By SWPA Executive Director Adam Stolberg and David M. Williams, Liberty Pumps
Correctly sizing a wastewater system requires assessing several component aspects as well as situational use. Below, SWPA Executive Director Adam Stolberg and David M. Williams, director of engineering for Liberty Pumps, discuss the importance of a properly sized wastewater system and how to best avoid common missteps before they require costly replacements or repairs.
What are some of the dangers from an undersized wastewater system? Is it possible to oversize?
An improperly sized wastewater system can lead to many issues. A system that is taxed beyond its designed capacity will suffer increased maintenance and downtime issues due to wear and tear on equipment, and at peak flows may simply not be able to keep up resulting in back-ups or overflows. An oversized system can also lead to significant issues. A pump that is too large for the application will run on the far-right side of the curve resulting in decreased efficiency, higher current draw, potential overload, or circuit breaker tripping, and likely long-term damage due to cavitation. An oversized basin can lead to infrequent cycling and eventual settling of material in the basin and piping, and the contents of the basin can turn septic and lead to odor issues and increased corrosion.
What role does a lift station perform in the waste stream and how can problems with a lift station domino throughout the entire system?
Lift stations play a key role in the overall wastewater system. A well designed and properly functioning lift station maintains a smooth flow and prevents back-up issues upstream and helps to prevent surges or clogging downstream. It also allows operators to focus resources on system management and improvement rather than having to deal with urgent maintenance issues and repairs.
How critical is the link between a pump’s discharge size and a sewage pipe system?
Besides the obvious physical connection compatibility between the pump discharge and piping, the major factor in determining sewage piping diameter is ensuring that it is properly sized to handle maximum flow while still maintaining adequate scouring velocities at lower flows.
Which performance attributes should one consider in selecting the right pump?
When selecting a pump, it is critical to have a good understanding of the hydraulics of the system, but it is also important to consider other factors such as potential for challenging materials in the wastewater that could lead to clogging, or special risks associated with downtime. For example, a main lift station for a hospital or nursing home would likely see challenging materials such as washcloths, wipes, and diapers, and would also be very sensitive to downtime. In this type of application, a grinder or chopper pump may be preferred over a solids-handling option to ensure optimum reliability.
How does pump selection influence component selection—like choosing a control panel or check valves?
It is critical that pumps and other components be selected to ensure proper function of the overall system. In the case of control panels, items such as circuit breakers, overloads, moisture detection hardware, thermostats, and start circuits (for single phase pumps) must all be correctly matched to the pump requirements. Further, in the case where hazardous location pumps are needed, intrinsically safe control panels and non-sparking guide rails will be required. Check valves must be chosen based on pump flow to ensure that maximum fluid velocities specified by the manufacturer are not exceeded. These are just a few of the items that must be properly selected when designing a pump station, and just one incorrectly specified component can be the difference between a trouble-free system and a maintenance headache. It is for these reasons that SWPA and its member companies preach the benefits of the systems approach to station design and promote taking advantage of complete packages designed and produced by manufacturers using proven hardware combinations.
If replacing an existing pump rather than starting from scratch, how does this change which factors to consider?
In some ways, selecting a replacement pump can be more difficult than a clean sheet design. When it is time for an existing pump to be replaced, it is critical to take a fresh look at the application as well as the physical condition of the equipment. A common issue is that replacement pumps are often selected and ordered based on old file documentation from the original station installation. In a perfect world, that would seem like logical path, but there are many ways in which this can lead to undesirable surprises. Here are a few key things that should be considered before selecting a replacement pump.
- Visit the site and inspect the existing equipment, comparing it to the documentation. It is often the case that the pump in the pit may be a different model or horsepower or sometimes even a different voltage than what is reflected in the records. Similarly, control panels may have been updated or replaced over time. Verify and document the condition of the panel as well as overload and circuit breaker sizes so you can be sure that they are adequately sized for the replacement pump. Some components in a control panel may be incompatible with a potential replacement pump. It is also important to inspect the wet well and verify the style and condition of the guiderail hardware. This is required to ensure proper selection of pump discharge configuration, but also it is frequently discovered that the guide rails may need to be replaced due to corrosion.
- Review the application and history of maintenance issues. Has the system loading increased significantly due to additional connections, or in the case of a force main, has there been a change to system working pressures? Is there a significant history of clogging issues that would suggest the need for a grinder pump rather than solids handling solution?
Taking time up front will provide a solid footing for selecting the optimal replacement pump and set the stage for a smooth retrofit and long-term station reliability.
MODERN PUMPING TODAY, May 2020
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