What Is Boiler Blowdown?

A boiler works hard, often in challenging conditions. Routine service is crucial for ensuring the boiler functions efficiently and offers optimal performance. A regular boiler blowdown also reduces the risk of unexpected equipment failures and mechanical issues. 

Boiler Blowdown Basics

A boiler generates hot water or steam. When boiler water turns to steam, the process leaves behind impurities, such as sediments, solids and chemical concentrations. An accumulation of these substances will insulate the heating surfaces on the underside of the boiler, reducing its operational capacity and efficiency. 

The boiler blowdown procedure uses pressure to drain the water and eliminate the scale. It also adds new water to replace the liquid removed during the process, improving water quality while preventing scale and rust. 

Blowdown can be continuous or intermittent. The former entails a slow, ongoing removal at the top of the water level and is effective for removing impurities with minimal water loss to drainage. The latter removes undissolved or suspended solids. 

The Blowdown Procedure

The blowdown process consists of several steps:

1. Operate the quick-opening valve: A boiler blowdown valve helps to clear sludge and impurities. Operating this mechanism causes significant changes in water flow without delays.
2. Operate the slow-opening valve: This boiler valve, which is typically farther away from the boiler than the quick-open version, requires five 360-degree turns to go from the closed to the fully opened position.
3. Implementation: Execute the process for the designated time by opening and closing the slow-opening valve. It may require multiple cycles to maintain the appropriate water level, which you can observe by monitoring the gauge glass. 
4. Closure: Close the slow valve, followed by the quick valve.
5. Valve opening: Open the slower valve again to drain the line between the two valves. 
6. Repeat closure: Close the slow valve again and check for a secure shutoff after cooling.

The blowdown separator plays an important role in the process. During operation, the water enters the top of the unit and flows over a striking plate that separates the steam and condensate, providing a safe place for the blowdown water to flash to steam.

Blowdown Frequency and Control

In most cases, a blowdown should occur at least once during each eight-hour work shift. Factors to consider when determining the appropriate frequency include the boiler’s capacity, the desired water quality and the nature of the operating conditions. 

Implementing appropriate control techniques is essential for preventing excessive or insufficient blowdowns. The different methods include:

• Manual: An instantaneous manual system operated approximately once per shift can reduce the total dissolved solids (TDS) to an acceptable level.
• Automatic: These automated systems optimize surface blowdown rates by regulating the water discharge volume.
• Conductivity: A conductivity-based control system uses a probe to measure boiler water conductivity by comparing it with a designated set point.

Blowdown Heat Recovery

A heat recovery system can capture some of the wasted energy generated during the blowdown process. Heat recovery increases overall efficiency and conserves resources. The types of boiler blowdown heat recovery systems include:

• Flash tanks: A tank converts a portion of the blowdown into low-pressure steam.
• Heat exchangers: An exchanger recovers heat by preheating the boiler makeup water.
• Economizer: An economizer transfers flue gas heat to the incoming feedwater, reducing the boiler’s fuel requirements.

Contact Smith Hughes to Learn More

As a leading nationwide provider of boiler-related products and services, Smith Hughes can help you understand every aspect of the boiler blowdown process, including the procedure, frequency and control methods and heat recovery. We’ll assist you in implementing best practices for your preventive maintenance procedures. Contact us for additional information today.