Extensive research into the processes that degrade pump performance is being matched by the development of application techniques for protective coatings. By gaining a greater understanding of both the pumping process and the factors that affect it, end users can make improve their maintenance strategies.

For those working with large industrial pumps, maintaining their performance in the face of a continuous threat from corrosion and erosion can be a particular challenge.

Corrosion

Corrosion is often defined as a chemical reaction between the component surface and the reacting fluid passing through a pump.

Flow accelerated corrosion

Flow accelerated corrosion (FAC) describes the removal of the protective oxide layer on a metal. The speed of this process is impacted by the oxygen content, flow velocity and, to some extent, the chloride content. The formation of a calcareous layer due to high carbonate hardness of the water reduces or even prevents FAC.

Erosion

Pumps used to transfer fluids containing abrasive substances (such as sand) can experience significant levels of erosion, especially in areas with high flow velocities. This can be seen in the oil and gas industry where injection pumps are employed to force water back into the oil field, thus maintaining the pressure needed to lift the oil to the surface. The sand particles act as an abrasive and the high working pressures only compound the issue.

Pump manufacturers have two options to minimise erosion, either reduce the flow velocities in every part of the pump or design the pump in such a way that the flow velocities through the close running clearances are low.

However, often, the specifications required for the application will prevent either of these solutions from being implemented.

Erosion-corrosion

In operating conditions where both erosion and corrosion are present, the degradation mechanism can become complex. Corrosion can create oxide layers with low adherence to the substrate that is prone to erosion. Alternatively, erosion may damage the passive layer, leading to an activation of the surface which accelerates corrosion.

Appropriate material selection

Corrosion problems are mitigated using an appropriate base material in the construction of the pump. For applications where use of carbon steel or cast iron is preferred, the corrosion rate can be estimated accurately. Based on a mutually accepted corrosion rate per year, the service life of the pump can be anticipated and factored into the maintenance costs of the application.

If the expected corrosion rate is not acceptable, the pump materials have to be upgraded to stainless steels, which leads to higher costs.

If stainless steel is selected for an application, the expected service life is much longer. However, this is only true if the appropriate stainless steel grade has been chosen for the specific application. Even stainless steel becomes susceptible to corrosion due to the passive layer being damaged and the base material becoming activated, which then starts to corrode.

Usually the passive layer can be re-established. However, if the chloride content is too high or the pH level too low, the material may remain in an active state and corrosion continues. Another frequent cause of corrosion in stainless steel pumps are stagnant conditions caused by process interruptions or intermittent operation.

Protective coatings

It is important to determine whether the application of coatings will actually improve the performance and the service life of a pump in the first instance and if the costs are really lower than a materials upgrade. In most cases, pump manufacturers aim to meet the requirements of a process by using the most appropriate materials for the application, with coatings being used only as a backup solution.

Polymeric coatings, such as fusion bonded epoxy, can be applied to pump components using a fluidised bed or electrostatic coating. They provide a good level of corrosion protection so long as the coating is not damaged. Polymer coatings are limited to low flow conditions and tend to be used in clean water applications, where it may also improve hydraulic performance by smoothening the pump surfaces.

Coatings that are appropriate for pipelines may not be suitable for pump applications where the flow velocities are much higher, narrow passages concentrate the flow and moving parts can be difficult to protect. Again, some methods that are commonly used in pipework (such as galvanic protection) are largely unsuitable for pumps.

In these cases, coatings are applied to specific areas where increased flow rates are expected or at points where impact damage is expected, such as 90 degree bends. A hard layer is usually applied using a spray coating method.

Not all coatings are the same

The improvements in performance and durability afforded by coating systems have given rise to a large number of businesses offering a coating service.

Coatings often take time to apply correctly, which will inevitably impact the final cost of refurbishment. However, increasing the deposition rate will increase the stresses within the coating. Over time, this can cause the coating to degrade and fail prematurely.

The procedures and settings used by companies such as Sulzer and its coating suppliers have been developed over many years. The procedure for coatings is closely guarded, ensuring that every client will receive the same quality of coating.

Modern coating technology can be applied to legacy equipment to extend the service life of a pump. Implementing a new coating as part of a refurbishment project can significantly improve the performance and reliability of existing equipment.

Ultimately, the key to a successful corrosion prevention scheme is to fully understand the application and to use all available information to determine the most appropriate action.

The future

As coating technologies continue to advance, end users will eventually be able to select coatings that can be applied during the manufacturing process.

Improvements in service intervals will lead to reduced maintenance costs and reduced costs attributed to lost production. Together with improved efficiency, these costs of ownership can be minimised through the appropriate use of base materials, protective coatings and the implementation of better pump design to deliver a comprehensive and cost effective pump solution.

Read the article online at: https://www.worldpipelines.com/business-news/13012017/sulzer-comments-on-corrosion-and-erosion/