Problem Description

A small (but quickly growing community) relies on groundwater to meet its water needs. But over pumping of the aquifer is causing deeper, highly mineralized groundwater to migrate to the shallow subsurface, contaminating the local supply.  The community has decided to switch to surface water supply and needs to build a drinking water distribution system that must satisfy certain requirements.

The community’s Development Company provided a preliminary design of the distribution system, but the community would like an external review of the design before proceeding with construction. You have been hired analyze the hydraulic feasibility of the preliminary design, particularly with respect to minimum pressure and flow velocity requirements: pressure at all junctions with non-zero demand should exceed the minimum allowable pressure of 15 psi but be less than 165 psi (note: all junctions should have positive pressure). Velocity in the pipes should exceed 1 ft/s but be less than 8 ft/s.

Technical Details

The proposed distribution system is gravity-fed: water is released from nearby reservoir (10 miles away) situated at a higher elevation (131m) to the system of 24 junctions and 25 pipes (see Figure 1). The preliminary design includes specifications for each pipe’s length, diameter and roughness, as well as the elevation and base water demand at each junction (see Tables 1 and 2). An expected 24-hour water demand pattern is also provided (Table 3)

To aid in your feasibility analysis, use the MAGNET Pipe Network Analysis (MAGNET PNA) web application to model the flow dynamics (velocity, head, pressure) across the proposed distribution system. If the preliminary design is not feasible, suggest and analyze ways in which the system could be modified the satisfy the requirements. See "MAGNET Pipe Network Analysis – Hints / Additional Instructions” below.

Deliverable

Please summarize your findings in a clear, well-organized memo. Please limit the memo to two page (single-space) and submit supporting documentation and technical details (including screenshots of model results) as an attachment.

MAGNET Pipe Network Analysis – Hints / Additional Instructions

Begin by constructing the water distribution network using the provided specifications. If you have not already completed the Quick Tutorial (Menu Bar > Help > Quick Tutorials), please do so before proceeding. The Quick Tutorial explains step-by-step how to add network objects, assign attributes, run the model, and visualize / analyze the results.

Carefully input the node and link attributes using the information provided in Tables 1 and 2. Note that unit conversions are required before inputting into MAGNET PNA. Double-check that your inputs are correct (better yet, have a different project partner / teammate check for you!).

Set up the model to perform a 24-hour analysis (Total Duration: 24:00) with a 1-hour time step (Hydraulic Time Step: 1:00) and a 6-hour pattern time step (Pattern Time Step: 6:00). Use the Pattern Editor to create the demand pattern presented in Table 3.

Run Analysis and visualize / analyze the results. Is there sufficient pressure at all nodes and at different times? Are there any negative pressures (i.e., the hydraulic grade line is lower than the pipe elevation)??? IF yes, you should consider adding a pump at the link location just before the first junction with non-zero demand and insufficient / negative pressure. (Remember, a pump replaces a pipe in the network, so delete the appropriate pipe before adding the pump). The community has enough funds to include in the distribution network a pump with the following performance relationship: 700GPM can be maintain 100ft of head.

After drawing the pump and creating the pump curve, Run Analysis again.  Analyze the new results to determine if the pressure in all nodes, for all time-steps, is sufficient or not. A properly placed pump should alleviate insufficient pressure issues (if any) in the distribution network.

Once node pressures are within the acceptable range, carefully check the velocities in all pipes and at different times. If velocities in one or more pipes is not acceptable for one or more time-steps, modify the pipe characteristics and re-run the model, analyze the results, and so on. Continue adjusting input parameters until all velocities (all pipes, all times) are within the acceptable range.  

Figure 1: Proposed water distribution network.

Table 1: Characteristics of the junctions in the preliminary design. (Note that the expected unit of elevation in MAGNET Pipe Network Analysis is feet; for demand, expected unit is GPM. A conversion prior to direct input is needed.)

Table 2: Characteristics of pipes in the preliminary design (Note that the expected unit of pipe length in MAGNET Pipe Network Analysis is feet; for diameter, expected unit is inches. A conversion prior to direct input is needed.)

Table 3: Demand pattern for 24-hour period (four 6-hr time periods).