Ensuring pump reliability in abrasive service

When a refinery was experiencing accelerated wear on its coke cutting jet pump, which was incurring significant costs with a mean time between failure (MTBF) of six to ten months, it turned to Sulzer.

The application required a pump that could deliver a high differential head and handle varied fluids that had a significant content of abrasive fines. 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 onto 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 that uses higher grade materials, replace the damaged components like-for-like, or implement some mechanical design changes and repair the damaged parts using a coating material that will improve durability. Of these three, the last is 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, provided they are applied by service centres that have expert knowledge and understanding. For applications designed to improve wear and corrosion resistance, high velocity oxygen fuel (HVOF) coatings are commonly used.

Different coatings have slightly varied bonding properties with different substrates, so it is essential to understand the conditions required to achieve a perfect bond. A coatings bond is one of the most critical aspects of its success in service. As such, it should be in focus during all processes associated with coating.

In every refurbishment project, establishing the process specifications are essential to the long-term success and durability of the coating. This involves detailing the approved equipment and process parameters as well as the properties required for the coating acceptance, such as its tensile strength, microstructure characteristics, hardness and surface roughness values.

By inspecting the microstructure and mechanical properties of the coating it is possible to verify that it was applied to required specifications and that it will provide all of the expected benefits in operation.

At the refinery, the 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 0.002 inches (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 minimizing downtime, by opting to upgrade the parts with a more durable coating and improving the mechanical design of some of the parts.

There are many pumping applications in the oil and gas sector involving fluids that contain abrasive media. Unless the pumps are designed and constructed to handle these challenging conditions, the reliability of this equipment can be substantially affected. Changes in process parameters and applications can cause legacy equipment to be adversely affected and without specialist intervention, or complete replacement, the maintenance costs can increase dramatically.

High energy pumps represent a considerable investment and they are expected to deliver efficient and reliable service, even in challenging environments. By working with a partner that has the expertise and facilities of an original equipment manufacturer (OEM), operators can benefit from a vast reserve of knowledge. Experienced assessment coupled with expert design analysis enable improvements in durability and reliability to be applied to high energy pumps. These cost-effective solutions represent an ideal opportunity to extend service life and minimise operating costs.

Sulzer (UK) Ltd

193 Camphill
Birmingham
B12 0JJ
UNITED KINGDOM

0121 766 6161

http://www.sulzer.com/

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