OneManWolfpack
Civil/Environmental
I have one of those dreaded "intermediate high point" issues for y'all.
Our client recently replaced an older submersible wastewater lift station with a new one pumping into the same 14-inch forcemain. The forcemain has an several intermediate high points, and combination air/vacuum valves installed along the line. When the new station was put online, the pumps would not operate without throttling the valves at the pump discharge. By throttling, I mean almost closing. They're 21-turn plug valves, and they must be closed 15 turns to keep the station from sounding like it's pumping gravel.
We've done some pretty extensive analysis, conducted drawdown testing, installed pressure logging equipment downstream of the throttled valves, and I think we've pretty much made sense of what's happening in the forcemain that's causing the problems for the pumps. In a nutshell, the line drains from the high point(s) in between pumping cycles, and when the pumps kick back on, they're only pumping against a fraction of the friction head for which they were designed.
The pressure logging data (examples attached) shows that most days, the line pressure at the pump station is around 30 psi when the pumps are running. The pressure gradually increases as the pumps run to 35-40 psi (as the line fills) and then drops off to static pressure when they shut down.
The system changes significantly after heavy rainfall. I assume because higher inflow to the station in question as well as the four other stations that pump into the forcemain allow the forcemain to stay full. After rainfall events, the pressure in the line is up around 70-75 psi during pumping.
The attached graph shows system curves for these two conditions (purple = dry weather, pumping to the high point of the forcemain; blue = wet weather, pumping into a full forcemain; red = wet weather, pumping into a full forcemain with all stations running). Each color, purple, blue, and red, has a heavy and a light line. The light line is adding the headloss through the throttled plug valves, and the heavy line is what the system would be with the valves open.
Obviously, throttling the plug valves is not an acceptable long-term solution, so we're trying to find a better way to operate the system. We've looked at several options.
1) Install a permanent throttling device (orifice plate? different type of valve?) at the pump station. This alternative would essentially make the system permanently operate on the three light system curves on the attached graph. This isn't my favorite solution, but it will likely end up being the cheapest.
2) Install some type of backpressure sustaining valve at the end of the line to keep it from draining between cycles. I'm not sure what type of valve/controls would be required, but the idea is that it would remain closed to a 5-psi or so backpressure and open when backpressure increases due to the pumps turning on.
3) Install a "gooseneck" at the end of the line to create a new high point (it would have to be approximately 8-feet above grade). This wouldn't be the prettiest sight driving by, but I feel like it is a good hydraulic solution, and it should be relatively inexpensive and maintenance free unlike option 2.
4) Install VFDs at the pump station. The attached graph shows where the pumps would operate at reduced speeds. At 50%, they would be able to operate at the low pressure/empty line condition. This would work, but is the most expensive alternative.
I would appreciate any comments or advice on these alternatives and any other solutions you guys and gals can come up with.
Thanks in advance, and a happy Friday to you all!
Attachments:
Our client recently replaced an older submersible wastewater lift station with a new one pumping into the same 14-inch forcemain. The forcemain has an several intermediate high points, and combination air/vacuum valves installed along the line. When the new station was put online, the pumps would not operate without throttling the valves at the pump discharge. By throttling, I mean almost closing. They're 21-turn plug valves, and they must be closed 15 turns to keep the station from sounding like it's pumping gravel.
We've done some pretty extensive analysis, conducted drawdown testing, installed pressure logging equipment downstream of the throttled valves, and I think we've pretty much made sense of what's happening in the forcemain that's causing the problems for the pumps. In a nutshell, the line drains from the high point(s) in between pumping cycles, and when the pumps kick back on, they're only pumping against a fraction of the friction head for which they were designed.
The pressure logging data (examples attached) shows that most days, the line pressure at the pump station is around 30 psi when the pumps are running. The pressure gradually increases as the pumps run to 35-40 psi (as the line fills) and then drops off to static pressure when they shut down.
The system changes significantly after heavy rainfall. I assume because higher inflow to the station in question as well as the four other stations that pump into the forcemain allow the forcemain to stay full. After rainfall events, the pressure in the line is up around 70-75 psi during pumping.
The attached graph shows system curves for these two conditions (purple = dry weather, pumping to the high point of the forcemain; blue = wet weather, pumping into a full forcemain; red = wet weather, pumping into a full forcemain with all stations running). Each color, purple, blue, and red, has a heavy and a light line. The light line is adding the headloss through the throttled plug valves, and the heavy line is what the system would be with the valves open.
Obviously, throttling the plug valves is not an acceptable long-term solution, so we're trying to find a better way to operate the system. We've looked at several options.
1) Install a permanent throttling device (orifice plate? different type of valve?) at the pump station. This alternative would essentially make the system permanently operate on the three light system curves on the attached graph. This isn't my favorite solution, but it will likely end up being the cheapest.
2) Install some type of backpressure sustaining valve at the end of the line to keep it from draining between cycles. I'm not sure what type of valve/controls would be required, but the idea is that it would remain closed to a 5-psi or so backpressure and open when backpressure increases due to the pumps turning on.
3) Install a "gooseneck" at the end of the line to create a new high point (it would have to be approximately 8-feet above grade). This wouldn't be the prettiest sight driving by, but I feel like it is a good hydraulic solution, and it should be relatively inexpensive and maintenance free unlike option 2.
4) Install VFDs at the pump station. The attached graph shows where the pumps would operate at reduced speeds. At 50%, they would be able to operate at the low pressure/empty line condition. This would work, but is the most expensive alternative.
I would appreciate any comments or advice on these alternatives and any other solutions you guys and gals can come up with.
Thanks in advance, and a happy Friday to you all!
Attachments:
