Once we have determined our hatch dimensions, we can then size the diameter of the wet well.
As an example, let’s assume that we have sized a minimum hatch 4′ long by 5′ wide. It might seem logical that a 6′ inside diameter manhole would work. This would be incorrect as it does not account for the circular shape of the manhole. That is why it is important to always model the hatch and manhole together to ensure compatibility.
As a general rule, it is better to error with a larger manhole. One reason is the discharge piping. Many times the hatch is sized within the manhole correctly, but the discharge piping is forgotten. In the following example, the hatch is correctly sized and the pumps are correctly spaced. However, the flange of the 90 bend exiting the wet well would not fit within the well. A larger manhole should be used. Also, keep in mind installation concerns, such as making sure the contractor has enough working room to bolt the flanges on the 90 bends.
Some final considerations when sizing the wet well diameter are site location limitations.
- Are there restraints on how deep the wet well can be, such as soil type or high groundwater?
- Site constraints that would force the well to have a small footprint?