5 D ISCUSSION
5.1 Scenario 1.1: Imposed steady draw at Ntuzuma (9) – Control 1.0
All reservoir control systems overridden: Only Ntuzuma (NR5) draws continuously, at a steady rate.
The imposed steady draw rate of Ntuzuma is that of its characteristic draw rate.
NR5 has the largest individual reservoir draw, and is at the extreme end of the aqueduct.
Draw rate based on projections for 2036 flows.
Purpose:
o Simple operational test to verify model performance and understand system interactions.
o Neglects the impact of reservoir consumer demands and control systems on the performance of the WA system.
o Provides insight to the effect of biased spatial allocations of demand (fire flows, new settlements etc.)
Umlaas Road Reservoir
Ashle y Road BPT (20Ml)
Wyebank Road BPT (20Ml)
1
2
3
4
5
7 6
8
9
Lumped Demand
0.53 km 140 0 mm 3.30 km 140 0 mm
1.06 km 140 0 mm
1.14 km 140 0 mm
3.26 km 140 0 mm
0.24 km 140 0 mm 0.43 km 140 0 mm
2.06 km 140 0 mm
8.00 km 140 0 mm 20.01 km
20.01 km
Scenario 1.1
Classification Normal/override Start time 00:00
Reservoir draws override (NR5 only) Sleeve valves
active
AD - 3 WR - 3 Globe valves
active
AD – 3 WR - 3 Control system Control 1.0
Imposed (override) reservoir draw Reservoir draw as per reservoir control scheme
No reservoir draw (overridden)
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Umlaas Road Reservoir
Ashle y Road BPT (20Ml)
Wyebank Road BPT (20Ml)
1
2
3
4
5
7 6
8
9
Lumped Demand
0.53 km 140 0 mm 3.30 km 140 0 mm
1.06 km 140 0 mm
1.14 km 140 0 mm
3.26 km 140 0 mm
0.24 km 140 0 mm 0.43 km 140 0 mm
2.06 km 140 0 mm
8.00 km 140 0 mm 20.01 km
20.01 km
All valves fully open initially
7.0 m settling level
BPT start level 10%
All valves fully open initially
BPT start level 10%
7.0 m settling level
Zoo med section o f os cillatory settling
level
Rates: 2036
1- Simple operational test to verify model performance and understand system
interactions.
2- Only NR5 drawing continuously at its steady characteristic flowrate.
Initial Conditions: All valves open, reservoirs 50 %, and BPTs 10% full.
Risk:
1- Valve oscillations - wear risk.
2- Settling level not at NOL (50% level) –overflow risk.
3- Globe valve active during normal operation – deviation from design intent
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 establish settling levels at 7 m. AD after 140 mins and WR, 60 mins. Globe valve 1 and sleeve valve 2 switch to maintain level in an oscillatory stable state.
Figure 30 - Scenario 1.1 results overview – Imposed steady draw at Ntuzuma a
b
c
d
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This scenario was envisaged to provide an uncomplicated set of results that could be used to analyse the system behavior in the absence of reservoir draw scheduling effects. It also enables a delineation and decoupling of the system components that interact in a complex manner during regular operation of the system. This test furthermore provides valuable insight into the operation of the system when the consumption demand is shifted toward a specific spatial location. The NR5 reservoir was selected to be overridden to draw constantly, due to it having the greatest consumer demand of the reservoirs downstream to the Wyebank BPT. Its average demand in 2036 represents 43.5% of the total anticipated demand downstream of the AD BPT in that year. It is also suited to the outcomes of this test due to it being at the extreme end of the aqueduct.
The BPT initial volumes (10%) are representative of a highly unlikely scenario, possibly a rebound from a failure state, or reboot from emergency maintenance. Under normal circumstances, the control system ensures that the volume of liquid within the BPT would remain within the upper 20% of the BPT capacity. The other possible, and an increasingly plausible situation it could accurately represent, is a shortage of potable water supply.
According to the control philosophy (Section 3.3.2), the valves would initially adopt a fully-open state in this situation.
Figure 30a demonstrates the ability of the WR BPT to enact a greater demand draw upon the AD BPT than the AD BPT is able to demand from the supply source. This is understood from the marginal, yet observable difference between the supply and demand flows (AD BPT), despite the absence of direct draws between the AD and WR BPTs, and the presence of a sizable draw downstream of the WR BPT. It is anticipated that this result will be more clearly visible in the presence of direct draw(s) from the AD BPT (to reservoirs that are supplied between the two BPTs e.g. Haygarth Road), and will thus be discussed in more detail in Section 5.2.
The constant WR BPT draw that is visible between 35 min and ~64 min in both Figure 30a (as the flow out of AD) and Figure 30b (as the flow into WR) corresponds to the lack of valve movements (WR BPT) within this period, as demonstrated in Figure 30d. An equivalent condition for the AD BPT is observed between 90 min and 140 min. This observation is explained by the BPT levels residing within the confines of a control system sub-range (6.5m<ℎ<7.0m), in which no valve changes are stipulated. The prolonged length of these periods is due to the gradual (slowed) closure of the valves in a manner that resembles the action of an integral controller. It is within these periods that the BPT levels
102 gradually climb to 7 m, where an apparent settling level is consequently established. The control system is then seen to maintain this apparent settling level, thus causing the levels and valve positions to become ‘oscillatory stable’. The lack of visibility of the oscillations in the BPT level curves indicates that valve movement frequency is high enough to attenuate the resultant level oscillation. The presence of the minute oscillations within the level curves is shown in the inset image in Figure 30b and the zoomed section in Figure 31.
The fitted-operation BPT control system mechanisms can be described through its determination of the systems behavior as presented in Figure 30 (a-d). As described in Section 3.3.2, the system consists of control sub-regions, at whose boundaries the adjustment of one or more control valves are stipulated, giving rise to ‘staggered valve movements’, as observed in Figure 30c and Figure 30d. This forms a system of localized bang-bang control where any flow imbalance will cause the BPT level to gravitate toward a specific sub-region boundary. This boundary is then established as the settling level until the downstream demand is altered to a sufficient extent to cause the level to drift to another sub-region boundary. Small flow imbalances will be compensated for through the aforementioned oscillatory movement of the valve around the established settling level.
The traversing of a system boundary sets any valve that is programmed to move on a set trajectory toward its required position, governed only by the preset valve movement rate.
It is thus likely that the control system will always establish BPT settling levels at these sub-region boundaries. The make-up of Control 1.0 resembles the principle of a proportional controller, where larger outlet flow demands will necessitate further opening of valves, and thus lower BPT levels.
Figure 31 - Zoomed section around the oscillatory settling level of the BPTs.
6,97 6,98 6,99 7 7,01 7,02
60 80 100 120 140 160 180
Levels (m)
Time (Minutes)
AD & WR BPT Levels - Zoomed Section
Level of AD BPT Level of WB BPT
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The irregular pattern of the oscillations depicted within Figure 31 are due to the difference between the flowrate change for the predetermined, fixed per-interval valve movement, and the outlet (demand) flowrate. The phenomenon will be discussed further in Section 5.2
The high number of valve movements, required to maintain the ‘oscillatory stable’ settling levels unintendedly resulting from the stipulated control philosophy (Control 1.0), are undesirable from both an operational, and a maintenance perspective. Such repeated valve movements are unnecessary and can increase the amount of wear on the valves.
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