Making Drainage Flow Uphill

 

By Timothy Allinson, P.E.

Murray Company, Long Beach, Calif.

The other day I was approached by a project manager from our underground (Civil) division. It seems that on one of his projects there was a deal-breaker problem with storm sewer piping that had to cross some highly sensitive electrical conduit and AT&T fiber optic cables. These obstructions could not be moved, and, as a result, the storm sewer serving the site would have to be pumped. The civil engineer wanted a mechanical engineer to specify and detail the lift station. Since it was outside the building, the MEP engineer wanted nothing to do with it, so all eyes turned to me.

Now, I have designed sump pits and pumps for nearly every project I have worked on, but I have never designed one to serve an entire site. The CE calculated the 25-year storm flow as 12.25 CFS. Multiplying by 448 gpm/CFS yields a flow of about 5,500 gpm - that's a lot of water. In discussing the challenge with my local trusted sump pump rep, the first thing we agreed on was that there should be two pumps - one to handle small flows of light rains and irrigation runoff and one large pump to handle peak storms. We arbitrarily assigned these pumps as 500 gpm and 5,000 gpm respectively.

Next, I sketched a detail of what the pump vault might look like for these pumps, respecting the fact that space available for the vault was limited by surrounding utilities (See Figure 1). I worked out some preliminary inverts and started coordinating with the civil engineer. He took the information I gave him and drew a profile of what the piping in and out of the lift station would look like (See Figure 2). Note that work on private property is shown dashed and the lift station vault is shown as a tall skinny object with invert 54.00. Storm water flows right to left in the profile.

The more I stared at this profile the more I thought, Why are we pumping all this water when most of it will flow by gravity? Basically, the lift station was just a giant trap, but under peak flows the water would flow through this trap by gravity.

With this realization I proposed to the CE and his hydraulist that we delete the 5,000-gpm pump and let the peak storms flow by gravity. We could keep the 500-gpm pump to handle light rain and irrigation runoff that would tend to sit in the trapped piping and eventually clog it with sediment. They agreed to run this past the city engineer, who agreed with our proposal and was far less concerned with having a pump than I was. A calculation revealed that the trapped piping had about 1,000 gallons of water that all agreed should not be allowed to sit and stagnate, plus there was the issue of sediment build-up.

In a second discussion with the CE, his hydraulist asked, "Why do we need to pump the trapped 1,000 gallons down in two minutes? We could do that in 30 minutes with a smaller pump." Acknowledging this point, we decided that a 50-gpm pump with 2" piping would accomplish the task of draining the trap and sediment removal quite adequately. So the lift station that started out as 5,500 gpm was reduced first to 500 gpm and, finally, to 50 gpm. This dramatic reduction not only saved money but it also reduced the electrical load as well as the generator load and greatly simplified required maintenance.

These sorts of unique problems are what I consider the true tenants of engineering. It is fairly easy to design the same types of systems day in and day out, but it is the practical solution of unique challenges that tests one's creative ability, practical knowledge and, most importantly, the ability to work together as a team in extracting the best solution.

Timothy Allinson is a Senior Professional Engineer with Murray Company, Mechanical Contractors, in Long Beach, Calif. Prior to entering the design-build industry he worked for Popov Engineers, Inc. in Irvine, Calif, and JB&B in New York City. Tim holds a BSME from Tufts University and an MBA from New York University. He is a professional engineer licensed in both mechanical and fire protection engineering in various states, and is a leed Accredited Professional. Tim is a past-president of ASPE, both the New York and Orange County Chapters, and sits on the board of the Society of American Military Engineers, Orange County Post.