This series discusses the control elements of a piping system, which improve the quality of the product. Part 1 (Pumps & Systems, July 2015) covered passive controls, such as overflow and bypass controls, on/off controls, and manual controls.
Active Control Operation
Active controls maintain a set value of a process variable under changing system operating conditions. They are often referred to as a control loop (see Figure 3). The level in the destination tank is measured by the level transmitter that sends the measured value (MV) to the level controller. The desired tank level set point (SP) is entered into the controller. The controller compares the MV to the SP, and a controller output (CO) is sent to the final control element—a control valve in this case. The flow rate into the tank (Qin) is adjusted to maintain the desired tank level.
The destination tank level can vary for several reasons, such as changes in:
- pump flow rate, caused by changes in the static head
- pump flow rate, caused by mechanical wear
- the flow rate out of the tank caused by a change in the system flow demand
- the tank level set point caused by the operator
Level control loop operation can be examined by looking at a low-level condition in the destination tank. The set point for the destination tank level is five feet above the tank bottom. Because of a change in system flow demand (Qout), the tank level drops to 4.9 feet. The level transmitter senses the measured value of 4.9 feet and sends the information to the level controller. Because the level controller SP is 5 feet, the controller compares the measured value of 4.9 feet to the set point of 5 feet.
The error between the measured value and the set point causes the control to send an open signal to the control valve. The valve positioner causes the valve to open. This results in less head loss across the control valve, causing an increase in the flow rate. If the resulting flow rate through the control valve is greater than the system flow demand, the level of the destination tank increases. Over time, the level in the destination tank reaches a steady state condition.
Level Control with a Control Valve
The pump curve in Figure 2, Part 1 of this series (Pumps & Systems, July 2015) shows that the pump produces 125 feet of head at 400 gallons per minute (gpm). The sum of the static and dynamic head for 400 gpm through the system is 76 feet. At 400 gpm, the pump produces 125 feet of head, but the system only needs 76 feet of head. As a result, the control valve must absorb 49 feet of head to limit the flow rate to the set value.
The advantage of level control with a control valve is the ability to minimize process variability by maintaining a more consistent tank level. The disadvantage is the added cost of the control loop, the need to tune and maintain the control loop, and the added head loss across the control valve necessary to maintain control.
The annual operating cost of a control valve appears in Table 3.
Level Control with a Variable Speed Drive
Figure 2 in Part 1 of this series shows that for the pump with the 10.5-inch impeller diameter running at 1,780 rpm, the maximum possible flow rate is 616 gpm. This is where the energy supplied by the pump equals the energy required by the process elements. Under this condition, no differential pressure is required across a control.
If we were able to adjust the pump performance so it only produces the head required by the process elements at the desired flow rate, the control valve could be eliminated. That is the function of a variable speed drive. By adjusting the pump's rotational speed, the head can be maintained to be equal to the head required to meet the flow rate.
Adjusting the rotation speed of the impeller changes the pump performance curve. Figure 4 shows the pump performance for a range of speeds. When the pump is running at 1,424 rpm, it produces 76 feet of head, which is equal to the head required by the process elements at 400 gpm. The pump efficiency lines are superimposed on the system curve as well.
The advantage of level control with a variable speed drive is to minimize process variability by maintaining a more consistent tank level and eliminate the excess head required across the control valve.
The disadvantage is the added capital cost of the control loop and variable speed drive. In addition, the losses across the variable speed drive need to be taken into account in power requirement.
Passive controls have simple designs and low installation costs, but they can have greater process variability and operating costs. Active controls can maintain a set point with minimal changes in the process variable. These systems have tighter control that comes at a higher original cost, but the operating cost can be much lower.