Time Between Repairs for Wastewater Pumps, Picking Parts Based on Temperatures & Factors Causing Excess Vibration

Q. What is the estimated time between repairs for common wastewater pumps? A. The optimal performance or life of pumping equipment will be achieved when the equipment is correctly designed, sized and selected for a given application and the units are operated (and maintained) at the specified conditions. The design life can be different for varying services and equipment types. In any application, shareholders in the overall outcome will influence the type of equipment selected and the way it is applied and maintained. Main shareholders and influencers in the design, selection, application and maintenance of the equipment include the following:

• Project engineers: minimize capital expenditures
• Maintenance engineers: minimize repair hours
• Shareholders/owners/general manager: maximize dividends and/or share price
• Production personnel: maximize uptime hours
• Reliability engineers: maximize equipment reliability to avoid failures
• Accounting staff: maximize project net present value

In order for users to obtain maximum reliability and mean time between repairs, pump equipment must be properly designed and selected for the intended application, and it must be operated according to the manufacturer's recommendations. Actual equipment life may not match the predicted value, which depends on the various ways of measuring and analyzing reliability. Common pump types for wastewater processes are vertical sump pumps or submersible pumps. In a controlled process, the estimated or mean time between repairs is typically two to eight years, depending on the severity of service for both types of pumps. For pumps that require pressures up to 285 pounds per square inch gauge (psig) and temperatures up to 450 F and are exposed to a harsh abrasive environment, the estimated or mean time between repairs is typically two to four years. If in practice the equipment life falls short of industry averages, then a thorough review should be conducted. For more information on the reliability of pump applications, reference HI's guidebook Reliability of Pumping Equipment: Guidelines for Maximizing Uptime, Availability and Reliability. Q. What should end users consider when selecting wetted parts for pumps based on the temperature of the liquid being pumped? A. The handling of liquids at temperatures below 32 F (0 C) or above 250 F (120 C) usually requires careful selection of the materials and corresponding attention to construction details. The corrosion-resistance and/or physical properties of many of these materials are affected by high or low temperatures. For this reason, end users must consider the temperature of the liquid being pumped. For material selections acceptable in the temperature range involved, consult the applicable codes and practices of the industry in which the pump will be used. Selection of materials for pumps operating at low temperatures should be made only after each component and its function have been considered. Many materials change from tough to brittle behavior with a decrease in temperature. As a starting point in the selection of a suitable ferric steel for low-temperature service, the user may consider a heat-treated, fine-grain, low-carbon alloy steel with moderate hardness and low phosphorus, nickel and molybdenum. This material usually offers better notch toughness at low temperatures than other ferric steels. Users should also consider the austenitic stainless steels and bronzes for possible use in low-temperature pumping applications. Austenitic stainless steels, fully annealed, show improving toughness with decreasing temperature, and they exhibit no transition point. Most bronzes and all aluminum alloys are not embrittled at low temperatures, and they may also serve in this type of service, if otherwise suitable for the application. End users must carefully consider other factors, such as cost, corrosion-resistance, availability, erosion-resistance, hardness, toughness and fatigue strength, before making a final selection of materials for high- or low-temperature services. Also note that other factors should be considered when selecting materials for wetted pump parts, including the user's experience, the manufacturer's experience, the expected pump life (such as temporary or long-term use), whether or not it is intermittent or continuous duty, pumping hazardous or toxic liquids, and the condition of the liquid. For more information on wetted parts materials selection, refer to ANSI/HI 9.1-9.5 Pumps – General Guidelines for Types, Definitions, Application, Sound Measurement and Decontamination. Q. What factors cause excess pump vibration, and how can the specific cause be identified? A. The most common factors affecting pump vibration include:

• Mechanical unbalance of rotating parts
• Mechanical unbalance from abrasive fluids wear
• Pump and driver natural frequency and resonance
• Miscellaneous mechanical problems
• Hydraulic disturbances
• Hydraulic resonance in piping
• Poor structural rigidity

Table 1 from ANSI/HI 9.6.4 can help identify possible sources of high pump vibration. For more information on pump vibration, reference ANSI/HI 9.6.4 Rotodynamic Pumps for Vibration Measurements and Allowable Values.