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Vasevida
August 16, 2016, 08:56 AM
Hi Patrick

I finally have a bit more time to look into my modeling issues and I have come up with a combination tank/vsp/pumped system question. I have a few pumped pressure zones in the municipal system. One of them is a well (2 VS pumps in an old mineshaft) that discharges into a closed pipe system and a storage tank supply pipe. The well pumps manifold together and subsequently split with one pipe supplying a small pumped pressure zone (avg demand less than 15 gpm) and the remainder heads up about 230' vertically uphill where it empties into a 2.5 mg storage tank and pressurizes a different pressure zone. The tank's pipe entry is about 5 or 6 feet above the water surface in the tank and it free falls into the tank. The tank puts no head pressure on the supply line from the well pumps, but the line has check valves and it does have the pressure of the full pipe the whole way up the hill. Basically the well pumps are split to supply the pressure zone and the tank, and the pumps run all the time, maintain line pressure about 125psi (the head of the tank supply line). When supply exceeds demand, the proxy pressure reducing valve is the free surface entry into the tank, so when supply outstrips demand the remainder of the well water pumped simply fills the tank. The well pump pressures are not monitored as the tank pipe entry acts to reduce excessive pressure and so there is no problem with over pressuring the small pumped pressure zone. I am getting negative pressures at the high point of the tank supply pipe that do not allow the model to run properly.

Any idea the best way to model this one? the free drop into the tank? The previous advice you guys have been giving has been spot on. This model is starting to get a bit frustrating and have already dumped considerable time effort and consultant dollars in to it to no avail. Thanks in advance for any and all input, I have to be out in the field for a while today, so I might not be able to respond right away. Thanks Jamie

Patrick Moore
August 16, 2016, 02:18 PM
Jamie,

In the Help file there is a section called "How-to?" and it suggests this solution for a tank inlet that discharges above the water surface.

(click if need larger image)
322

Basically they suggest using a PSV set at the elevation of the inlet discharge and with a setting of zero:
The tank's inlet consists of a Pressure Sustaining Valve followed by a short
length of large diameter pipe. The pressure setting of the PSV should be 0, and
the elevation of its end nodes should equal the elevation at which the true pipe
connects to the tank. Use a Check Valve on the tank's outlet line to prevent
reverse flow through it. There should be another small length of pipe (a dummy
pipe) separating the tank and check valve.


Note: I believe they meant to say in regards to the check valve to add a pipe and a node between the tank and the pipe that has the check valve on the discharge. It is always best to separate control elements from being directly connected to any tank because EPANET will close pipes when the tank empties or fills to 0% or 100% full.


Negative pressures in the model indicate that the hydraulic grade line (HGL or Head) is below the elevation of that element. In that case you need to check what is controlling the HGL near the point of negative pressure. If you did not have the PSV as indicated the model would only use the water line in the tank as the known HGL and could create negative pressures in the inlet line. By adding the PSV you increase the "back-pressure" on the valve as the valve will only flow when the pressure upstream is at zero which will increase the HGL higher than if the PSV was not there.

Try that, it should resolve your issue. If not reply back and we can help you further.

Patrick Moore


Hi Patrick

I finally have a bit more time to look into my modeling issues and I have come up with a combination tank/vsp/pumped system question. I have a few pumped pressure zones in the municipal system. One of them is a well (2 VS pumps in an old mineshaft) that discharges into a closed pipe system and a storage tank supply pipe. The well pumps manifold together and subsequently split with one pipe supplying a small pumped pressure zone (avg demand less than 15 gpm) and the remainder heads up about 230' vertically uphill where it empties into a 2.5 mg storage tank and pressurizes a different pressure zone. The tank's pipe entry is about 5 or 6 feet above the water surface in the tank and it free falls into the tank. The tank puts no head pressure on the supply line from the well pumps, but the line has check valves and it does have the pressure of the full pipe the whole way up the hill. Basically the well pumps are split to supply the pressure zone and the tank, and the pumps run all the time, maintain line pressure about 125psi (the head of the tank supply line). When supply exceeds demand, the proxy pressure reducing valve is the free surface entry into the tank, so when supply outstrips demand the remainder of the well water pumped simply fills the tank. The well pump pressures are not monitored as the tank pipe entry acts to reduce excessive pressure and so there is no problem with over pressuring the small pumped pressure zone. I am getting negative pressures at the high point of the tank supply pipe that do not allow the model to run properly.

Any idea the best way to model this one? the free drop into the tank? The previous advice you guys have been giving has been spot on. This model is starting to get a bit frustrating and have already dumped considerable time effort and consultant dollars in to it to no avail. Thanks in advance for any and all input, I have to be out in the field for a while today, so I might not be able to respond right away. Thanks Jamie

Patrick Moore
September 12, 2016, 01:41 PM
Moderator note: Moved this to it's own thread. Link to original thread for this post: http://forums.innovyze.com/threads/317-System-Curve-for-Closed-System

Patrick Moore