Existing Pump Station Problem

[uartsims]

Hey guys. Wanted to get a few thoughts on this situation. I have a client who has a small sanitary sewer pump station designed for a residential development. To make a long story short, he has expanded the subdivision a couple of different times and now the station is inadequate for the anticipated flows.

Currently, the station is pumping approx. 40 gpm based on recent field tests we conducted. The required flow (based on peak conditions) is about 90 gpm. The simple solution here (and maybe the only one) is to replace the system. However, upon review and in the interest of my client, I have discovered an unusual piping configuration for the discharge line(s). A 4" line runs from the station for about 1000', then necks down to a 3" line for about 1200', then connects to an 8" force main about 920' long, and finally to a 4535' 12" line that makes its way to the treatment plant. Friction losses in the discharge pipe system equate to about 1/2 of the TDH.

I have found an alternate route for the discharge system in which a line from the pump station could be connected directly into the last leg of the 12" discharge pipe that empties into the treatment plant. This alternate route bypasses approximately 3300 lf of pipe. However, my dilemma stems from the fact that a 4" discharge line still doesn't quite allow the required flow from the pumps. But, if the line was increased to a 6" force main instead, the design flow would meet the requirements to obtain 90 gpm flow.

Has anyone ever encountered adjusting discharge pipe sizing solely to modify anticipated friction head losses? Is this a good idea? Laying another 1,000 lf of 6" discharge line would certainly be less expensive than upgrading the existing pump station to accomodate the existing configuration. ANy thoughts? Thanks in advance.

 

[jmrosell]

i would consider simply replacing the pumps. it may be cheaper than the cost of new 6" forcemain, abandoning the old forcemain, and additional easements.

 

[francesca]

Ultimately the decision will lie with the body that owns the forcemain -- it's their property. They are likely to require the increase in size of the lines as yes, it is the best solution. (Lower lifetime pump costs, etc.) The city should reimburse your client for the cost in some way as it is not on his land.

Remember that the lines and pumps should have originally been sized for the full development potential of the drainage area. Don't replace the pipes with a 6" if that will only work for a few years... but also remember that you need to keep 2 fps scour velocity, so your pumps will need to pump at 90 gpm for the 6" and over 155 gpm for an 8". (I can't remember the numbers off the top of my head.)

 

[LHA]

What you are considering is workable, but proceed with caution. Treat it like a new design.

Keep in mind that, by altering the system curve, you are now taking liability for the design. The previous designer is only liable for operations at the operating point that engineer designed to. And I would be very surprised to learn an engineer designed for 1000 lf of 4" to 1200 lf of 3", etc. It sounds like field adjustments were made. This may or may not be the cause of the problems. And this system is going to fail if it keeps being so overloaded.

Make sure to check the new pump efficiency. It may fall out of the recommended operating range. This may (or may not) require a painless fix of adjusting speed or impellers, or might require new pumps anyway.

Engineering is the practice of the art of science - Steve

 

[bimr]

You can have the pumps and controls replaced for $10-15 K. That is going to be much more cost effective than replacing any pipe.

If the piping cost is $50 per foot, that would only allow you to replace 200 feet or so.

There is also a permitting issue. If you replace the force main, you probably will be required to get a permit. That will take a long time.

It will only take a week or two to replace the pumps.

 

[LHA]

bimr makes valid points. I'm not sure I agree with 10-15k for new pumps and controls, assuming dual duplex and even assuming no retrofit of the fiberglass/steel appertenances. But, his $50/lf for 6" is a good round estimate. So lets work backwards from your 1000 lf @ $50. You need USD50k, and you could certainly retrofit and rewire pumps and controls for less than that.

Permitting might be required, will likely be a hassle and will definitely take a long time if required.

Engineering is the practice of the art of science - Steve