The pump was an eight-stage barrel diffuser (BB5) operating at 4,200rpm, with flow of approx. 273m3/hr and discharge pressure greater than 275 barg.
The pump was operating with decoking water, which is primarily composed of wastewater from other refinery operations. Typically, the water could contain coke fines at 2,500 ppm and residual corrosive media such as hydrogen sulfide.
Frequent operation of the pump at low flow rates had resulted in suction/discharge recirculation damage. After about six months of operation, a loss of material from the thrust bearing caused the rotor to shift on to the thrust pads, resulting in high bearing temperatures. Further inspections found:
• Impeller inlet vane eroded due sub-optimal flow conditions
• High-velocity abrasion at the diffuser inlet
• Corrosion erosion at the wear ring side faces
• Severe erosion of the shaft at impeller waterways
• Significant loss of material at running surfaces
• Clearances were 300% greater than the original design
The options for improving this situation include replacing the pump with a new product using higher-grade materials, replacing the damaged components like-for-like, or implementing some mechanical design changes and repairing the damaged parts using a coating material that will improve durability. Of these three, the last was the most cost-effective solution over the mid- to long-term.
Root cause analysis is essential if situations like this are going to be improved. Unless there was a defect in the original materials, replacing parts like-for-like will result in the same outcome. By understanding the cause of the problem, it is possible to deliver a long-term solution. In this case, the impeller clearances were increased by the abrasive nature of the media, which caused the pump body to be worn away. In addition, sharp corners and high angles of incidence of the media flow paths caused the abrasive particles to wear away the base material.
The first improvement came from redesigning the thrust rings to reduce the angles of incidence and to remove any sharp corners. In addition, radii were added to the wearing parts.
However, greater durability can be achieved through the addition of specialist coatings, which can improve performance and reliability. For applications designed to improve wear and corrosion resistance, high-velocity oxygen fuel (HVOF) coatings are commonly used.
HVOF is a line-of-sight process and to optimise the application accuracy, the coatings were applied using a robotic arm equipped with automated thermal spray equipment. The external, and as far as possible the internal, surfaces of the impeller were treated with an erosion-resistant pump coating while the shaft bore had an anti-galling coating applied to it.
To refurbish the pump shaft to original dimensions, it was rebuilt using the HVOF coating process. The coating was then surface-ground to the original dimensions before being reassembled. The refurbished pump was reinstalled and monitored carefully to determine the increase in MTBF.
In fact, the pump continued to run for two full years before it was re-inspected for wear. By reusing the original parts from the pump and enhancing their wear characteristics with upgraded coatings and components better suited to the requirements of the application, the clearances on the high-pressure bushing were only 0.05 mm oversize after two years of service.
Within the same timeframe, if the operator had continued using replacement parts, the cartridge would have been changed twice at this point. Instead, the refinery had made a significant saving in maintenance costs, as well as minimising downtime, by opting to upgrade the parts with a more durable coating and improving the mechanical design of some of the parts.