Boiler feed pumps provide the boiler with the appropriate amount of water needed to maintain safe operation. The water being pumped into the system may be feedwater, condensate returns or a combination of the two depending on the system. Boiler feed pumps are generally multistage pumps. These pumps must generate enough pressure to overcome the steam pressure generated by the boiler. To do this, they use multiple impellers with bearings on the end of each shaft to overcome boiler pressure.
Horizontal Boiler Feed Pump – Picture Courtesy of Pro Pump
Horizontal Pumps
These pumps tend to be easier to install and maintain than their vertical counterparts. This is largely due to the fact that their internal parts are easier to access. While this a major benefit, the horizontal positioning also takes up more floor space in a boiler room. These pumps can typically handle up to 1500 PSIG, which is slightly lower than vertical pumps.
Vertical Pumps
These pumps take up less floor space but require more vertical space. As mentioned above these can handle higher pressures (and temperatures) than the horizontal pumps. Due to there design, these pumps also are much harder to maintain.
Vertical Multi-Stage Boiler Feed Pump
On/Off vs Modulating Continuous Feed Pumps
Boiler feed pumps can operate using two distinct methods: on/off control and modulating continuous control .
- On/Off Pumps: These pumps operate by switching on when the boiler’s water level drops, which is detected by a low-water cutoff float. While simple and cost-effective, these systems can create problems. On such problem is thermal shock due to large temperature differences between the boiler water and the incoming feedwater. This can cause damage to surrounding metal components. The frequent cycling of these pumps also leads to more wear and tear.
- Modulating Continuous Pumps: By using more advanced controls, modulating pumps adjust the feedwater flow based on the boiler’s steam rate. This ensures more consistent water levels, reducing the risk of thermal shock and minimizing mechanical stress on the pump. Though they come at a higher installation cost, modulating continuous pumps improve long-term reliability and energy efficiency.
Like this article? Check out our recommendations for Centrifugal Pump Maintenance here.
Feedwater And Pumps
Feed pumps are designed to handle the high temperature requirements of the boiler systems. Water turns to steam at 212 degrees Fahrenheit at atmospheric pressure. However, by pressurizing the deaerator, water will stay in its liquid form at higher temperatures. This means some pumps are designed to handle temperatures from 225 to 350 degrees Fahrenheit.
Pumps are commonly stainless steel or ductile iron. Depending on the chemical additives in the water, you may want to choose one over the other. If the wrong material is in use, it can cause corrosion on pump components over time.
Pressure Requirements
Use the list below from an article written by Industrial Steam of pressure drops to consider when calculating discharge head for a pump:
“The Total Pump Discharge Pressure must overcome the following:
System Pressure – The pressure required by the steam distribution system.
Non-Return Valve Loss – how much drop occurs across the non-return valve? The boiler must operate at a pressure high enough to overcome this and satisfy the system pressure requirements.
Economizer – The pressure drop across the waterside of the economizer.
Super Heater – The pressure drop across the waterside of the super heater.
Modulating Level Control Valve – The pressure drop across the level control valve. (not required with on-off pump control)
Feed Stop & Check Loss – Drop across the feedwater stop and check valves (if used).
Piping Loss to Boiler Height to boiler water level – The drum may be substantially higher than the boiler feedwater pumps. The pumps must be able to lift the water up to the drum level.
Total Discharge Pressure –The sum of all pressure drops the feed pump must satisfy. After the total pressure drop is calculated, it must be converted to feet of discharge head in order to select the pump. The correction factor of 2.31 converts PSIG to feet of head. We correct for the density of boiling water (227 degrees F) by dividing the head by .96.
TDH = total discharge pressure (PSIG) x 2.31 .96″
Choosing The Right Pump
Choosing the Right Boiler Feed Pump
When selecting a boiler feed pump, several key factors should be considered:
- Pressure Requirements: Ensure the pump can generate enough pressure to overcome the boiler’s internal pressure.
- NPSH Requirements: Meet the Net Positive Suction Head (NPSH) to avoid pump cavitation, which occurs when the pump’s pressure is too low. Cavitation causes the formation of vapor bubbles, which implode and can lead to pump vibration, mechanical damage, and potentially catastrophic pump failure.
- Pump Configuration: Decide whether a horizontal or vertical pump is more suitable based on available space and pressure needs.
- Modulating vs On/Off Control: Evaluate whether a modulating continuous pump or a simpler on/off system is appropriate for your application.
Boiler Feed Pump Redundancy
It is essential that a boiler has the water it needs to continue operating effectively. Decrease flow, and you will have a low water condition. Increase flow, and you end up with carryover. Due to this fact, most systems will have redundancy built in. As a boiler operator, it is imperative that you switch back and forth between these pumps on a regular basis. This helps ensure pumps don’t lock up from slitting idle for too long. It also will help you spot an issue before it is an emergency situation.
Maintenance Requirements for Boiler Feed Pumps
Proper maintenance of boiler feed pumps is relatively simple but crucial for extending pump life and avoiding costly downtime. Regular tasks include:
- Bearing Lubrication: Ensuring bearings are properly lubricated to reduce friction and wear.
- Seal Replacement: Inspecting and replacing seals as necessary to prevent leaks.
- Float Level Control: Checking the float level controls to ensure they are functioning properly.
- Leak Detection: Periodically inspecting piping for leaks, which could indicate corrosion or other system issues.
By staying on top of routine maintenance, you can extend the life of your boiler feed pumps and avoid unexpected failures.
Why Boiler Feed Pumps Are Important for Boiler Operation
A boiler feed pump is essential for keeping a boiler running safely and efficiently. Without it, the boiler can’t keep the right water levels, which can cause shutdowns or even damage. The pump sends water into the boiler at the right pressure to balance the steam pressure. It also helps the system run smoothly, preventing problems like sudden temperature changes, pump damage, and wear and tear. Choosing the right pump is key to reducing downtime, extending the life of the equipment, and keeping the boiler working properly.
