Improving lifetimes of pumps

VAD Souza and A Neville from the University of Leeds detail their research into boosting the lifetime of wastewater pumps' service through the use of thermal spray coatings

The use of pumps in wastewater treatment is crucial in all phases of the process and therefore it is important to have pumps, which provide long service life to reduce costs. The use of stainless steel in internal parts of the pumps (impellers, wear ring, etc.) has provided successful performance over the years. However, material waste can present different pH values, and solid particles entrained in the fluid and the wide range of environments, can make stainless steel become vulnerable to high rates of material wastage. There is often a requirement to adopt materials or surface treatments to provide higher resistance because of this.

For this article, some laboratory tests were carried out in order to investigate the improvement in performance offered by high-velocity oxy-fuel (HVOF) thermal spray coatings against stainless steels. The tests were performed in an erosion corrosion environment (silica sand in 3.5% NaCl) at three sand loadings (200, 500 and 1,000mg/l) on WC-based HVOF coatings, austenitic stainless steel (UNS S31603) and super duplex stainless steel (UNS S32750).

The results showed the HVOF WC-based thermal spray coating can enhance the performance compared to both stainless steels by 80%. Coating technology for alleviating erosion corrosion damage in wastewater applications, therefore, has some potential.

The collection and conveyance of spent water to a WwTW presents an extremely difficult environment for materials to operate in. Wastewater can be defined according to the origin, such as residential and non-residential wastewater. The residential wastewater comprises mainly sewage and this includes wastewater from toilets and other parts of the household. Non-residential wastewater originates from industries, businesses and institutions and also storm, rains and floods1.
Pumping station equipment must handle sewage and waste of almost every form and description, including both solids in suspension and in solution, therefore having access to reliable pumps is one of the most critical issues in processing waste water2. Often the reliability of pumps is limited by degradation of the materials by processes such as corrosion and wear.

Sewage is wastewater containing ions in solution as well as organic and inorganic particles in suspension. The range of pH values can be from five-eight and other factors such as dissolved oxygen content, the hydrodynamic regime in the pumping process, means corrosion and erosion present real challenges for the operation of pumps. In order to resist chemical attack it is often necessary to use stainless steels, at least for the key components. Duplex (austenite-ferrite) steel is widely used because of its high corrosion resistance and yield strength3.

In anaerobic digestion systems the temperature is up to 65°C 4 which, in combination with a lack of oxygen, may require a CrNiMo duplex steel containing Cu. If corrosion is minimised by appropriate alloying, erosive wear will often determine the life of pump components3.

Centrifugal pumps are normally chosen for three main reasons5:

  • they can endure periods of low to near zero flow conditions,
  • they can pump a reasonable amount of solid waste,
  • pump selection is relatively simple with centrifugal pumps because they can be choked without damaging the pump, thereby allowing operation over a wide discharge range.

    Due to the increase in water consumption, together with the demographic explosion, the whole optimisation process for industrial and domestic wastewater treatment has to move forward – and since the pumping system is the ‘heart’ of the wastewater treatment process, it is necessary to be constantly seeking longer lifetime pumps and more resistance to varied environmental conditions.

    Stainless steels offer resistance to corrosion due to their thin, tenacious, Cr2O3-rich passive film (Figure 1), which prevents charge transfer at the fluid/material interface. In instances where there are solids present, or high-shearing flows, the passive film integrity can be breached and this enables high rates of corrosion to ensue.

    Under these circumstances one material selection solution is to adopt surface engineering systems, which can offer the additional protection of resistance to erosion to the component. In surface engineering systems the surface of the material is modified to provide enhanced properties and in this case erosion and corrosion resistance. Surface engineering systems can basically be divided into ‘line of sight’ and ‘non-line of sight’ categories. The line of site processes refer to techniques where access to the surface to be treated is restricted to geometry, whereas non-line of sight does not depend on geometry and can access any part of surface.

    In non-line of sight processes the electroless plating technique and thermo-chemically formed ceramic coatings applied by dip process can be considered, while line of sight processes generally include evaporation deposition (PVD), ion implantation (IM), ion beam mixing (IBM) and thermal spray coating. Among thermal spray coating processes the HVOF technique has evolved over the years and now provides coatings with optimum compaction and low chemical decomposition, especially for WC-based coatings6.

    One of the great advantages of the HVOF process is the higher velocity reached by the particles and the low temperatures involved, which minimises any potentially damaging effects to the coating and substrate7. As it is shown in Figure 2, the HVOF process comprises a mixture of fuel (propane, propylene, hydrogen or acetylene) and oxygen, which are burned in the chamber and because of the expansion the gas velocity can become supersonic (Mach 5).

    Powder is introduced axially, heated, melted and accelerated. The powder normally reaches velocities around 550m/s. A great advantage of HVOF on conventional thermal spraying is the lower temperature (1,900-3,000K), which prevents carbide particles from changing or oxidising8. WC-based coatings have been applied successfully on stainless steel substrates by the HVOF process in internal parts of centrifugal pumps in erosion corrosion environments increasing the lifetime up to 600% 9.WC-Co-Cr thermal spray coatings have an extremely complex microstructure (Figure 3), which is dependent on a number of factors relating to the powder feedstock, the processing method and the application conditions.

