gold hand dredge pump

Failure analysis of a high chromium carbon steel impeller operating in phosphoric acid slurry

Abstract

The aim of this study is to identify the failure mechanism behind the damage found on the impeller of a slurry pump operating in a phosphoric acid production plant. The failure analysis was carried out first by identifying the solid particles found in the slurry, then by analysing the damaged impeller itself. The observed damage was non-uniform throughout the different regions of the impeller, especially in the back shroud. This is why a series of samples was extracted from different regions to perform hardness tests, microhardness profiles along its cross-section as well as cross-sectional microstructure examinations. Slurry erosion and cavitation erosion were the main damage sources and were particularly effective due to the uncharacteristically low surface hardness value. This was due to decarburization and the continuous network of carbides that facilitated crack propagation in the grain-carbide interface in near surface areas.

Introduction

Slurry pumps are widely used in chemical production plants, hydrometallurgy process plants, the mining industry and the petroleum industry. The failure of any of those could cause costly extended periods of production shutdown. Their internal components are exposed to a constant flow of solid particles in a corrosive medium at high velocities. This is why they are made from erosion-corrosion resistant alloys. Austenitic and duplex stainless steels (DSS) as well as high chromium cast irons (HCCI) are the most commonly used materials for these components, especially impellers. The impeller examined in this paper is made of a high chromium carbon steel. The common use of these alloys in erosion-corrosion applications stems from the fact that compared to stainless steels, high chromium carbon steels and cast irons present better wear resistance due their higher hardness for a smaller price due to the low content in expensive alloying elements such as Nickel. Erosion-corrosion resistance involves not only good mechanical properties to resist particle impingement but a sufficient corrosion resistance as well. Corrosion, unlike erosion, is an electrochemical process that involving the oxidation and loss of matter of metallic surfaces. Alloys cited above are protected against the effects of corrosion by an insoluble and protective passive layer made of the metallic oxides. The damage erosive processes have on these layers can, however, negate their protective properties and accelerate corrosion and mass loss. Impellers are amongst the critical components to a pump's performance and their wear is generally non uniform since the complex geometries lead to different flow patterns and velocities. The use of non-specified materials can be a root cause of unexpected impeller failure. Casting defects are also amongst the most common root causes of impeller failures. These works show the non-uniformity of wear damage in erosive-corrosive environments due the complex geometry of the impeller and the nature of the wear phenomena. The root cause of failure is most likely fabrication defects or the non-conformity of the material. However, few of the failure analysis studies have identified heat treatment caused phenomena as a root cause for the failure of slurry pump impellers. One of the root causes of failure this paper examines is a heat treatment caused phenomena, namely surface decarburization. The effect of the combination of decarburization and the microstructural change beneath the decarburized layer on the erosion-corrosion resistance and hardness of an impeller operating in a phosphoric acid production plant is presented in this paper.

Experimental procedure

The failed impeller and a 5 L sample of the slurry that circulated inside the pump were received from a phosphoric acid production plant. The impeller operates at 80°C and at a flowrate of 380 m 3/h and a rotational speed of 1200 rpm. It is a closed impeller with three inner vanes with a diameter of 400 mm and an eye's diameter of 70 mm. The liquid to solid ratio is 2.5:1, the liquid phase being a phosphoric acid solution at 29% concentration and a pH=1.

Visual inspection

Before the visual inspection of the impeller a white and brittle deposit covering most of the impeller's surfaces had to be cleared. A sample was taken and was analyzed by XRD and it was found that it was Sodium Fluorosilicate as well as an SiO 2 phase. This is understandable, since the impeller is downstream from the phosphoric acid production reactor. In fact, this deposit is a by-product of a series of chemical reactions which begin with the initial

Wear damage

The back shroud was the most damaged part of the impeller and wear damage is non-uniform. Beyond a circling radius value of 120–130 mm, cavitation erosion damage becomes the dominating wear pattern and is especially noticeable between the back-shroud's vanes and on their sides, meaning cavitation occurs at a velocity of around 15 m/s. Erosion patterns are also observable near the base of the

Conclusion

Under the service conditions of the phosphoric acid production plant, the impeller of a slurry pump was affected by slurry erosion, corrosion and cavitation erosion. Upon closer examination it was found that uncharacteristically low surface hardness values led to the impeller's failure. A microscopic analysis of multiple samples showed that a 2 mm deep layer was beneath nominal hardness values and that the layer itself was affected by decarburization for depth between 1 mm and 1.4 mm.