Automatic water supply system for large industrial boilers

System Overview

Large industrial boilers typically use an automated closed-loop system to maintain drum water level within tight limits, compensating for steam demand fluctuations. The system architecture comprises a feedwater source (condensate return and makeup water), a deaerator or feedtank to remove dissolved gases, feedwater pumps, control valves, flow meters, level detectors, and a central controller (PLC or DCS). Redundancy is often built in with multiple pumps and parallel control valves to ensure uninterrupted operation during maintenance or pump failures.

Control Strategies

Single-Element Control

In single-element control, only the drum water level is used to modulate the feedwater valve. This approach is simple but can suffer from slow response to rapid steam demand changes, leading to level oscillations and inefficiencies.

Two-Element Control

Two-element schemes add steam flow measurement to the level controller, allowing feedwater flow to track steam output more closely. This reduces level deviations during load changes but can still be affected by feedpump dynamics and supply pressure variations.

Three-Element Control

Three-element control incorporates drum level, steam flow, and feedwater flow into a weighted control algorithm, providing the most stable response during rapid load swings. Industry practice mandates three-element loops for boilers above 5 t/h to meet safety and efficiency standards.

Key Components

Feedwater Pumps

Centrifugal pumps, driven by electric motors or steam turbines, must deliver variable flow rates with minimal pulsation. Pump control often uses variable frequency drives (VFDs) to match feedwater flow to demand, improving energy efficiency.

Feedwater Control Valve

High-precision control valves, such as sliding gate or globe valves, regulate water flow into the boiler drum. Valves are sized to handle maximum demand and feature fast-acting actuators to maintain tight level control.

Level Detection

Conductivity probes, floats, or differential pressure transmitters provide real-time level data. Three-probe conductivity arrays are common in modern systems, offering adjustable on/off thresholds with built-in hysteresis.

Feedtank and Deaerator

Feedtanks serve as buffer vessels and semi-deaerators, removing dissolved oxygen and CO₂ to prevent corrosion. Design must account for adequate residence time, spray nozzles, and venting to achieve >95 % deaeration efficiency.

Water Treatment Integration

Effective water treatment is essential to protect boiler internals and maintain thermal efficiency. Makeup water is pre-treated via softening, demineralization, or reverse osmosis based on boiler pressure class. Continuous conductivity monitoring and automatic blowdown systems maintain dissolved solids within limits, reducing scale and carryover. Inline chemical dosing stations adjust pH and scavenge oxygen, coordinated through the central control system.

Safety & Maintenance

Regular calibration of level sensors and flowmeters is mandatory to ensure accuracy. Valve stroking tests and pump performance checks should be scheduled during low-load periods. Common issues include control valve sticking, probe fouling, and deaerator vent blockage; troubleshooting involves loop checks, cleaning probes, and verifying actuator response.

The boiler system of Fangkuai boiler relies on PLC control system, which can accurately control the automatic water supply device to avoid abnormal water level.

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