Load Cycling

Boiler Load Cycling

In response to changes in fuel costs, environmental issues, alternative energy supplies and other drivers, many plant owners often consider cycling their boilers to operate on/offline or at reduced loads during low demand. Most units currently in service and supplied pre-1980 were designed to operate at base load, so cycling operation and faster ramp rates could adversely impact critical boiler components. 

Since the 1970s, B&W has analyzed, tested, engineered solutions and developed best practices for cycling units originally designed for base load service. Whether then, during the rising interest in nuclear power and the high cost of natural gas and oil, or now, with low natural gas prices and supplemental solar and wind power, a thorough condition assessment is essential to determining potential equipment modifications that can make the unit more flexible and achieve cycle goals.

There are four types of operation related to boiler cycling which are of interest:

Load cycling      

The boiler remains in service, but load is varied widely, running at maximum load for part of the day and a low load for the remainder.

On/Off Cycling     

Boiler is brought offline for periods of time and then returned to service.

Fast Load Ramping

Boiler is required to respond quickly and ramp load at high rates.

Extended Load Range

Continuous operation with minimum stable heat input to the boiler; for example, less than 25% of rated heat input.

Each method for cycling presents a different set of challenges. In reviewing options for addressing them, B&W considers overall equipment operation including:

  1. Combustion system flame stability
  2. Fuel delivery system 
  3. Air quality control / emissions
  4. Circulation 
  5. Pressure part life assessment

Turndown Impacts

Turndown rates are classified as mild (40 to 100% of maximum continuous rating, or MCR), moderate (25 to 40% MCR) or deep (below 25%) load cycling operation. Relative system impacts (fuel dependent) include:

Retrofits and modifications are available to minimize the impacts of cycling on boiler systems. These may include:

Request a Condition Assessment

Cycling Considerations Primer

Most of these units were originally designed for base loaded operation and not designed to accommodate frequent on/off cycles. The stresses placed on the weld joints, headers, drums and attachments between expanding and contracting parts, can cause failures after a finite number of cycles.