Pumps are the primary drivers of any industrial plant's process flow system, providing process chemicals, slurry transport, cooling water and waste transport to the various parts of a facility. These pumps must operate reliably and efficiently to meet production goals and ensure safe operation.
Pumps operate under fluctuating conditions including positive and negative pressures, elevated temperatures, corrosive solutions and abrasive slurry mixtures. The effects on the wet-end components of a pump's critical tolerances and the resulting impact on performance are numerous, and processes can impact these critical tolerances. For example:
- Insufficient discharge pressure can result in recirculation, leading to accelerated abrasive conditions if particulates are in suspension.
- Higher-than-expected temperatures can accelerate corrosive conditions of a medium, affecting a pump's metallurgical condition.
- Increasing amounts of solids in suspension can accelerate erosive wear of critical tolerances, reducing discharge pressures and increasing wear at suction spools, wear ring seats, cutwaters and volutes faces (see Image 1).
- Cavitation can impact impeller vanes, accelerating metal loss and weakening the vanes themselves (see Image 2) as well as damaging suction spools.
- Corrosion on pump wet-end internals from media changes or metallurgical deficiencies can lead to scale and corrosion deposits and/or pitting with resultant increased surface roughness that promotes pipe frictional forces and hydraulic drag (see Image 3).
Image 1. Damaging effects on pumps can include solids in suspension accelerating erosive wear as seen in Image 1. (Images and graphics courtesy of A.W. Chesterton Companies)
Image 2. Cavitation affecting impeller vanes in Image 2.
Image 3. Media changes leading to scale and corrosion deposits or pitting in Image 3.Corrosion and erosion are the primary causes of metal loss in pumps unless abrasion or cavitation is present. Corrosion is primarily an electrochemical reaction occurring between the metal and the environment that causes the anodic metallic ions to go into solution. To prevent this, a film or barrier coating may be applied that dielectrically insulates the metal surface, which contains anodic and cathodic regions, from the corrosive media.
Solutions
The use of coatings to maintain a new pump as well as to improve an in-service centrifugal pump's performance and energy efficiency is not new. In 1948, A.J. Stepanoff discussed the use and benefits derived from porcelain enameling a pump's wet-end in his book Centrifugal and Axial Flow Pumps. If these coatings can resist the effect of corrosion/erosion, chemical attack and abrasion, the flow through the hydraulic passage becomes more efficient and the dimensional tolerances are able to be maintained for an extended period, resulting in longer life and more reliable operation of the pump.
To determine how effectively a coating system can impact a pump's performance, one must first establish a performance reference point. This involves measuring flow and energy consumption over several duty points to establish a pump curve. Ultrasonic devices can accurately measure pump flow rates.
Installing pressure gauges at suction and discharge establishes inlet and discharge pressures. By using amperage meters, one can measure the energy demand to the pump motor during flow.
Once these steps are taken, the energy per unit flow (specific energy) can be established at a given discharge pressure. If the pump is impacted by corrosion involving scale buildup, reducing tolerances or increasing frictional drag, then a coating process can reduce corrosion scale buildup and frictional drag.
If erosion or abrasion accelerated by corrosive chemical exposures has opened up critical tolerances in the wet end of the pump, a resurfacing grade coating that uses abrasion-resistant reinforcements such as aluminum oxide (Al2O3) or silicon carbide (SiC) may be applied to restore these tolerances to design specifications and provide extended wear resistance.
There are specific considerations that need to be factored in to any coating material's selection such as adhesion of coating, resistance to chemical/corrosive attack, resistance to erosive/abrasive wear and resistance to operating thermal stresses. Any coating process also requires care in surface preparation to ensure the desired performance of the coating itself. This typically involves inspection and decontamination of the surface to remove impurities that impair adhesion, surface cleaning to remove scale and corrosion deposits, and grit blasting to increase the available surface area for maximization of the applied coatings adhesion.
Application may require multiple steps to rebuild or smooth worn surfaces and to achieve the required total dry film thickness.
These steps must be taken in a controlled environment and should only be undertaken by personnel experienced in the application of industrial maintenance coatings, who have been qualified by the manufacturer. Finally, inspection and re-assembly of the pump components must be done with care to ensure that tolerance fits and clearances are maintained.
Case Study
In one example, a wastewater treatment plant where pump efficiency and reliability were a concern identified a pump for maintenance based on assessing losses in flow volume, reduced suction and discharge pressure, and amperage draw. After the pump was disassembled, areas of high wear and corrosion losses were noted (see Image 4).
Image 4. Areas of high wear and corrosion are evident in a pump at a wastewater treatment plant in Image 4.
Image 5. Pump elements' exposed surfaces where corrosion or scale formation could occur were coated with two coats of a low coefficient of friction, ceramic reinforced epoxy coating.The pump elements were then decontaminated by steam cleaning, followed by abrasive grit blasting to SP5 (Sa2.5) White Metal cleanliness with a 3+ mil (75+ micron) angular profile. The sections where metal loss had occurred were rebuilt to original tolerances based on the original equipment manufacturer's design specifications using a high-build, advanced ceramic reinforced epoxy coating, with some areas receiving as much as 0.240 inch (6 millimeters) of coating to rebuild to original dimensions.
Following this stage, all exposed surfaces where corrosion or scale formation could occur were coated with two coats of a low coefficient of friction, brush- or spray-applied ceramic reinforced epoxy coating (see Image 5).
The coatings used were reinforced with ceramic particles for increased resistance to erosive and abrasive flow as the influent flow contained suspended grit from storm drainage and other organic waste. After repairs were completed, all areas were inspected for any discontinuities, and the pump was reassembled. Performance measurements were taken to reestablish performance, energy consumption per unit flow and efficiency curves (see Table 1).
Table 1. Performance measurements were taken to reestablish performance, energy consumption per unit flow and efficiency curves of the pump.This example shows how protective coatings can improve reliability and performance by significantly reducing the surface roughness over the base metal component and decreasing surface energy. The result is less wetting of the pump's internal surfaces from the process flow, reducing overall friction losses.
The application of the protective coating interrupted the erosion/corrosion process, resulting in smoother flow passages and less friction while increasing the bare metals resistance to the effects of abrasion in applications with solids or slurries.
Protective coatings may be used to protect new pump components from damage or to upgrade and return worn in-service pump components to new or near-new conditions at a fraction of the cost of spare parts or whole replacement costs. The process of upgrading the wet-end components with protective coatings can improve not only performance but also, in some instances, energy efficiency.
Pump wet-end coating can be important for almost any pump maintenance practice. For a minimal investment, it can lower operating costs and extend the asset value of the pump over a longer period of time than if left unprotected. It can result in lower life-cycle maintenance and associated process costs through extended mean time between repairs.