Boiler blowdown is a critical part of boiler operation. It refers to the removal of a certain amount of boiler water to maintain the water quality in the boiler.
Maintaining good boiler water quality is essential. Excessive impurities in boiler water can cause problems such as scaling, corrosion, and reduced steam quality. These problems not only reduce boiler efficiency, but also pose safety risks and even shorten the life of the equipment.
There are two main ways to blow down a boiler: continuous blowdown and Regular Blowdown. Understanding the difference between these two methods is important for achieving efficient boiler operation and maintenance.
Continuous blowdown is the continuous removal of boiler water from the surface of the steam drum. This is the area where salts such as calcium and magnesium and other substances accumulate as water evaporates.
The main purpose of continuous blowdown is to control the concentration of dissolved solids (such as calcium and magnesium salts) to ensure the purity of the produced steam. In addition, it helps maintain boiler water alkalinity to prevent steam-water co-boiling and steam water carryover. Steam and water co-boiling produces wet steam, which reduces the efficiency of steam power plants, while steam carryover carries impurities into the steam system, damaging downstream components.
Regular Blowdown is the intermittent discharge of boiler water from the bottom of the boiler. The bottom of the boiler is the area where sludge, sediment and insoluble impurities accumulate due to gravity.
The core goal of Regular Blowdown is to remove these solid impurities. In this way, scaling is prevented, which forms an insulating layer on the heat transfer surfaces and reduces boiler efficiency. It also avoids localized corrosion, which can cause corrosion by forming areas of different electrochemical potentials due to the accumulation of sludge.
Continuous blowdown is carried out continuously during boiler operation to achieve continuous control of the boiler water chemistry.
The blowdown rate is usually about 1% of the boiler steam production. However, this rate can be adjusted based on the results of water chemistry testing. For example, if the concentration of dissolved solids in the boiler water is higher than the recommended level, the blowdown rate can be appropriately increased.
To improve energy efficiency, continuous blowdown systems are often equipped with flash tanks (blowdown separators) for heat recovery. Flash tanks vaporize high-pressure blowdown water to form steam that can be reused elsewhere in the system, such as in deaerators.
Regular Blowdowns can be performed manually or automatically at set intervals (e.g., every 8 hours). The exact interval depends on factors such as boiler type, feed water quality, and expected impurity accumulation.
The duration of each Regular Blowdown is generally between 30 seconds and 1 minute, and the blowdown rate is relatively low, about 0.1% - 0.5% of steam production.
Regular Blowdowns are usually performed during low-load periods to reduce thermal stress on the boiler. When the boiler is at low load, the temperature and pressure changes during the blowdown process are smaller, which reduces the risk of damage to boiler components.
The water source for continuous blowdown is from the steam drum, specifically the steam-water interface, which is an area rich in dissolved solids.
The main equipment of a continuous blowdown system includes a blowdown control valve (to regulate the blowdown flow), sampling points (to monitor water quality), and a blowdown tank or expander (flash tank).
The steam produced by the flash tank can be reused in the deaerator to achieve partial energy recovery. This not only saves energy but also reduces the overall operating cost of the boiler system.
The water source for Regular Blowdown is taken from the lowest points in the boiler system, such as mud bags, water wall headers, and economizers, where sludge and sediment accumulate.
The key equipment of a Regular Blowdown system is the blowdown valve (manual or automatic) and the blowdown tank.
Due to the high amount of sludge in the blowdown water, it is difficult to recover heat or reuse the water. The high impurity content makes it very challenging to treat this blowdown water for reuse, and the short blowdown time also limits the effectiveness of heat recovery methods.
Continuous blowdown helps keep the boiler water chemistry stable. By controlling dissolved solids, the risk of steam carryover and wet steam is reduced, ensuring that the steam produced is of high quality and suitable for a wide range of industrial applications.
However, excessive and continuous blowdown increases fuel consumption. For example, for every 1% increase in blowdown rate, fuel consumption may increase by about 0.3%. This is because more water containing heat is discharged from the boiler, and the boiler needs to consume more energy to replenish the heat.
Regular blowdown can effectively remove solid impurities and sludge from the boiler, helping to maintain the integrity of boiler components and prevent scaling and localized corrosion.
Before regular blowdown, the water level must be strictly controlled. If the water level is not properly controlled, it may cause operational risks such as water hammer or boiler dry burning.
Due to the high impurity content of the discharged wastewater and the short blowdown time, the heat loss during regular blowdown is large. Energy that could have been used to generate steam is wasted as these hot water rich in impurities are discharged.
|
Item |
Continuous Blowdown |
Periodic Blowdown |
|
Purpose |
Remove dissolved solids |
Remove sludge/sediment |
|
Operation Mode |
Continuous |
Intermittent (e.g., every 8 hours) |
|
Discharge Location |
Steam drum (top) |
Mud drum / bottom header |
|
Blowdown Rate |
About 1% of steam production |
About 0.1% – 0.5% of steam production |
|
Heat Recovery |
Partially recovered (via flash tank) |
Very little or none |
|
Typical Equipment |
Control valve, flash tank |
Blowdown valve, expansion tank |
|
Water Quality |
Clear but high in dissolved solids |
Turbid, with a large amount of solids |
To achieve effective boiler water quality management, a combination of continuous and scheduled blowdowns is recommended. This integrated approach addresses both dissolved solids and sludge issues in boiler water.
Blowdown strategies should be based on regular water quality monitoring results. By regularly analyzing the water chemistry, operators can adjust the blowdown rate and frequency to ensure optimal boiler operation.
Optimizing the chemical treatment of boiler water can also reduce the frequency of blowdowns and improve energy efficiency. Chemical agents can be used to prevent scaling and control the pH of the water, reducing unnecessary blowdowns.
Blowdown processes can be made more precise by using automated blowdown controllers. These controllers adjust blowdown operations based on real-time water quality data to ensure that blowdown occurs at the right time and in the right amount.
Both continuous and scheduled blowdowns are essential parts of boiler operation. Each method plays a unique role in maintaining boiler water quality, ensuring boiler efficiency, and improving equipment safety and longevity.
Choosing the best blowdown method depends on a variety of factors, including boiler type, feed water quality, and the specific operational needs of the facility. By deeply understanding the differences between continuous and periodic blowdown and implementing reasonable blowdown measures, boiler operators can optimize boiler performance, reduce operating costs, and ensure safe and reliable operation of the boiler.
