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The proper sizing of a submersible pumping system is likely the most common concern among pump users as it delineates both the technical and the economic realities that users face in their specific application. Below, SWPA Executive Director Adam Stolberg joins Lisa Riles, global business unit director for residential and specialty at Xylem Applied Water Systems, to discuss the myriad factors of proper sizing and their effects. Riles has three decades of experience with Xylem in various roles of leadership and increasing responsibility and offers a depth of understanding on this issue.

What are some of the first steps of determining the necessary size of the pump with regard to flow rate and the required amount of lift?

The required flow is dependent on the type of application you are working with. For a standard sewage pump station, the flow required is dependent on the number of homes or buildings that will be connected to the system. Stormwater systems are based around storm projections, how much water is expected during a worst-case storm. Combined sewer and stormwater systems tend to have the greatest challenges when determining the required flow as they need to be sized for storm events but then also handle normal everyday flows in an efficient manner. The amount of lift (or pressure) a station needs to overcome is divided into two parts. The first part is the elevation that the water needs to be lifted from the starting point to the end point, and the second part is the frictional losses that occur between these two points as the water is being transported.

How important is friction loss in properly sizing a submersible pumping system? 

Friction loss is, of course, a very important aspect of sizing a submersible pumping system. The higher the friction loss the more pressure a pump will have to produce. The more pressure the pump needs the more power and, therefore, the larger the motor that will be required. This increases the cost of the pump, the controls, as well as the highest cost for most pump stations over their life: the cost of the energy necessary to run the pump station.

What role does that play in selecting components of the total system to calculate total dynamic head (TDH)?

This creates a situation where a system needs to be sized between different competing interests. A larger pipe size reduces friction losses but has a higher cost (particularly with valves and other components) and also can create too low of a velocity in a pipe causing solids to settle. A small pipe is likely less expensive and has high velocities that will prevent solids from settling but then also makes for a larger required pump and motor. Smaller pipes also create a situation where if the flow for a pump station needs to be increased in the future, the amount that you can increase the flow can be very minimal due to the very large increase in friction losses small pipes create. Finding the right balance between losses, pipe velocities, and expense can be one of the key challenges faced when sizing a new submersible pump system.

What’s the ideal “shopping list” you’d make to create an informed pump purchaser? What questions should they have ready for pump suppliers?

Ideally, a pump customer would have the flow required, total head, and static head for a system. If this information is unknown, a pipe diagram showing the projected route can be used to determine most pressure related characteristics of a system. Some other information that is always good to know is expected run times, any special challenges a pump station may have (sand, site challenges, seawater intrusion, grease, and/or chemicals being some common examples) and the control strategy a customer would like. 

What online tools or reference aids are available to assist these consumers?

The engineering toolbox (www.engineeringtoolbox.com) is a great place to see many formulas related to pumps on the internet. I am, however, most familiar with tools and aids provided by Xylem. Xylect (www.xylect.com) provides both a pump sizing tool and also includes an application guide that can help give information on different station types in many applications. Xylem also has guides and recommendations on many topics, such as stormwater, pressure sewer systems, and many types of pump station designs. Many of these guides are becoming available online and also paper versions can be received for use.

How would you best advise these consumers on incorporating projected maintenance into their selections?

Having a station with levels of redundancy allows customers to feel comfortable to perform required maintenance. Having a spare pump that can be adapted to a customer’s requirements in a shop or warehouse, for example, allows a pump to be removed for service without adding risk to the operation of the station. Another example of having an extra layer of redundancy is using a diesel operated backup pump installed at critical pump stations. This allows a submersible pump to be removed for maintenance and still have full station capability. Documentation is, of course, another key aspect for incorporating maintenance. Once a baseline has been established by recording information after installation of new equipment, changes from this baseline can be seen and a decision can be made on maintenance. This information compared with historical operational data can be used together to determine also when the best time for maintenance is, again, when the risk of problems is the lowest.


FOR MORE INFORMATION

Registration for SWPA’s 2019 Pumping Systems and Controls Training Seminar is now open! The event takes place March 12 and 13, 2019, in Rosemont, Illinois (just outside Chicago). SWPA’s one-of-a-kind educational programs provide attendees the opportunity to interact with dozens of SWPA Member Industry Experts while tapping into their extensive experience and wide-ranging expertise. Register online at www.swpa.org/swpa-training-seminar.


MODERN PUMPING TODAY, March 2019
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