Rock Products

FEB 2018

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36 • ROCK products • February 2018 www.rockproducts.com and depends on the type of substrate and the fluid chemistry. Corrosion may create oxide layers with low adher- ence to the substrate which is prone to erosion, or erosion may damage the passive layer, leading to an activation of the surface which accelerates corrosion. In this case surface protection regimes are often the best and sole option. Cavitation – Most commonly seen on the pump impeller, cavitation is caused by a pressure difference, either on the pump body or the impeller. A sudden pressure drop in the fluid causes the liquid to flash to vapor when the local pressure falls below the saturation pressure for the fluid being pumped. Any vapor bubbles formed by the pres- sure drop are swept along the impeller vanes by the flow of the fluid. When the bubbles enter a region where the local pressure is greater than saturation pressure, the vapor bubbles abruptly collapse, creating a shockwave that, over time, can cause significant damage to the impeller and/or pump housing. In most cases it is better to prevent cav- itation rather than trying to reduce the effects on the pumping equipment. This is normally achieved by one of the three actions: • Increase the suction head. • Lower the fluid temperature. •  Decrease the Net Positive Suction Head Required (NPSHR). For situations where cavitation is unavoidable or the pumping system suffers from internal recirculation or excessive turbulence, it may be nec- essary to review the pump design or minimize the potential for damage using a bespoke coating system. Appropriate Material Selection For pump manufacturers, the key is to mitigate the corrosion problems by using the most appropriate base mate- rial in the construction of the pump. For applications where the use of carbon steel or cast iron is preferred due to cost reasons, the corrosion rate can be estimated very accurately. Based on the mutually accepted corrosion rate per year, the service life of the pump can be anticipated and factored into the main- tenance 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. In cases where this cost increase is prohibitive, the alternative is to use advanced coat- ings that can be tailored to suit each application. If stainless steel is selected for an appli- cation, the expected service life is much longer, in some cases infinite. However, this is only true as long as the appro- priate stainless steel grade has been chosen for the specific application, it has been produced carefully and is used within the agreed fluid specifications. Special care is required as soon as par- ticles are introduced into the fluid. In this case 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. Normally the passive layer can be re-established, but if the chloride content is too high or the pH level is too low, the material may remain in an active state and the corrosion continues. Another frequent cause of corrosion in stainless steel pumps are stagnant conditions caused by process interruptions or intermit- tent operation. A further threat for stainless steel is chlorine, which is used to combat biological growth in the pump or the connected pipelines. Low level con- centrations, around 2 ppm, will have little impact on stainless steel, but it is important to understand how and where the chlorine is introduced into the water flow, to avoid spot concentrations that will damage the protective layer. Unexpected corrosion can easily negate the anticipated improvement in dura- bility of stainless steel compared to the much cheaper carbon steel variant. Protective Coatings It is important to determine if the appli- cation of coatings will actually improve the performance and the service life of the pump in the first instance and if the costs are really lower than a materials upgrade. In most cases, pump manufac- turers aim to meet the requirements of a process by using the most appropriate materials for the application and use coatings only as back-up solution. Polymeric coatings like fusion bonded epoxy can be applied to pump com- ponents using a fluidized bed or electrostatic coating. They provide a good level of corrosion protection as long as the coating isn't damaged. As a polymer coating it is limited to low flow conditions and normally used in clean water applications where it may also improve the hydraulic performance by smoothening the pump surfaces. However, coatings which are appropri- ate for pipelines may not be suitable for pump applications where the flow velocities are much higher, narrow pas- sages concentrate the flow and moving parts can be difficult to protect. Again, some methods, such as galvanic protec- tion, commonly used in pipework, are largely unsuitable for pumps. In these cases coatings are applied to PUMP EFFICIENCY Cavitation damage should be pre- vented by changing the pumping system characteristics. Where cavitation is unavoidable, a bespoke coating system should be used.

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