In modern power generation units, like those now commonly found in refineries and petrochemical facilities, boiler feedwater pumps consume a relatively small amount of power — an estimated 1-3% of the input heat from the boiler — thus contributing small rises in enthalpy throughout the power cycle.
However, boiler feed pump reliability is one of the main factors affecting power plant availability, as well as overall profitability of the power generation station.
Power plant availability is determined by subtracting the planned outage time and forced outage time, both measured in hours, from the planned operating time of 8,760 hours in one year. The reliability of a power plant is defined as the planned time of operation minus the forced outage time. The typical availability of power plants is in the range of 88-95%, whereas reliability is in the range of 94-99%. So, forced outages occur in even the most reliable plants.
Modern petrochemical processing facilities have opted to build dedicated cogeneration units for a number of reasons. Steam is plentiful, as these plants generate exothermic reactions resulting in heat that is otherwise lost. An integrated unit leads to more predictable external electricity costs or eliminates them altogether. Considering that many of the units are small, the manpower required to run an integrated power plant is fairly small. Once the integrated unit is ready, the biggest risk faced by the refinery is a forced outage, subjecting it to base load and peak load electricity costs until the outage is resolved. This leads to the subject of boiler feed pump reliability.
In recent history, boiler feedwater pumps have been selected as multi-stage, between-bearing machines. The preferred prime mover for a boiler feed pump is a steam turbine, utilizing excess steam from the boiler or steam supply. An electric motor driver is not uncommon but has the added costs of electricity transmission.
The hydraulic components of most well-operated centrifugal pumps work normally during operation, and that extends to boiler feed pumps. However, with the aging of a pump and the wear of its components, mechanical seals and bearings can be subject to transient loads outside of their designed operating conditions. This occurs because as clearances in a pump are enlarged, the natural hydraulic radial and axial thrust loads fall out of specification, pushing bearings and seals to their mechanical limits. This is exacerbated in peaking plants, where the load swings frequently or vanishes completely. The result is less than optimal hydraulic performance, parasitic energy loss in the form of mechanical vibration and noise and of course, unplanned outage time which exposes the plant to greater electrical costs.
Our experience has allowed us to develop a standard upgrade for between-bearing boiler feedwater pumps up to about 5000 brake horsepower, or 3.6 MW. The Conhagen solution involves:
- Upgrading the stationary wear ring materials at the impeller hub and eye rings, as well as the balance drum to a non-galling, dry-running solution
- Mechanically locking the impeller wear rings in place without the use of fasteners, using Conhagen’s Clic-Loc tongue and groove design
- Adding pressure-reducing grooves to the high-pressure bushing to lessen its influence on the pump rotor dynamics and allowing for the bearings to resolve the static and dynamic loads
- Performing rotodynamic analysis to confirm compliance to API 610 stability requirements for the upgrade. Should the pump for some reason fail to meet the stability criteria, modifications to shaft and bearing designs can be done in-house.
Conhagen has spent the last few decades studying its own service records and has found that these options, combined with thoughtful commissioning and pump operation, lead to robust pump reliability. We believe a pump’s best day should not be on the test stand — it should be in the field. We would be happy for the opportunity to service your boiler feedwater pumps.
For more information, visit conhagen.com.
