Some boilers utilize a reasonably steady load so only drum level control from single element drum level measurement is possible.
Single element control is used on boilers during startup or low load regardless of capacity or rapid load swings. Single element control has often been unsatisfactory because some of the newer boiler designs have minimum water storage compared to the steaming rate of the boiler.
A majority of the larger sized units and those subject to rapidly fluctuating loads require different methods of control. A two element system controlling the feedwater control valve from the steam flow signal and resetting the drum level signal is able to handle some of the less difficult systems.
Larger units with small storage capacity related to throughput, and units experiencing severe, rapid load swings, usually require three element control, whereby water flow is matched with steam flow and reset from the drum level signal.
Single element feedwater level control
Single element drum level control measures drum level only. This is a simple feedback control loop (Figure 1).
The mass of the water flow and the steam flow must be regulated so mass water flow equals the mass steam flow to maintain drum level. The feedwater control regulates the mass water flow to the boiler. The effects of the input control actions interact, since firing rate also affects steam temperature and feedwater flow affects the steam pressure, which is the final arbiter of firing rate demand.
The overall system must be applied and coordinated in a manner to minimize the effects of these interactions. The interactions can be greatly affected by the control system design. If the boiler operates under varying steam pressure, the calibration of the liquid level transmitter will also vary with steam density.
When a single element control system is implemented, the level transmitter (LT) sends a signal to the level controller (Figures 1 and 2). The process variable (input signal) to the controller is compared to the set point (SP). Single element control is the minimum feedwater control system. When the system consists of single and two or three element control, two controllers are required because the controller tuning is different.
The output signal is modified and adjusts the final control device. The diagram for a P&ID would typically be in ISA symbols (Figure 2).The final control device can be a control valve, pump speed control, or a combination of both. For simplicity, redundant transmitters are not shown on the P&ID drawings.
Two element feedwater level control
The two element control scheme utilizes steam flow in addition to drum level. This is a simple feedback plus feedforward control system (Figure 4) with a secondary variable that has a predictable relationship with the manipulated variable. The secondary variable, steam flow, causes the manipulated variable to change the primary variable. The steam flow adjusts the feedwater control valve based on steam flow signal and the drum level controller signal. As the steam flow increases or decreases, the steam flow adjusts the output of the summer and directly sets the feedwater final element. (See “Summer” section for further discussion.)
If the conditions are ideal, as in Figure 4, the feedwater flow would be equal to steam flow, and the level in the drum would be maintained. If feedwater pressure is not constant, as seen in Figure 5, the changes in pressure would affect feedwater control.
Three element feedwater level control
Three element control utilizes steam and water flow in addition to drum level (Figure 7). This is a simple feedback, feedforward, and cascade control loop. The steam flow adjusts the feedwater control valve based on the steam flow signal and the drum level controller signal. As the steam flow increases or decreases, the steam flow adjusts the output of the summer and directly sets the feedwater controller set point. By adding feedwater flow, the measured variable is the feedback to the controller, therefore measuring what is being controlled. Control is improved by adding mass flow compensation to drum level, steam flow, and water flow.
Three element feedwater adds the feedwater flow measurement to the control loop. The significant advantage in adding a feedwater controller is the measurement of the controlled variable. By measuring the feedwater flow and controlling it to a flow set point, the feedwater equals steam flow to maintain drum level over the boiler control range. The desired feedwater is maintained and overrides shrink and swell and variations in feedwater header pressure (Figure 8).
Control system configuration
The drum level controller is configured to be either a reverse- or direct-acting controller. This depends on the configuration of the final control device and the failsafe mode of the control valve. If the control valve fails closed, the controller is configured to be a reverse-acting controller. If the control valve fails open, the controller is configured to be a direct-acting controller. The final control element may be a control valve, speed control, or a combination of both. Speed control is used to reduce line pressure drop and is also used in combination with a control valve to establish greater turn down.
A summer occurs when two or more values come into an equation and the output equals the sum of the inputs in percent based on the K values. The functions are referred to as function blocks or algorithms. In Figures 4 and 7, the summer equation is used although the equation is not functioning as a summer. Some companies use multiplication or division equations. The summer can be configured using a basic summer equation. The equations used will vary with the control systems specified, including the number of inputs. The equation below represents three inputs: a, b and c. Only two inputs are used in Figure 7. This is a common type of equation that may have two or more inputs depending on the vendor algorithm/function block. A basic equation performing the same function should be available with most control systems.
K (a) + K (b) + K © ± Bias = output
Considering two inputs to the summer, set both K values to 1. Set the bias to -50%. With the drum level controller in manual mode and the drum level at the desired set point, set the controller output to 50%.
The 50% output is offset by the -50% bias in the summer equation. Therefore, the steam flow signal input to the summer will equal the summer output. Considering a pound of steam equals a pound of water, the drum level would be maintained during all load changes. Even though swell and shrink may occur on load swing, the steam flow modifies the feedwater rate. The drum level controller becomes a trim controller adjusting the output of the summer to control the level at the set point.
ABOUT THE AUTHOR
G. F. (Jerry) Gilman 's career has spanned more than 37 years with Procter & Gamble (P&G) and its subsidiaries. A licensed professional engineer, Gilman provides training for ISA and consulting to utility plants, industrial power plants and engineering contractors. Gilman is the developer of the “ISA Boiler Control Systems Engineering” and “ISA Burner Management Systems Engineering” training courses.