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Regarding Modelling closed system water network and system head curve using Bentley WaterGems

Regarding Modelling closed system water network and system head curve using Bentley WaterGems

Regarding Modelling closed system water network and system head curve using Bentley WaterGems

Dear All,

I have some questions to clarify few confusions regarding system head curve and pump selection in Bentley watergems modelling environment.

First, a bit of background. The system is in a developing country, serving low-income communities (standpipe at household level). Currently, the set-up is, storage of water from supply in underground reservoir, then lifting of water to overhead tank and from there, distribution. The pump operation is done by manual operator. I am attaching the existing model for your reference as well.

The local NGO operating the system received two more access points for water supply from municipality water works and they want to expand their service network. They also have water scarcity and capacity deficiencies in the existing network. So, they want to connect the new network with existing one. Topography is quite flat and they don’t have funds for any pressure controlled valve for boundary set-up between the two phases.

Due to capital cost limitations, the NGO is opting for pumping directly to new network (though opinions about life cycle cost, inefficient operating cost was communicated). Given land-rights issues, they are reluctant to make high capital investments. Objective is to meet demand at 1 bar pressure minimum.

The pumps considered are low cost household level centrifugal pumps (usually used for lifting water to high-storied buildings). For the new supply sources, water stored at sump/equalization tank and then, pumping is considered.

From local consumption survey in existing network, the consumption pattern shows significant peaking during day time. To address that, at each source, two pumps in parallel are considered. The second pump will kick in only during the peak hour operations. Initially, only the reservoir was used for pipe sizing and identifying flow variation at each new source. Then, pumps were introduced. The pumps are identical in size. Since one pump will operate alone at each source for about 13.5 hours (parallel pumping for 5.5 hours), pump selection was done such that best efficiency point lies between low and peak demand region during solo operation.

My understanding is, it is going to act like a closed system. Though the existing tank at discharge side should make it an open system, when I tried to generate the system head curve, the curves had the same characteristic errors of closed system head curve (generated in the usual way). Any explanation why it behaved like that?
I devised a pressure dependent demand scenario and run the model to devise system head curves. That's the method for closed system (Walski et al.). The system head curves are generated for the peak and least demands during parallel pumping and solo pumping. They are also attached.

My primary concern is, during peak hour combined operation (13:00 hour); the flow leaving the pump station (i.e. discharge side pipe flow) is higher relative to the pump curve-system head curve intersection point’s corresponding flow. Also, the pressure at downstream node in the model is even lower. I am suspecting my selected pump capacity is low (for peak hours). I deliberately did that keeping in mind flow is available from existing overhead tank as well. I’ll appreciate it if anybody can explain the phenomenon, any remediation needed.

Despite the inherent inefficiency of the system choice, I am prioritizing pump operation efficiency, water supply at 1 bar minimum pressure.



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