Boiler Feed Pumps
Boiler feed pumps deliver treated water into the boiler at pressures above the boiler operating pressure. Learn about pump types, sizing, common failures, and the critical importance of NPSH.
Role of the Feed Pump
The boiler feed pump must deliver water into the boiler at a pressure higher than the boiler operating pressure, overcoming the static head and friction losses. The pump must also handle hot water (typically 80 to 105 degrees C), which creates NPSH challenges.
Feed pumps are critical equipment. If a feed pump fails and the boiler runs dry, tubes overheat and rupture within minutes. Most boiler codes require at least two feed pumps, each sized for full boiler capacity, for redundancy.
Pump Types
- Centrifugal (multi-stage): Most common for medium to large boilers. Multiple impeller stages build the high pressure needed. Efficient and reliable. Can handle moderate temperatures if NPSH is adequate.
- Regenerative turbine: Used for small, high-pressure applications. Compact and can produce high pressure from a single stage. Sensitive to wear and cavitation.
- Positive displacement: Used for very high pressure or very low flow applications. Can handle high temperatures without cavitation issues but requires pressure relief protection.
Sizing
- Flow rate: Must exceed maximum boiler evaporation rate by at least 20% for safety margin.
- Discharge pressure: Boiler operating pressure plus 10 to 20% margin plus static head and pipe friction losses.
- Temperature: Feedwater temperature determines NPSH available. Hotter water = lower NPSH margin = higher cavitation risk.
- Redundancy: Always install at least two pumps. One operating, one standby. Each sized for full capacity.
Common Failures
- Cavitation: Caused by inadequate NPSH. The pump suction is starved, forming and collapsing vapour bubbles that erode the impeller. Solution: increase suction head, lower water temperature, or install a larger suction pipe.
- Seal failure: High temperature and pressure stress mechanical seals. Use seals rated for the temperature. Consider cooling flush for the seal.
- Bearing failure: Caused by misalignment, imbalance, or inadequate lubrication. Check alignment after installation and after any maintenance work.
- Motor overload: If the pump is operating too far right on its curve (too much flow), the motor draws excess current. Install a flow restrictor or reduce impeller diameter.
Never run a boiler feed pump dry
Dry running destroys mechanical seals in seconds. Always verify water level in the feedwater tank before starting the pump. Install a low-level interlock that stops the pump if the tank runs low.
Tip: Monitor feed pump motor current. A gradual increase over time often indicates wear (bearing, seal, or impeller degradation). Trend the data in PlantLogica to catch problems before failure.