Boiler water supply pump precautions

Boiler feed pumps are used to pump water into a pressurized boiler, and their application considerations can vary greatly depending on the boiler's design and purpose. For example, a pump used on a domestic boiler will be very different from a pump used on a boiler used for large-scale power generation. We will focus on the latter here, but even in the power generation sector, there are different plant designs and power generation capabilities, resulting in a wide range of flows, pressures, and temperatures, all of which affect pump selection.

Coal-fired power plants with a capacity of more than 1 MW are usually equipped with a pair of boiler feed pumps, but in some cases a single boiler feed pump is used. The boiler feed pumps used in these applications are highly engineered and are among the largest pumps in the world from a power perspective due to the high flow and differential pressure requirements.

Combined cycle power plants use a gas turbine to generate electricity, coupled with a steam cycle (boiler and steam turbine) that utilizes the high temperature exhaust gases. The generation load of a combined cycle power plant can be as large as a coal-fired power plant, but because it uses multiple methods of generating electricity, the flow and differential pressure of the boiler feed pump are generally lower than that of a coal-fired power plant.

The most common pump types used for boiler feed service are bearing multistage pumps of the axially split (BB3), radially split single-casing (BB4), and radially split double-casing types. For a summary of pump type descriptions and definitions, visit datatool.pumps.org. BB3, BB4 and BB5 pumps for boiler water service will be custom designed for the application, so various factors will be considered in consultation with the pump manufacturer when selecting a boiler water service pump, such as steam generation capability, temperature and pressure requirements, material compatibility and maintenance requirements. In addition, how the pump will be operated is also an important consideration that needs to be discussed with the manufacturer. From an operational perspective, there are four important considerations: whether the pump needs to operate in hot standby mode, typical load fluctuations, start-up and shutdown frequency, and cold start requirements.

Hot standby is when a pump is kept ready for use when not in use, avoiding cold starts. In hot standby operation, boiler feed water is kept close to operating temperature and constantly circulated, allowing the plant to respond quickly to load demands.

Load fluctuations are changes in the demand on the pump during daily operations. Combined cycle natural gas plants typically operate at multiple load points over a 24-hour period. In this case, the pump selected should be designed to reliably accommodate load fluctuations, and the control system should facilitate meeting the different load points while maintaining optimal efficiency and reliability.

Frequent shutdowns and starts stress the internal components of the pump and can cause faster wear than in continuous or hot standby operation. In addition, frequent cycling of large motors can also cause them to fail. Materials of construction, internal design clearances, and motor starting methods can be optimized to mitigate the stress and wear caused by starting and stopping the pump.

Cold starting is starting the pump when the system is at or near ambient temperature. As with the considerations for frequent shutdown and startup operations, pumps that require cold starts can be preheated, heated slowly, and designed with materials that have limited expansion and contraction due to temperature.