Economizer Protection Measures During Boiler Startup and Shutdown

Economizer Protection Measures During Boiler Startup and Shutdown

## 1. Pre-ignition Preparation and Potential Issues

Before igniting the boiler, fill the boiler water to the lowest visible level on the water gauge. After ignition, as the boiler water temperature rises, the water volume expands, causing the water level to rise. When steam begins to generate within the water-cooled walls, the water level rises further. Therefore, additional water replenishment is generally not required for a long period after ignition.

## 2. Economizer Risks During Boiler Startup and Shutdown

The economizer recirculation valve is a key device for protecting the economizer. Its core function is to open the recirculation valve during startup or shutdown, when feedwater is interrupted. This valve utilizes the difference in specific gravity of the working fluid between the steam drum and the economizer to create natural circulation, continuously replenishing the economizer with water and preventing damage from overheating due to water depletion.

During startup and shutdown, the boiler often experiences intermittent water inflow, during which time the water flow within the economizer stagnates. If protective measures are not taken, high-temperature flue gas will continuously heat the economizer, causing water inside the tubes to evaporate or even dry out. The conflict between high temperatures outside the tubes and the lack of cooling inside the tubes can directly lead to overheating and damage of the economizer tubes. Furthermore, residual water inside the tubes may boil and vaporize due to the high temperature, generating superheated steam, further exacerbating the risk.

## III. Correct Operation of the Economizer Recirculation Valve

### 1. When to Open

- The recirculation valve must be opened during the initial ignition of the boiler (when water is not being continuously fed) and after shutdown (when feedwater is interrupted). During this period, water in the steam drum flows into the economizer through the recirculation pipe, where it is heated and then returned to the drum, forming a natural circulation loop. This continuous water flow removes heat and ensures effective cooling of the economizer.

### 2. When to Close

- The recirculation valve must be closed immediately when the boiler enters normal operation with continuous water feeding. Leaving it open at this point will cause a "feedwater short circuit": some feedwater, without being heated by the economizer, enters the steam drum directly through the recirculation pipe. This not only reduces the cooling effect of the economizer but also affects the thermal efficiency of the boiler.

## IV. Hazards of Improper Operation

1. **Failure to Close the Recirculation Door in a Timely Manner**

- If the recirculation door is forgotten to be closed during water filling, the recirculation pipe resistance is much lower than that of the economizer, allowing most of the feed water to flow directly through the recirculation pipe into the steam drum. This causes a sharp decrease in the amount of water flowing through the economizer, preventing effective cooling of the tube walls and potentially causing overheating and damage to the economizer.

- If the recirculation door is not closed during normal operation, allowing low-temperature feed water to enter the steam drum directly will lower the local temperature of the boiler water, disrupt the steam-water separation process, and affect steam quality. Furthermore, the temperature difference between the recirculation pipe opening and the steam drum will generate stress, which may cause circumferential cracks in the steam drum during long-term operation.

2. **Recirculation Door Not Closed Tightly**

- If the recirculation door is not properly sealed during operation, low-temperature water that has not been heated by the economizer will seep into the steam drum, causing cracks in the drum metal due to uneven heating and cooling, threatening boiler safety.

3. **Improper Operation After an Economizer Leak**

- After a shutdown due to an economizer tube leak, it is strictly forbidden to open the recirculation door. During the initial shutdown period, the furnace wall temperature remains high. If recirculation is activated, water in the drum will escape through leaks, exacerbating damage. Furthermore, premature water discharge (when the water temperature exceeds 80°C) can cause thermal stress in the convection tubes and water-cooled walls due to rapid cooling, leading to expansion or weld leaks.

## V. Key Measures for Economizer Protection

1. **Strictly Control Recirculation Door Operation**

- The recirculation door must be opened during intermittent water supply, during initial startup, or when water supply is interrupted after shutdown. After continuous water supply or normal operation, it must be closed immediately to prevent water short-circuiting.

- Regularly check the tightness of the recirculation door to ensure it is leak-free after closing.

2. **Special Protection After Shutdown**

- After an economizer leaks and shutdown, reduce the makeup water flow rate and extend the water supply time to avoid damage caused by sudden changes in water velocity in the tubes.

- Boiler water may be discharged only after 24 hours of shutdown and when the boiler water temperature drops below 80°C to prevent thermal stress damage caused by high-temperature water discharge.

3. **Monitoring During Startup and Operation Transition** - During the transition from startup to normal operation, ensure that the recirculation valve is closed to prevent low-temperature water from directly entering the steam drum, which could affect steam quality and equipment life.

By standardizing recirculation valve operation and paying attention to post-shutdown status control, economizer damage due to overheating and thermal stress during startup and shutdown can be effectively avoided, ensuring safe and stable operation of the boiler system.