The generation of power stations built in the UK nearly 40 years ago are now all operated by private companies. Originally designed for continuous operation, modern conditions require that the stations are routinely brought on and off load (two-shift operation) placing heavy demands on the feed pumps and reducing reliability. Sulzer Pumps undertook a major retrofit of the pumps at the Ratcliffe power station to upgrade them to a design suitable for this new mode of operation.
Problems
The original pumps (not supplied by Sulzer) were designed for operation at their maximum continuous rating delivering 1660 m3/h at 196 bar. For many years the station operated in this way with excellent reliability from the pumps. However privatisation led to operation between 180 and 520 MW with much higher stop start cycle rates. This new operating mode caused problems with the feed pumps, specifically
- Low pump flow produces much higher pump bearing vibration levels, typically 12 mm/s.
- Low pump flow accentuates ‘blade passing’ hydraulic pulsations generating undesirable vibration levels, sometimes >20 mm/s.
- Greater risk of bearing wear/failure and balance disk wear/failure.
- Greater risk of impeller failure. Failure at the back shroud/hub radius due to fatigue.
Low speed and therefore low flow operation of large boiler feed pumps can significantly increase the risk of failure of components such as bearings and the balance disc. ‘Blade passing frequency’ effects have also proved to be particularly troublesome.
New cartridge
As the station is expected to be generating for at least the next 20 years a complete replacement of the existing cartridge but retaining the pressure containing barrel was proposed. The new cartridge would be specifically designed to address the challenges of the new operating regime and featured:
• Retention of existing drivers (cost)
• Minimum impact on pipework and foundations (cost)
• Hydraulic sized for real world duty requirements
• Optimised inlet geometry for improved suction performance
• Minimise vibration due to blade passing over the operating range of the pump
• Increase in peak and off load efficiency
• Balance drum in place of the existing balance disc
• Mechanical seal in place of existing labyrinth seals
• Bearings suitable for a barring speed of 28 rpm
• Cover and new elastic pressure bolting to interface with the existing barrel
• Service life approaching 60,000 hours
Other train related modifications included a modern non lubricated coupling between the pump and driver, a grounding device to help prevent shorting across the seal faces and an upgrade of the existing oil supply.
The chosen solution was a Sulzer HPT 350-540 six-stage pump designed to operate at 1690 m3/h at 200 bar but with a preferred operating range covering 1345 to 1960 m3/h. This design flexibility allows the pump to be reliably operated over a wide range of station output levels.
Building and testing
Essentially the cartridge manufacture and assembly followed the same process as for a new pump. The key difference in this instance was the refurbishment of the existing barrel casing to accept new studs plus the refurbishment and resize of the sealing faces. Inspection of the casing indicated the existing faces were damaged and needed a full weld repair and re-machining. To ensure a high degree of accuracy, the casing was removed, welded up and re-machined on a vertical borer.
To allow testing without the original barrel, a low pressure test barrel was manufactured to allow the new cartridge to be tested at reduced speed. The test would however allow accurate calculation of full speed performance as well as assessing the cartridges mechanical performance including axial thrust loads.
The tests confirmed the new design covered the required duty envelope with better performance than initially predicted including a power saving of 2.5% at the 100% load condition compared to the proposed performance.
After testing the cartridge was assembled into the refurbished barrel and installed on site ready for re-commissioning. Performance after commissioning reflected the test data and the pump continues to deliver the flexibility the operator initially specified.
Conclusion
The constraints of designing a modern day, state-of-the-art, cartridge to fit an existing ageing barrel are the biggest challenges that face pump manufacturers when designing retrofit cartridges. Items such as the existing site NPSH available and essential services such as lube oil and cooling water can, in some cases, limit the design scope or solution.
The real challenge for the Ratcliffe Project was not including all the modern day mechanical features, but providing a hydraulic solution that allowed the pump to operate efficiently over a wide operating envelope. An effective design solution was demonstrated during the slave tests and the pump has gone on to perform as predicted in the field, reducing the operators costs through improved efficiency, extended time between maintenance and much improved reliability. After the expected benefits were proven to the customer’s satisfaction they proceeded to upgrade the remaining units in a similar manner.