5 D ISCUSSION
5.3 Scenario 5.1: Triple step override test – Control 1.0
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Umlaas Road Reservoir
Ashle y Road BPT (20Ml)
Wyebank Road BPT (20Ml)
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3
4
5
7 6
8
9
Lumped Demand
0.53 km 140 0 mm 3.30 km 140 0 mm
1.06 km 1400 mm
1.14 km 140 0 mm
3.26 km 1400 mm
0.24 km 140 0 mm 0.43 km 1400 mm
2.06 km 140 0 mm
8.00 km 1400 mm 20.01 km
20.01 km
All valves fully open initially
All reservoirs start drawing at 180 mins
All reservoirs cease drawing at 360 mins
All valves fully open initially
All reservoirs start drawing
at 180 mins All reservoirs
cease drawing at 360 mins
Rates: 2036
1- Represents a string of worst- case scenario situations 2- Used to analyse the WA’s
capability to cope with large user demand variations Initial Conditions: All valves open, reservoirs 50 %, and BPTs 10% full.
Risk:
1- Valve oscillations - wear risk.
2- Overflow risk on step 1 and step 3
3- Settling level not at 50% level –overflow risk.
AD Initial: WR Initial
S1= 0.85 G1= 1 LADBPT= 50% of capacity S1=0.85 G1=1 S2= 0.85 G2= 1 LWRBPT=50% of capacity S2=0.85 G2=1 S3= 0.85 G3= 1 Lres=50% of capacity S3=0.85 G3=1
Other Observations:
Both BPTs exceed 8.0 m when all downstream demands are ceased. Resumption of downstream demands causes the settling level in AD and WR to be 6.0 m and 6.5m respectively.
System copes well with maximum demand
Figure 36 - Scenario 5.1 results overview – Triple step override test – Control 1.0.
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d a
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Scenario 5.1 was designed in order to assess the system’s response to rapid changes in demand, and to assess the volume changes within the BPTs under such circumstances. The triple-step stress test represents a string of worst-case scenario tests. This analysis is intended to provide valuable emergency preparedness for situations such as dispersed, runaway wild fires that would require massive, instantaneous water draws from numerous reservoirs around the affected regions. It also enables the evaluation of the valve closure times which aid in preventing transient overpressure, but can render the system sluggish in its reaction to demand cessations, thus causing catastrophic overflows within the BPTs.
The system response to the lack of demand flows is observed within the starting period in Figure 36a and Figure 36b. The WR BPT (Figure 36d) can be seen to initially rapidly shut its valves in order to decrease the incoming flow, yet the valve closures are slowed between 20 mins and 40 mins. This is due to the restricted valve movement rates and staggered control arrangement and it allows the BPT level to rise to above 100% (8 m), although an overflow will not occur as an allowance as been provided within the BPT height (8.3 m is the overflow level). The filling of the BPTs to above their maximum level is undesirable, and action should be taken to prevent this occurrence.
The final settling level is thus determined by the initial position of the globe valves and the rate to which their movement is restricted. Since no demand flows are stipulated in these test periods, the BPT level cannot not decrease. Thus, if the valves are initially all set to their fully open positions, and the BPT exit flow suddenly stops, a variable settling level, above its target settling level, is likely to occur.
The similar response observed, from Figure 36a for the AD BPT, is notably slower. While it only takes ~39 mins for the WR BPT to reach its maximum level (stability), the AD BPT takes ~85 mins to reach stability. This can be explained through the volume differences between the two BPTs and their series arrangement. The WR BPT volume is half that of the AD BPT while their depths are equal. This combined with the WR BPTs ability to demand higher rates of inlet flows than the AD BPT, results in the WR BPT achieving an increase in level much faster than the AD BPT. The AD BPT response is thus slowed by its responsibility to supply the WR BPT, at a high rate. The WR BPT draw from the AD BPT is able to exceed the AD BPT inlet draw is because the BPT draws are governed only by the BPT levels and the interplay between the line losses and valve resistances, and not by any outlet flow control. The AD BPT inlet flow will thus always trail its outlet flow (Figure 36a).
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At the 180 min step, the outlet draw from the WR BPT instantly increases from zero to the sum of the characteristic flows (2.96 m3/s) for the reservoirs downstream of the BPT. The WR BPT can then be observed to establish its new settling level at 6.5 m within ~26 mins (Figure 35b). The AD BPT (Figure 35a) however, due to its aforementioned responsibility to indirectly respond to the WR BPT level, does not exhibit the clarity of the described step increase in outlet flow. The average magnitude of the subsequent increase however, can be seen to be the sum of the characteristic flows of all the reservoirs downstream of it (4.33 m3/s). The AD BPT reaches stability within ~60 mins of the step, at a level of 6 m.
The discrepancy between the stability levels of the BPTs can be explained by the difference between the supply and demand capabilities of the two BPTs. The WR BPT is able to demand more inlet flow than the AD BPT, yet must supply a smaller outflow, allowing its stability level to be higher than that of the AD BPT (according to the PLC scheme in Figure 18, lower levels correspond to lager inflows). The AD BPT’s PLC control system must thus satisfy the demand with a lower settling level.
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