Wastewater pump selection – a material approach

Published:  10 September, 2015

Due to the unpredictable nature of modern wastewater, sewage pumps are subject to vastly variable levels of corrosion and abrasion. Selecting the most suitable material is therefore imperative to obtain a reliable, long-lasting and cost-effective solution. PWE spoke to Tony Price, market development manager at Xylem Water Solutions who explained the measures that can be undertaken to protect pumps and prolong their lifetime.

Over the past few decades, the method of transferring wastewater has become increasingly more complex due to geographical extensions, which have increased the corrosive and abrasive effects of wastewater.

Although wastewater treatment has improved, the amount of oxygen in wastewater, together with chemical treatment of wastewater generally, leads to a higher degree of corrosion of volutes, pump impellers and propellers.

As a result, higher demands are placed on the correct selection of pump material. This is a critical measure in the protection of pumps to ensure optimal long-term performance resulting in sustained high efficiency and ultimately leading to significant energy savings.”

The role of wastewater in a pumps lifetime

Typically, the factors that affect the nature of wastewater are chloride content, pH value, temperature, oxygen content and levels of abrasion. All of these variables should be considered when selecting the pump material and protection.

Untreated wastewater does not normally contain dissolved oxygen. However, if oxygen is present, even in low quantities, then unacceptable levels of corrosion can occur. In untreated wastewater, abrasive particles are often present increasing the risk of wear on the hydraulic parts.

Pump parts, such as impellers, propellers and suction covers which are in direct contact with the transferred liquid, are at the highest risk of erosive wear. Wastewater velocity is directly proportional to the wear rate. This needs to be minimised as when the velocity of the liquid transfer increases, so does the wear rate.

Controlling the impact

The choice of pump material primarily depends upon the application and ensuring the correct selection is the key to achieving a long pumping lifetime. Being the component with the highest level of contact with transferred liquid, the material used in the impeller is a crucial factor as it is heavily affected by wear and erosion - corrosion.

The most frequently used pump materials, especially for the impeller, are grey iron, stainless steel and hard-iron. Grey iron is known for its excellent casting and machining properties and is a suitable choice of material if there are no requirements for corrosion protection or extra wear requirements. Grey iron can be hardened where required, for example on an impeller’s leading edge. Stainless steel demonstrates a higher resistance to types of corrosion but where abrasive particles are present there is a lower resistance to wear.

Hard iron demonstrates medium corrosion resistance while displaying very good wear resistance properties. In instances where there is a high level of abrasive particles but low Chloride content (hence minimal corrosive effect), the use of standard pump materials combined with a hard iron impeller and Insert Ring will prolong the expected lifetime of the pump. This is a common solution for handling wastewater applications with abrasive particles.

Should there be a high chloride content within the wastewater, the high corrosion effects can be countered by using an epoxy coating on the pump combined with Zinc anodes in addition to the Hard Iron components as previously stated.

Taking the hard line on long term protection

Hard-iron is a high-strength alloy containing 25% chromium and 3% carbon. After solidification this makes Hard Iron highly resistant to abrasive wear and erosion – corrosion. In terms of relative wear, Grey Iron shows an index of 1.3 (3.3 if hardened) whereas stainless steel is 2.0; hard iron shows a relative wear index of 10 and is resistant to chloride levels of up to 300ppm.

Tony Price explained his company has conducted extensive accelerated wear tests on impellers at its laboratory, and observed that grey iron and stainless steel wore at approximately the same rate; 2mm in 50-63 hours of accelerated testing. However, hard iron displayed this level of wear at 190 hours of accelerated testing thus demonstrating the suitability of this material for abrasive applications as found within modern wastewater.

Price concludes that he believes hard-iron pumps offer a long-term solution to the transportation of corrosive and abrasive wastewater, which not only can meet the highest pumping requirements, but can also reduce maintenance costs and provide sustained high efficiency levels.

For more information please visit: www.xylemwatersolutions.com/uk

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