Pressure system in SewerCAD

[ksmith436]

I am modelling a grinder pressure system in SewerCAD and am running into some problems. The main issues seem to be:

1. When the lift station pumps are turned on (by wet well elevations) they run immediately at max discharge, they are either off or on full blast. Consequently, it seems that flow instantaneously moves through the system. Besides the pump curve data and relative speed settings, are there any other way to control the pumps?

2. How should the grinder pumps be represented schematically? Since I only have the 1000 pipe version of SewerCAD, I am selectively representing the grinders at main lines (3-in and up). I am representing grinders by assuming grinders are in series and using one pump with TDH simply factored by # of residential units. Population load is placed at a node behind the pump. Is this wrong?

3. Analysis of the wet wells shows the wells filling up (I can see calculated level, % fill, etc.) but total flow (discharge) still reads zero over a 24-hr extended period run. The lift station pumps just upstream of the wet well also read zero for total flow.

Thanks alot!

 

[francesca]

1. Unless you have specified variable speed pumps, the only thing controling the speed of the pumps is the TDH.

2. Does the 1000 pipe license limit the number of lift stations? I worked on a model with a 1000 pipe license and had in excess of 40 lift stations with the true number of pumps represented.

What do you mean "population load is placed at a node behind the pump?" You should have the inflow hydrograph set on your wet well or else load your wet well by gravity pipe from a manhole. Your pumps should just be connected to the wet well.

3. Make sure that your pipes/valves are in the correct direction and that you don't have any controls on them. If it is as you describe, your pumps shouldn't come on at all and your wet wells should overflow. Are you sure you're running a 24-hour simulation?

How is your pressure-gravity interface going? Have you got negative pressures in your pressure pipes beyond the control point?

 

[ksmith436]

Francesca, thanks for the quick response. Let me ask you a question. It sounds like that you are not recommending analysis of pressure lines and grinders upstream of the lift stations. Rather, I should just be placing the load at the wet well and focusing on downstream lines.

I think alot of the problems I have encountered are due to having pressure lines upstream of lift stations - this is why I mentioned the 1,000 pipe limit. Since you were able to model a system with 40 lift stations all under 1,000 pipes, I assume you were not looking at upstream pipes. Also, I am adding additional pumps upstream to represent the grinders. This seems to be gumming up the SewerCAD analysis. The "population loads" are placed behind these pumps, not the lift station pumps. If I understand your reply, I should not complicate the model with these additional pumps. Instead, I could just put a large fixed load (representing the total sewer shed load) behind the wet well?

Thanks again.

 

[francesca]

I don't think I understood your scenario correctly. Do your grinder pumps not have wet wells? Surely they would? Or are they individual residential pumps? I think SewerCAD needs a wet well feeding a pump. Whether or not you include them depends on the "big picture." Are they essential to modeling your system? Add your loads wherever they make sense. If you're doing a 24-hour simulation, you want the inflow to match real-world conditions as closely as possible.

My model included an extensive gravity system feeding wet wells, with some forcemains terminating in gravity lines feeding other wet wells, as well as a trunk forcemain and individual forcemains feeding it. The control point on the trunk FM shifted depending on which pumps were running. This is a nightmare condition in SewerCAD, which handles the interface between pressure and gravity flow appallingly - or not at all.

SewerCAD cannot handle gravity flow in a pressure pipe. I'm sure it made sense, but the real world situation is that beyond the control point in a forcemain, flow occurs by gravity, yes in the pressure pipe. SewerCAD's response to this situation is negative pressures at any point (with HGL) higher than the top of the last pressure pipe. In order to get it to model the controlling HGL elevation, you have to stick your pressure pipe up in the air with the top of the pipe matching the HGL and use bolted manholes for the rest of the pressure pipe. (Bolted manholes don't overflow so SewerCAD can handle pressure flow in a gravity system with a bolted manhole.) You have to iterate by hand to get a converged solution of the HGL.

 

[bimr]

You have not said so, but one must assume that you are talking about the low pressure sewer residential system with multiple pumps.

The E/One pump used in these application is a semi-positive displacement grinder pump. The low pressure sewer system has a specific design pressure but the system operating prssure varies due to the randomness of the number of pumps operating. The system design pressure is 138 TDH.

If the operating pressure exceeds the 138 feet TDH, then the pump capacity will be reduced somewhat as described on the semi-positive displacement pump curve.

With the semi-positive displacement, your concern would be sizing the low pressure sewer system adequately based on the number of pumps installed. E/One has used statistics to determine how many pumps should be installed per section of piping.