Controlling a drainage pump with floats has been used for many years. It works like this: When the water in the sump rises to a set height, the float turns the pump on, and when the water drops to a certain level, the pump shuts off (Figure 7).
Figure 7. Float-controlled pumps allow for automatic operation. In this illustration, he water has risen to the level where the pump-on float will turn on the pump. As the pump removes water from the sump, the pump-off float will drop down 30 to 36 inches, at which time it will shut off the pump.
This is the same as the sump pump in the basement of many houses; the only difference is the volume of water and the size of the pump. Unlike a household sump pump, these pumps are not designed to run for very short periods of time (less than 30 seconds). To increase the pumping time, a storage volume must be included in the design.
Sump Storage Volume
The required storage volume is based on the number of times the pump turns on per hour. A pump cycle is the time in minutes when the pump turns on, shuts off and then turns on again. Most pump manufacturers do not recommend more than 10 cycles per hour, which means the pump turns on about every six minutes. The required volume of storage can be calculated with the following formula:
Storage volume (cubic feet per acre)
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=
|
2 * max flow rate of the pump (gpm)
(Number of cycles per hour)
|
(2) |
Ideally, the maximum flow rate of the pump should equal the drainage coefficient selected for the tile drainage project (Table 1).
Example: Design a lift station for 100 acres where the tile system is designed for a ⅜-inch drainage coefficient and 10 is the maximum number of pump cycles per hour.
Solution: Table 4 shows that for 10 cycles per hour and a ⅜-inch drainage coefficient, 1.4 cubic feet of storage is required per acre; thus, the amount of pump storage from the pump-on water level to the pump-off water level must be 140 cubic feet (100 x 1.4).
Storage volume can be achieved in a vertical direction, with horizontal pipe or a combination of the two (Figure 8).
Figure 8. This illustrates common options for creating water storage in the sump.
Vertical Storage
Most sumps are circular, and storage in the sump will be the volume between the pump-on and pump-off levels. The diameter of the sump must be selected to achieve the desired storage volume.
A common distance between pump-on and pump-off is 30 to 36 inches. The required diameter for vertical storage is given in Table 5, assuming a maximum of 10 pump cycles per hour. For larger acreages, the diameter of the sump is so large that excavation and installation become a problem.
Table 5. Sump diameter required to provide the necessary storage so that the pump does not come on more than 10 times per hour at design flow rate.
Sump Diameter for a Float-controlled Pump (36-inch on/off)(feet) |
|
Area Drained (Acres) |
Drainage Coefficient |
40 |
80 |
120 |
160 |
240 |
320 |
1/4 |
4 |
6 |
7 |
8 |
10 |
11 |
3/8 |
5 |
7 |
8 |
10 |
12 |
14 |
1/2 |
6 |
8 |
10 |
11 |
14 |
16 |
3/4 |
7 |
10 |
12 |
14 |
17 |
19 |
Combination Horizontal and Vertical Storage
To reduce the diameter of vertical storage, many tile installers and farmers use horizontal storage with a small diameter (3 to 4 feet) vertical sump (Figure 8).
Horizontal storage is much easier to install than large-diameter vertical storage. Horizontal storage is a length of buried 18- to 24-inch-diameter dual-wall corrugated plastic pipe connected to the vertical sump (Figure 9). The length is dependent on the number of acres draining into the pump station.
Figure 9. Two-foot-diameter corrugated plastic tile provides horizontal storage.
Horizontal storage (feet of length)
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=
|
8 * q * A
Π * Dv2 * N
|
-
|
(Dv / Dh)2 * d
|
(3)
|
Where:
q = Drainage coefficient flow rate per acre (gpm/ac from table 1)
A = Area to be drained (acres)
Dv = Diameter of the vertical sump (feet)
Π = 3.14159
Dh = Diameter of the horizontal storage pipe (feet)
N = Number of pump cycles per hour
d = Distance between the pump on/off floats
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The tile main drains into the top of the horizontal storage pipe. The length of required horizontal storage can be calculated with the following formula:
For example, the required length of 2-foot-diameter horizontal storage connected to a 4-foot vertical sump is shown in Table 6, assuming a maximum of 10 pump cycles per hour.
Table 6.
2-foot-diameter Pipe for Horizontal Storage, Float-operated Pump (3-ft on/off)(feet) |
|
Area Drained (Acres) |
Drainage Coefficient |
40 |
80 |
120 |
160 |
240 |
320 |
1/4 |
Not needed |
11 |
22 |
34 |
57 |
80 |
3/8 |
6 |
24 |
41 |
59 |
95 |
131 |
1/2 |
12 |
36 |
61 |
85 |
133 |
182 |
Because the flow rate of most field lift pumps is more than 500 gallons per minute, the pump often will run for less than two minutes. In this short pumping time, when the required horizontal storage is more than 60 feet of linear length, all the water in the horizontal storage cannot get to the pump. Thus, for larger acreages requiring more than 60 feet of horizontal storage, installers will make a tee or cross at the inlet to the vertical sump by placing 30 or more feet of horizontal storage in one direction and 30 or more feet in the other direction.
Pump Controlled With a Variable-frequency Motor Drive (VFD)
A recent innovation is the use of variable-frequency motor controllers, commonly called VFDs, to control the lift pump motor. A VFD converts single or three-phase input electrical power to variable-frequency three-phase output power.
A pump driven with a three-phase motor provides several advantages vs. single-phase motors because three-phase motors are generally less expensive and physically smaller than single-phase motors. With three-phase power, soft-start options can be used to reduce the in-rush current and put less strain on the motor so it lasts longer.
With the ability to vary the frequency of the motor, the pump rotation speed (revolutions per minute, or rpms) will vary, thus changing the pump flow characteristics. A water level sensor provides feedback information from the sump to the VFD, which determines the pump rpm to maintain a set water level in the sump (Figure 10).
Figure 10. A pressure transducer box and pipe are part of this VFD-controlled pump.
When the drainage flow is low, the pump’s rpms are low, and when the drainage flow is high, the pump’s rpms are high. Thus, the flow rate of the pump can be matched to the flow rate of the drainage water entering the sump.
A VFD-controlled pump runs continuously until very low drainage flow occurs. At this point, it acts like a float-operated pump and may turn on for short periods of time. This significantly reduces the required storage volume (Figure 11).
Figure 11. Variable-frequency drive (VFD)-controlled pumps still need some storage but not as much as a float-controlled pump. The water level commonly is sensed using a pressure transducer at the top of a 2-inch pipe. As the water level changes in the sump, the air pressure will change in the pipe. The pressure transducer senses this and provides a signal to the VFD to vary the rotation speed of the pump.