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Darrell D
September 16, 2015, 06:58 AM
Background:

I am modeling our storm water collection system using InfoWater. We pump excess storm water from ponds (tanks with negative demand nodes) around the county to a ground storage tank (GST) before treating it and distributing it for irrigation. The modeled system ends at the GST.

Issue:

The GST has a free surface intake at an elevation of 26 feet modeled using a PSV set to zero as suggested in the FAQs. Many of the pumps and ponds are at elevations well above 26 feet (as high as 90+ feet). The result is a negative pressure at any node above 26 feet suggesting a siphoning of all contents above that elevation. This causes the pumps to operate outside of their curve or at pressures well below their actual operating condition. This siphoning effect is also seen in lines and at pumps where there is an intermediate maximum elevation. I have tried experimenting with Vacuum Breaker Valves and Pressure Sustaining Valves with no success in overcoming the siphoning effect.

Has anyone been able to overcome such a scenario and how? Any suggestions would be appreciated.

Patrick Moore
September 17, 2015, 03:11 PM
Darrell,

It is difficult to tell exactly what is occurring without actually seeing your model. It does look like the PSV is working properly (as it is supposed to work), but you may not be able to use InfoWater to get this answer if the pipes are not really running full in the real world.

But here are a few guesses:


The PSV will limit the flow such that the upstream pressure remains at or above the setting which in this case is zero. This may be a higher flow than what the pumps are capable of doing and pushing the pumps to the far right on the pump curve. If the only Head value being set is zero psi upstream of the PSV, then its no wonder that pressures at nodes above 26 ft are negative. The problem seems to be that the PSV alone is setting the hydraulic grade line based on its maximum flow so that it can maintain zero psi upstream. The problem I believe is that in a typical water system the head that is filling a tank is going to be much higher than what is available in a gravity drainage system. Also in the gravity system the lines would not really have negative pressures, but rather the pipes would flow less than full. This is an assumption that InfoWater cannot make.

If this was a fully pressurized system set up with limited head on the upstream side and nothing the control the flow on the downstream side except the PSV, you would likely see very large flows (even slightly larger if the PSV was not used). This high flow could bee too much for the pumps (causing the pumps to run to the far right of the curve) if the pipes must flow full.

My guess would be that if this system was modeled using a gravity flow system like InfoSWMM which can allow for partial pipe flow, you would not see the negative pressures, but would rather see partial pipe flow and no siphoning. It's simply that when you are assuming full pipe flow (as required by EPANET the hydraulic engine) without anything to restrict the flow, the PSV can send too high of a flow while maintaining zero PSI and this is too much flow for the system and at a head that would indicate siphoning.

One last Note: the InfoWater help was based on a storage tank rather than a "reservoir". While it does somewhat apply, it is not 100% exactly what it was based on, but will work to simulate discharge above a water surface in a tank under typically 40 plus PSI suction pressure on the PSV. which is not exactly what you probably have in this case.

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If you would like to submit your model to support@innovyze.com to discuss further please send it over.


Innovyze Support



Background:

I am modeling our storm water collection system using InfoWater. We pump excess storm water from ponds (tanks with negative demand nodes) around the county to a ground storage tank (GST) before treating it and distributing it for irrigation. The modeled system ends at the GST.

Issue:

The GST has a free surface intake at an elevation of 26 feet modeled using a PSV set to zero as suggested in the FAQs. Many of the pumps and ponds are at elevations well above 26 feet (as high as 90+ feet). The result is a negative pressure at any node above 26 feet suggesting a siphoning of all contents above that elevation. This causes the pumps to operate outside of their curve or at pressures well below their actual operating condition. This siphoning effect is also seen in lines and at pumps where there is an intermediate maximum elevation. I have tried experimenting with Vacuum Breaker Valves and Pressure Sustaining Valves with no success in overcoming the siphoning effect.

Has anyone been able to overcome such a scenario and how? Any suggestions would be appreciated.