a low specific speed, cover the characteristics in Figure 2. Actual characteristics are scattered over a wide range with a clear tendency toward steeper characteristics similar to Curve II with increasing specific speed.3
The pump system can be characterized by three parameters. The first describes the relationship between pressure loss of the control valve and that of the system (see Equation 1).
The second allocates a certain dimensionless travel Ys (see Equation 2) to the pressure loss of control valve ΔpRV,S at design conditions.
The dynamic pressure loss at design flow rate Δpdyn,S is divided also by the sum of the dynamic pressure loss plus static pressure difference Δpstatic. For example, Δpstatic = 0 reflects a closed loop system, B = 1. Changing the travel of a control valve will change flow rate. The relationship between the capacity or flow rate versus travel follows a specific curve. Equation 4 demonstrates the equal-percentage characteristic, which is found in the majority of valves.
Figure 3 reflects this in dimensionless terms, for example, Cv divided by rated Cv over the travel/rated travel, with the parameter being the rangeability R = 25 and R = 50, respectively.
Influence of the System’s Parameter
A calculation of the system in Figure 1 was performed with some simplifications (for example, a negligible pressure drop on the suction side and also from the pressure side of the pump to the fork-off point of the bypass).