Pump Flow

Question

Hello 
 I have a system which is supplied by a reservoir then pumped to ( Network (1) as junctions ) and ground tank which further pump the water to (another network (2) as junctions and a treatment plant as 1 junction). Each network has a pattern for flow shifts and controls and the treatment plant works 24 hours. 
 When pumping flow to Network (1) i should supply the network and also the ground tank because it supplies the treatment plant which works 24 hours. When applying EPS analysis the pump discharge higher flow than both of Network (1) and the flow needed in the treatment plant is some hours and this flow goes directly to the tank which cause negative pressure in network (1). 
 The treatment plant needs 200 m3/h and Network (1) needs 1200 m3/h but the pump may supply 2600 m3/h and this cause a negative pressure in network (1). So wanna know why the behavior of pump changes rapidly and cause fluctuations in the pump curve and how can i solve this? 
 Regards

Jesse Dringoli · Accepted Answer

The flow supplied by pump station 1 is not equal to the "WTP" demand plus the pump station 2 flow, because you have storage in between (tank T-2) which essentially separates the system into two networks (flow in to the tank is not necessarily equal to flow out). 
 Pump station 1 adds enough energy to the system to "lift" the water from the upstream hydraulic boundary (the "Water Source" reservoir, at an HGL of 20 m), to the downstream boundary hydraulic grade (T-2, at a HGL of 70-72 m), and overcome friction losses between. The flow from the pump station is the flow from the pump definition corresponding to the head needed to overcome this static and dynamic lift. See: How are pumps handled in each OpenFlow (Haestad) product? 
 You do not have any controls in place to prevent the tank from becoming full or empty, so it becomes 100% full during this simulation (since the head that pump station 1 needs to add corresponds to a flow that is greater than the pump station 2 flow), which closes the adjacent pipe, and causes system disconnections (hence you see many red notifications and unbalanced timesteps). 
 Further, you have the initial elevation of tank T-2 set equal to the top elevation, so the upstream pipe is closed even in the first timestep ( automatic altitude valve ). 
 Another issue is that you have a control to close pipe P-271, which disconnects the "WTP" demand from the system. Since WaterGEMS is demand-driven, it will still try to satisfy that demand and the flow will go through the closed pipe. You will get a message about disconnected demands in that case. See: Disconnected Demand Nodes user notification when computing model 
 Lastly, you have controls set to turn pump station 2 off at certain times of the day, yet you only have demands downstream of it. So, when the pumps turn off, you will be in the same disconnected demand situation (see above). If you remove those controls though, there are times when there is zero demand downstream, because the demands use a pattern that has a multiplier of zero at certain times. A description of what this represents and how the real system works (or will work) will help us better guide you to the best modeling approach here. 
 If you fix the above issues, you can better visualize what I explained above about pump station 1's operating point. At a minimum, set the initial elevation of T-2 slightly below the top elevation, then view a profile at time zero between pump station 1 and T-2. You will see that there is quite a bit of dynamic lift needed, due to some large pipe headlosses. You can see from the profile view that P-10 has a large headloss (more than 75 m) because it has a smaller diameter than the upstream pipes. To illustrate the impact of this, try changing P-10's diameter to 860 mm to match the upstream pipes and re-compute. Since pump station 1 needs to add less head, it is able to add more flow (shifting on its pump curve), and thus you will see an increase in the flow into T-2. You can also see the opposite - if you reduce the diameter of the upstream pipes to 450 mm to match P-10, pump station 1 will add less flow. If it adds less flow that pump station 2, the tank may become empty.

Sushma Choure · Answer

Hello Gamal, 
 If you want pump to supply required amount of water only, then you should apply controls to pump, if not done so already. 
 It could be based on Tank HGL, working hours etc. 
 Creating Controls - Conditions, Actions, and Control Sets (CONNECT Edition and V8i SELECTseries 6) 
 Also you could use flow control valve i.e. FCV which will supply only required amount of flow.

Jesse Dringoli · Answer

Here is a link with information on providing the model files if needed: Sharing Hydraulic Model Files on the Haestad Forum 
 As Tom said, a pump will operate somewhere on its curve. For a closed system with only downstream demands, the sum of the demands will dictate the pump operating point, but for a system with storage, the pump flow is based on the static and dynamic head needed to "lift" the water between the two boundary hydraulic grades. See more here: 
 How are pumps handled in each OpenFlow (Haestad) product? 
 General Pump Selection Process 
 Understanding System Head Curves in WaterGEMS, WaterCAD, and SewerCAD 
 If the tank becomes full or empty, it will automatically close the adjacent pipe, which can cause problems: What happens when a tank becomes empty or full? 
 For more specific help, we will need to see the model.

Pump Flow

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