Abstract: Addressing the issues with the original design's emergency power supply and feedwater pump electrical control, the hardware, including the emergency power supply, was reconfigured. Automatic control of the boiler feedwater pump was achieved through a DCS system, effectively preventing safety incidents caused by boiler water shortages during power outages.
Keywords: Boiler feed water pump; DCS system; Automatic control
0 Introduction
The boiler feedwater pump is a critical piece of equipment supplying water to the boiler. A lack of water in the boiler can lead to a major safety accident, such as a boiler explosion. Our plant is located in a lightning-prone area, and power outages caused by lightning strikes occur frequently. Therefore, implementing automatic control of the boiler feedwater pump is of great significance for ensuring its normal operation, preventing boiler water shortages, and ensuring the safe production of the sulfuric acid plant.
1. Problems with the original water pump electrical control
1.1 Power Supply Issue: This sulfuric acid plant has one 35kV main power supply line, one 6kV backup power supply, and one 3000kW waste heat generator set. When the 35kV main power supply line and generator set trip due to an accident, the backup power supply takes approximately 20 minutes to provide power. However, our plant's actual measurements show that the steam tank level drops from 80% to zero in about 6 minutes. Therefore, the originally designed backup power supply is completely ineffective. This power supply problem poses a significant safety hazard to production.
1.2 Electrical control issues: There are two water pumps, which serve as backups for each other. The electrical control of the water pumps is a simple manual control circuit. When a fault trips, it is necessary to manually go to the site to restart them, which increases the restart time.
2. Reconfiguration of security power supply hardware
2.1 A Swedish VOLVO diesel generator, model HC. 1444DI, with an apparent power of 260KVA, was purchased. This unit features easy operation, reliable protection, accurate generator status display, password-protected menu, a user-friendly interface, precise voltage and current detection, and time programming. The unit uses the Canadian MEC2 control system. During normal operation, the unit displays parameters such as water temperature, oil pressure, speed, battery voltage, current, and apparent power. In case of a fault, the fault cause and alarm are displayed, and the unit can automatically shut down if necessary. There are manual and automatic operation options. In manual operation, press the RUN button first. The unit will start, accelerate, and reach a certain speed according to the set program before closing the circuit breaker to supply power to the load. To stop, press OFF. In automatic operation, press the AUTO button once to put the unit into automatic standby mode. When the mains power fails, after the start signal is input, the unit will start, accelerate, and enter normal operation according to the set program after a delay. When the mains power is restored, the unit enters the chiller program. It will automatically shut down after running under no-load for 1 minute.
2.2 A dual-power automatic transfer switch is configured. The dual-power automatic transfer switch model is WHK 630-M/4P, which adopts an electromagnetic drive and electromechanical interlocking mechanism. The main circuit contacts have a two-stationary-one-moving structure, and the moving contact adopts a "V"-shaped design. This switch has an automatic switching function. When the main power supply side voltage relay detects a voltage signal (such as undervoltage or loss of voltage), the backup power supply side voltage relay activates; after a delay, the dual-power automatic transfer switch activates, and the backup power circuit is connected. When the main power supply returns to normal, the main power supply voltage relay activates, and after a delay, the switch activates, restoring the main power supply, while the backup power supply terminates.
2.3 A soft starter device was installed, using AB company's product, model: SMC PLUS90KW. Since the water pump motor capacity is 90KW, direct starting of the motor would require a significantly larger diesel generator capacity. Therefore, a soft starter device was added. This serves several purposes: first, it reduces the motor's starting current; second, it reduces the impact on the motor and mechanical equipment during startup; and third, it reduces investment costs.
3. DCS system realizes automatic control of boiler feed water pump
3.1 Boiler Feed Pump Electrical Control Principle: The original manual control is maintained. An interlock control for the boiler feed pump is added to ensure that only one feed pump can operate at a time. Automatic control of the boiler feed pump is added to the DCS system. When the power grid fails, the diesel generator automatically starts, ensuring that the feed pump can start automatically. Figure 2 shows the electrical control principle diagram of the boiler feed pump.
3.2 DCS Control Principle and Control Ladder Diagram for Feedwater Pumps. The control signals of the two feedwater pumps are sent to the DCS system. Input signals include: interlocking signal, closing signal, fault signal, voltage monitoring signal, and soft-start closing signal. Output signals include: feedwater pump start/stop output and short-circuit soft-start output. The DCS system handles the logical relationships of various signals and issues start and stop signals to the feedwater pumps according to the DCS control ladder diagram.
3.3 Automatic Control Process of Feedwater Pump. There are two starting methods for the automatic control of the feedwater pump: First, the interlock signal starting method. When the feedwater pump is fault-free and not running, when the selector switch is in the interlock position, a starting pulse is sent, and the feedwater pump contactor engages. The auxiliary contacts of the feedwater pump contactor activate the soft starter, and the pump gradually starts. After 12 seconds of starting, when the feedwater pump approaches its rated speed, the DCS system sends a short-circuit soft start command, causing the feedwater pump to run at full pressure. Simultaneously, a normal feedwater pump operation signal is sent to the DCS system, allowing central control personnel to monitor the pump's operation. Second, the normal voltage starting method: When the feedwater pump is fault-free and the selector switch is in the interlock position, when the power grid fails, the VOLVO diesel generator automatically starts, and the dual power automatic transfer switch automatically switches to the diesel generator set for power supply. When the voltage monitoring signal detects normal voltage, a starting pulse is sent. The subsequent process is the same as the interlock signal starting method. When the power grid voltage returns to normal, the VOLVO diesel generator automatically stops. After the dual power supply automatic transfer switch automatically switches to grid power, the water pump restarts in the normal voltage start-up mode.
References:
1. Liu Baolin, *Handbook for Selection, Construction, Installation, Design and Application of Electrical Equipment*, China Water Resources and Hydropower Press, 1998, pp. 577-734.
2. *Hollysy MACS™ Hardware Manual*, edited and published by Beijing Hollysys Systems Engineering Co., Ltd., 2002, pp. 230-345.
3. *Hollysy MACS™ Software Manual*, edited and published by Beijing Hollysys Systems Engineering Co., Ltd., 2002, Section 3, pp. 67-169.
