Over the last few decades wastewater has become more complex. The material for a wastewater pump, therefore, must be carefully selected based on the composition of the wastewater to ensure the best performance at the lowest possible cost. Impeller spoke with Johanna Johansson, Application Engineer at Xylem, who has recently written a new white paper about how to select the optimal pump materials in wastewater applications.
“We receive a lot of questions about material selection and when to choose what material,” Johansson explains. “This new white paper will help our customers make decisions about the most reliable, long-lasting solution for their specific needs.”
The new white paper “Material selection for wastewater pumps: raise performance and extend system lifetime” is based on years of research and tests, both from the field and Xylem’s Flygt lab.
“We want our customers to achieve the optimal product lifetime at the lowest possible cost,” Johansson says. “When we talk about cost, we’re referring to total cost – this includes both the initial purchase and the ongoing need for maintenance, as well as sustained high efficiency. If you choose the right material from the outset, you’ll have fewer problems, resulting in lower overall costs and, ultimately, a more effective solution.”
More sand and grit in wastewater
Material choice should depend on the type of wastewater the pump is being exposed to, Johansson says. In recent decades the complexity of wastewater has increased for a number of reasons. For example, paved sections in cities and towns have increased in many areas, which leads to a greater concentration of sand and grit in the wastewater, and consequently a higher degree of abrasive wear on the hydraulic parts of pumps. In other areas, the growing cost of labor has made it more cost effective for the pumps to transport the particles, instead of using vacuum trucks to remove sand and grit from the system.
Given this increased abrasion, Xylem has carried out extensive wear tests with sand at its Flygt lab. The tests, which compare the material wear of grey iron, stainless steel and hard-iron, have demonstrated that stainless steel and grey iron impellers wear out at approximately the same rate, while a hard-iron impeller lasts approximately three times longer.
Selecting the best material
The most common wastewater applications have a low amount of corrosives and abrasives. Here a grey iron impeller is the best solution, Johansson says. If there is a high amount of abrasive particles in the wastewater, however, a hard-iron impeller is the best choice. Hard-iron has an extremely high wear resistance due to its embedded hard chromium carbides.
In addition to abrasives, other factors that affect the selection of material are chloride content, pH value, temperature and oxygen content. If there are corrosives in the wastewater, the pump needs to be protected with both a special epoxy coating and zinc anodes regardless of the material choice of the impeller.
“Sometimes there’s a misconception that a stainless steel impeller is the most effective option if there are chlorides in the wastewater,” she says. “However, hard-iron is a more suitable choice – particularly for applications requiring greater robustness and durability. One of the reasons for this is the low electro-chemical potential of hard-iron, which reduces the risk of galvanic corrosion.”
Download the white paper Material selection for wastewater pumps.