The Importance of Pump Basin Sizing

Did you know that basin selection and sizing is as critical to pump life as the hydraulics are? In the wastewater industry, it is common to spend a good deal of time time sizing a sump, effluent or sewage pump and thoughtfully selecting the proper controls for the application. Proper basin selection and sizing are sometimes overlooked. Failure to properly select and size the basin could not only decrease the life of the pump, but it could result in inlet backups and possible basin failure resulting in wastewater flooding into the surrounding area. The two most important things to consider when selecting a wet well or basin are material and dimensions:

Material – Basins are constructed primarily of concrete or plastic. These materials vary by function and cost and there are pros and cons to both.

  • Concrete: Precast Concrete is popular in the civil arena due to its availability and compression strength making it ideal for larger diameter or deeper bury depths.

  • Plastic – Fiberglass is most popular on larger basins, but other plastics such as polyethylene and HDPE are also utilized for smaller commercial or residential applications. Fiberglass is popular due to it’s weight, strength and value. In addition, it is easily pliable making custom depths, widths and attached valve boxes viable.

Dimensions – The width and depth of the basin will not only determine storage gallons, but also float spacing, and future serviceability.

  • Width - Fractional through 2 HP duplex basin packages should always allow for a minimum width of 16 inches between the two pumps and 6 inches from each side. This allows for adequate room for rail and plumbing installations inside the tank and easy future servicing issues in the tank. 3 inch and larger flanged pumps should require even more room depending on base elbow and rail configurations, as well as plumbing inside the tank.

  • Depth - The starting place for basin depth should always be inlet location. If the basin is in a remote area with a long run of inlet piping, slopes and inverts may impact the location of the inlet on the basin. In addition, if the basin is installed outside in a cold climate, inlet and discharge freeze bury depths must also be taken into consideration. It is important to consult with local authorities to determine these depths for proper sizing. In upstate New York and Vermont, freeze depths are typically 4 to 6 feet so the starting point for float space sizing should start there. It is important to allow for adequate depth below the inlet to allow all floats to operate effectively. There should also be a minimum of 6-8-inch spacing between each control float to allow for an adequate pump drink cycle (run time), prolonging the pump life. In addition, the top float should be a minimum of 6 inches below the inlet. This means that in a simplex 3 float system, there should be a minimum of 24 inches below the inlet (3 floats at 6-inch spacing plus 6 inches below the inlet). A duplex 4 float system should have a minimum of 30 inches below the inlet. For example, in a building where the inlet is 4 foot down from grade, the duplex wet well should be at least 7 feet deep. Note: If there is a construction obstacle preventing increased depth, special non-moving floats may be utilized, but these floats should only be used when necessary as they may be less reliable and allow only minimal drink cycle time adjustments.

Reserve Capacity - It is also important to give the wet well adequate reserve or safety storage above the alarm float to prevent basin failure. This “extra” storage above the inlet gives the owner time to react in the event of an alarm situation. While it is strongly advised to eliminate (or dramatically limit) the inflow of liquid during an alarm, not all applications allow for this so. It is essential that these applications incorporate extra reserve capacity, thus increased basin width or depth.

Properly selecting material and tank dimensions will increase pump life, future serviceability, provide reserve capacity in alarm situations and decrease chances of wastewater backups or flooding. It is important to coordinate inlet inverts, depth restrictions and possible construction obstructions with other trades to be sure there are no on-site issues ensure that the basin you select will work in the application. This will allow for a seamless, easy, quality installation and will assist in providing the longest life possible for your system.

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