3.71 Pumping water to high tank
Pumping water to high tank
Pumping water to high tank such as from a water supply plant to a high reservoir. The clean water collected in a low reservoir, is to be pumped up to a high reservoir. The high and low water levels in the two reservoirs and the altitude of the site for the location of the pump are shown in figure 1.
The pipe linking the reservoirs is long, about 20 km, and during much of the day water is drawn from the pipe with consequent loss of pressure.
The daily consumption of water varies considerably. Furthermore, other water resources are jointly operated. For these reasons and also with regard to the economy (saving energy, etc.) it must be possible to adjust the pumping from the water works in question from between about 50 to 115 litres per second. In cases of emergency (shortage of water) it should be possible to draw about 170 litres per second.
Figure 1 The system configuration
System curves
The system curves are shown in figure 3.71b. It proved possible to place the pumps 1 metre below the lowest level in the low reservoir so that a pressure of 1-3 metres was obtained on the suction side. At the high reservoir the difference in height between high water and low water is 5 metres. The static delivery head in the system therefore varies between 47 m (+16 in the low reservoir to +63 in the high reservoir) and 40 m (+18 and +58 meters respectively).
Figure 3.71b System curve
The losses in the pipe system are shown by curves A and B. From time to time the high reservoir may be disconnected for cleaning purposes or as the result of a breakdown. Pumping must then be directed to another more remote reservoir and this results in an increase of pipe losses by 3 m. Curve C illustrates the upper limit of the pressure requirement in this situation. The normal operating zone for the pumps is clearly indicated in the figure. The highest degree of efficiency should therefore lie within this zone.
Choice of pump
On the basis of the great variations in operating conditions found in this case a combination of a number of pumps was deemed necessary. However, it was scarcely possible to find place for more than three. Heavy demands on reliability of operation required that a back-up unit should, at least to some degree, be available. The necessary delivery heads implied that virtually only centrifugal pumps (turbine pumps with radial impellers) be considered. From the running and maintenance points of view it was desirable that all the pumping units should be identical.
It proved difficult to find suitable pumps of standard type on the market. The reducing of impeller O.D. (machining down) would have been feasible, but the resultant considerable loss of efficiency made this alternative unattractive. Finally three centrifugal pumps of equal size were chosen. These were all supplied with two stages. One of the pumps being speed-regulated by means of a a.c. motor and frequency converter.
The pump curve is shown in figure 3.71c. Curves worked out according to the laws of affinity for the speeds 1200 and 1300 r/min are also shown in the figure, as well as a combined curve for two pumps at full speed. The curve indicating efficiency is also included in the figure.
Figure 3.71c Pump curves
In figure 3.71d the pump curves have been incorporated into the system curves according to figure 3.71b. As may be seen, the speed-regulated pump provides the desired minimum flow at a speed between 1200 and 1300 r/min. Maximum flow was obtained with a speed plus the speed-regulated pump. One unit was then kept in reserve. A case of emergency implying a temporary need of 170 litres per second can be met with all three pumps in operation.
Pump curves
1 : Speed-regutated pump, 1200 r/min
2: Speed-regulated pump, 1450 r/min (full speed)
3: Speed.regulated pump, r/min
4: Constant speed pump + spæd-regulated pump 1200 r/min
5: Constant pump + speed-regulated pump 1300 r/min
6: Constant speed pump + speed-reguiated pump full speed
Figure 3.71d. Pump curve and system curve intersection at running conditions
The efficiency of the speed pumps is not the best in their normal operating zone, only 70%, but it is better than that obtainable with other pumps having modified impellers.
The maximum power requirement for the pumps selected occurs at head H=46 m and flow Q=340 m³/h (see figure 3.71c) and is
then 61 kW. The efficiency of the electric motor is estimated to and the pumping units are supplied with 75 kW standard motors.
Erection of pumps
Figure 3.71e schematically indicates how the pumps, pumping water to a high tank, were erected. Thus the pumps work in parallel with the speed-regulated pump as first unit. When this reaches full speed, one of the two constant speed pumps is switched on, the constant speed pumps operate alternatively, and the speed-regulated pump drops down to its lowest speed. In case of increasing flow requirements it increases speed again. The procedure is the reverse in case of decreasing water requirements.
Control of the speed as well as the starting and stopping of the constant speed pumps is subject to a theoretical daily graph which in turn is constantly corrected by regulation from the water level in the high reservoir.
The delivery pipes of the pump are fitted With hydraulically controlled slow-operating valves to reduce water hammer when starting and stopping. Pumps are thus always started With the valve closed and when stopping the valve loses first. For safety reasons the pumps are also supplied With automatic non-return valves.
Figure 3.71e Erection of pumps