    The thickness can reach values of 500µm but in the work reported in this article, the HVOF coating used was 250µm thick. In Figure 4, a SEM image of the as-polished WC-Co-Cr coating applied by the G-Gun HVOF system (Greenhey) used in this work is shown. The coating presents a distinct hard phase characterised by the angular-shaped particles embedded in the binder. The presence of pores is also evident (specified to be less than 1%).

    The chemical composition determined by energy dispersive X-ray analysis (EDX) in wt% is 86WC, 10Co and 4Cr with no indication of eta phases and a low formation of W2C as presented in XRD results in a previous communication10. In erosion corrosion the mechanisms of damage on a material are derived from mechanical erosion processes, electrochemical corrosion processes and interactions between the two.
    On a range of metal alloys it has been shown there are synergistic interactions between erosion and corrosion such that the conjoint action leads to more damage than the processes separately. In thermal spray coatings the complexity of the microstructure ensures the erosion corrosion processes are equally complicated. In Figure 5 the key material degradation processes which can occur in ceramic/metal thermal spray coatings are shown.

    The processes that lead to the total material degradation can be summarised as:

  • corrosion (C) – electrochemical charge transfer leads to material loss. The charge transfer rate can be accentuated by the increased mass transfer or mechanical impacts as a result of an impinging flow and can take place on hard phase and matrix,
  • erosion (E) – mechanical damage due to impacts of a high energy flow or suspended solids within a flow. The material removal does not involve corrosion processes,
  • interactions or synergy (S) – the enhanced material damage as a result of corrosion enhancing erosion. In this instance the corrosion processes affect the integrity of the material and render it more susceptible to mechanical damage. The regions of highest susceptibility are at the hard phase/matrix interface,
  • galvanic interaction between coating and substrate due to either corrosion or erosion damage allowing the fluid to reach the substrate.

    In this article, some results from an experimental programme are reported in which thermal spray coatings as potential surface engineering treatment in pump components are evaluated. Erosion corrosion tests in this work were conducted using an impinging jet apparatus, which comprised a liquid-solid jet generated from a recirculating system. The rig comprised a dual nozzle system as presented elsewhere11.

    The velocity of the jet for this study was kept constant at 17m/s. The solid loading in the study was 3.5%NaCl used just to represent a saline fluid and appreciating that in wastewater systems there are a range of fluids used with various ionic strengths. The solid loadings used were 200, 500 and 1,000mg/l and fluid temperature of 20 and 50°C, covering a range of typical solid particles present in municipal wastewater as presented in Table 1. The sand used for all tests was HST Congleton silica, with approximately 60% of particle size concentrated in the range 250-350µm. For all tests the angle of impingement was 90°. The surface area of the samples exposed to the jet was 3.8cm2. The degree to which the HVOF WC-Co-Cr coating offers an enhanced performance compared to the uncoated base stainless steels is evaluated in Table 2 by defining the improvement percentage (IP). In this respect it is important to point out the low improvement provided by the coating at 200mg/l-500°C over UNS S32760.

    This indicates that at a low-solid loading the high corrosion resistance and recovery efficiency of the super duplex stainless steel is predominant, appearing as an advantage over the coating. At all sand loadings and temperature the HVOF performs better than both stainless steel and this means surface engineering has the potential to improve the pump lifetime and offer cost-savings in terms of reduced maintenance. To determine whether the initial investment in supply of HVOF-coated parts is justified or not, it is necessary to conduct a full-life cycle-cost analysis, which is beyond the scope of this article. For design and application purposes the higher performance of HVOF over stainless steel is mainly due to hardness values, although the materials fall in different groups (stainless steels and cermets).

    Figure 6 shows the relationship of volume loss and hardness for HVOF WC-Co-Cr, UNS S31603 and UNS S32760. In this graph the high hardness of WC-Co-Cr HVOF gives the lowest volume loss values but the comparable hardnesses for stainless steels show wide variation in erosion corrosion resistance. Hardness, as reported elsewhere12-15 can only give a very broad indication of how a material will resist erosion or erosion-corrosion and other parameters are equally as important (such as resistance to electrochemical corrosion).

    This is in agreement with Wentzel et al.16-17, who also reported a loose correlation between erosion corrosion resistance and wear. Although thermal spray coatings present the lowest volume loss, the stainless steels have proved to highly promote a high rate of film recovery in erosion corrosion environments – and this is an important feature in terms of material degradation reduction. The mechanisms of degradation of thermal spray coatings differ to stainless steel due to the fact the passive film formation is different and has less self-healing capability compared to stainless steel. HVOF coatings therefore offer improvements in surface durability in conditions where severe wear and corrosion occur.

    Their erosion corrosion resistance depends on a number of factors to do with the hard phase concentration and distribution, the matrix content and ability to repassivate and conceal wear/corrosion interactions. Understanding how the materials degrade is crucial to optimise performance in wastewater processes. The conclusions reached include:

  • surface engineering can be employed to improve the lifetime of centrifugal pumps used in waste water treatment,
  • HVOF coating technology is a versatile process potentially of benefit for harsh erosion-corrosion environments.
    The authors acknowledge Weir Pumps, Glasgow and Greenhey Engineering Services, England for the financial support provided to V.A.D. Souza.
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