5 D ISCUSSION
5.8 Scenario 0x: Normal operation (Base case) test – Control 2.0
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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
0 1 2 3 4 5 6 7 8 9 10
0 20 40 60 80 100 120 140 160 180
Flow (m3/s) | Level (m)
Time (Minutes)
Ashley Drive BPT - Flow & Level
Flow (into AD)
Flow (out of AD)
Level BPT level
initially 70% BPT level in
deadband
0 1 2 3 4 5 6 7 8 9 10
0 20 40 60 80 100 120 140 160 180
Flow (m3/s) // Level (m)
Time (Minutes)
Wyebank Road BPT - Flow & Level
Level Flow (into WR)
Flow (out of WR) BPT level
initially 70%
Overflow
BPT level in deadband
0 0,2 0,4 0,6 0,8 1
0 20 40 60 80 100 120 140 160 180
Valve Position (fraction of stroke)
Time (Minutes)
Ashley Drive BPT - Valves
Globe valve 2
Sleeve valve 1
Globe valve 3 Globe valve 1
Sleeve valve 3 Sleeve valve 2 Stepwise
sequential valve movements
Valves shut on high level
AD BPT level in deadband
Rates: 2036
1- Assess the performance of the new control system 2- Represents the most-
encountered operational scenario.
Initial Conditions: All valves open, reservoirs 50 %, and BPTs 10% full.
0 0,2 0,4 0,6 0,8 1
0 20 40 60 80 100 120 140 160 180
Valve Position (fraction of stroke)
Time (Minutes)
Wyebank Road BPT - Valves
Globe valve 3 Globe valve 1Globe valve 2
Sleeve valve 2 Sleeve valve 3 Sleeve valve 1
Globe valve intervention – WR BPT level exceeds high-
high
WR BPT level in deadband Valves shut on
high level
Risk:
1- Overflow (WR) - Sleeve valve response slow, globe valves activate too late.
AD Initial: WR Initial
S1= 0.85 G1= 1 LADBPT= 70% of capacity S1=0.85 G1=1 S2= 0.85 G2= 1 LWRBPT=70% of capacity S2=0.85 G2=1 S3= 0.85 G3= 1 Lres=50% of capacity S3=0.85 G3=1
Other Observations:
Sleeve valve reaction.is too slow to prevent an overflow in the WR BPT from the fully open (initial) state. Upon establishing controllability, the system performance is improved.
Figure 44 - Scenario 0x results overview – Normal operation (base case) test – Control 2.0.
b
c
d a
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The revised control system was proposed in order to allay the concerns regarding the oscillatory behavior of the valves due to the localized bang-bang control philosophy of the original proposal. The revised control system (Control 2.0) is devised around a central deadband region, between 3.5 m and 4.5 m (BPT level), wherein no valve movements are required. This is consistent with the design intent, of maintaining a BPT level of 50%
(~4.0 m). Sleeve valve movements, when prompted to occur, occur individually in sequence, with individual movements of 5% (of the valve stroke) per 30 second interval.
The movement from fully closed to 25% however, occurs in a single interval. The arrangement of the valve movements can be seen in Figure 44(c-d).
The presence of the deadband can be observed as plateaued regions of the sleeve valves in both the AD BPT (Figure 44(c) between 142 mins and 148 mins) and the WR BPT (Figure 44(c) between 120 mins and 140 mins)
The control system, fulfills the intent of preventing the valve oscillations that is a distinguishing defect in the original control philosophy. The result is that amount of valve movements is significantly decreased (Figure 44(c-d)), and the level curves (Figure 44(a- b)) are smoother, and lack the plateau regions.
The reduction in the amount of requisite valve movements, and the elimination of the oscillatory stable state, reduces the generation of water hammer overpressures within the pipelines. The calculated waterhammer overpressures for the revised control system under Scenario 0x conditions are presented in Figure 45 and Figure 46.
Control 2.0 is also able to satisfactorily cope with routine demand variations.
134 Figure 45- Simulation results (Scenario 0x) for the waterhammer transient phenomenon at the AD BPT (left axis). Valve positions are represented on the right axis – Control 2.0.
Figure 46- Simulation results (Scenario 0x) for the waterhammer transient phenomenon at the WR BPT (left axis). Valve positions are represented on the right axis – Control 2.0.
The sequenced arrangement of valve movements, paired with the 5% per interval movement specification, causes the control mechanisms to respond sluggishly. Although this is desirable, in terms of waterhammer concerns, the overflow situation encountered between 15 mins and 40 mins in Figure 44(b) shows that it could result in an overflow, if the initial conditions are unfavourable (all valves initially fully open as per test conditions).
A similar peak is observed at 35 mins for the AD BPT (Figure 44(a)), although an overflow is prevented due to the aforementioned interaction between the BPTs and the ability of the
-1500 -1000 -500 0 500 1000 1500
0 0,2 0,4 0,6 0,8 1
0 20 40 60 80 100 120 140 160 180
Water Hammer Overpressure (kPa)
Valve Position (Fraction of Stroke)
Time (Minutes)
Ashley Drive BPT Valves & Water Hammer Overpressure
Globe Valve 1 Globe Valve 2 Globe Valve 3
Sleeve Valve 1 Sleeve Valve 2 Sleeve Valve 3
AD Water Hammer (Joukowsky) AD Water Hammer (Steel Pipe)
-1500 -1000 -500 0 500 1000 1500
0 0,2 0,4 0,6 0,8 1
0 20 40 60 80 100 120 140 160 180
Water Hammer Overpressure (kPa)
Valve Position (Stroke Fraction)
Time (Minutes)
Wyebank Road BPT Valves & Water Hammer Overpressure
Globe Valve 1 Globe Valve 2 Globe Valve 3
Sleeve Valve 1 Sleeve Valve 2 Sleeve Valve 3
WR Water Hammer (Joukowsky) WR Water Hammer (Steel Pipe)
135 WR BPT to demand a higher inlet flow (from the AD BPT) than the AD BPT is able to acquire from the supply source.
The configuration of the revised control system, that specifies the simultaneous intervention of the globe valves only at the high-high level (8.3 m), at the original regulated movement rate, is inadequate. The globe valves are unable to prevent the overflow situation due to this configuration. Increasing the rate of closure of the globe valves, to mitigate this, would drastically increase the magnitude of waterhammer overpressures generated. The only feasible adjustment, is thus to decrease the level at which the globe valves begin their intervention. The optimum intervention level should be tested through repetitive runs of this simulation at varying intervention levels.
The revised control system has the ability to allow the globe valves to assume a bigger proportion of the throttling function during normal operation. This is seen to occur, due to the effect of the sinusoidal-like ripple of the globe valves on the inlet flow to the WR BPT, at ~32 mins in Figure 44(a,b,d). This cannot occur within the original control system’s operation, as the sleeve valve and globe valve ranges are separated so that all sleeve valves are shut before the globe valves are shut (to avoid interference of the globe valves on normal control).
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