Upgraded Boiler Feedwater Pump Improves Efficiency and Adds Flexibility
Power plant operators face operational challenges that stress their equipment in ways not anticipated at commissioning. Power plants designed and constructed as baseline plants are now balancing the load from intermittent power sources such as solar and wind generation. For many plants, this creates new operational challenges for machinery which was not designed for stop/start operation.
For example, this intermittent operation places new demands on boiler feed water pumps. When a plant is dedicated to base load power, boiler feed pumps make extended runs, often running continuously for years. When these plants change operation to balance the intermittent production from sources like wind and solar, the pumps need to cycle frequently. When the pump is idle, there is a phenomenon called “rotor sag” or “shaft sag”—basically the weight of the rotor causing shaft deflection. In a pump with a long, thin rotor, a typical value for rotor sag would be 0.010” (0.25 mm) at the center of the pump. The rotor sag goes away when the pump is running and hydraulic forces at the wear rings are established due to the Lomakin Effect. During start-up, however, the rotor sag can lead to contact at the pump center bushing and wear rings, creating the risk of galling and seizure.
This was the specific concern several years ago at Colorado’s Fort St. Vrain Generating Station. The plant’s original Ingersoll Rand Model DA boiler feedwater pumps were originally specified to make extended runs. The 11-stage, type BB3 pumps had a long, slender shaft with a center bushing between stages 5 and 11. With operations now requiring frequent cycling, the plant engineer recognized the increased risk of metal parts galling and the potential for pump seizure. The challenge was finding a solution which would allow operational flexibility without sacrificing pump performance.
There are basically 3 approaches to reduce the risk of galling and seizing in a pump:
- Increase internal clearances
- Apply hard-coated metals
- Apply composite material wear components
Increasing the clearance at the wear rings and center bushing in the pump is the worst option. Excessive wear ring clearance can lead to a wide range of pump problems including efficiency losses, loss of rotor stability, shaft breakage, driver overloading, bearing overheating or failure, unequal load sharing in parallel operation, noise and damage associated with pump cavitation, and possible total pump destruction.
Because the downside of excessive clearance is known, many pump shops offer different types of hardened wear ring materials to reduce the risk of seizure. These components generally run at the original design clearance in the pump and are certainly an improvement over increasing the clearance.
Boulden Company believes that the application of composite material wear components is the ideal solution for boiler feed water pumps. In this option, the stationary wear rings, center bushing, and throttle bushing are upgraded to a quality composite material like DuPont™ Vespel® CR-6100 and the rotating parts remain metal. With this simple upgrade, the metal-to-metal contact inside the pump now becomes metal-to-composite contact. Because a composite material like Vespel® CR-6100 is low-friction, non-galling, and non-seizing the risk of pump seizure is essentially eliminated.
Because the risk of seizure is so low with the composite upgrade, the internal clearance in the pump can be reduced below the original design clearance, often by as much as 50%. This reduction in clearance increases pump efficiency, reducing operating costs. The Lomakin Effect is increased, reducing vibration and extending the life of critical components like mechanical seals and bearings.
In a high-energy pump like a boiler feed water pump, Boulden will also apply our patented PERF-Seal® design to these components. This design improves the performance of these components in a high-pressure environment and imparts beneficial rotordynamic effects to make the pump even more reliable.
This was the solution chosen by the Fort St. Vrain facility. At full load operation, they already had issues with motors overheating. Diminishing pump performance with increased clearance would have made this problem worse. Being able to reduce the clearance and improve efficiency not only helped avoid seizure, but also helped the motors run without overheating. The reduction in clearance combined with the PERF-Seal® design produced a significant efficiency gain (Table A) in these pumps which have now been running for several years. Since the original upgrade, the remaining pumps in this service have subsequently been upgraded.
If you are interested in upgrading your boiler feed water pumps, contact Boulden today. We will be happy to work with whoever is overhauling your pump—whether it is your local shop or the OEM service center.
The Fort St. Vrain facility has a unique history. It was initially commissioned as a nuclear power station with an innovative high-temperature gas-cooled design. Unfortunately, the cutting-edge design created excessive operating costs, and the nuclear plant shut down in 1989.
But that wasn’t the end…To learn more about how an upgraded boiler feedwater pump improves efficiency and adds flexibility, download the Boiler Feed Pump PDF or contact Boulden today to learn more about how we can help you with your next project.