Wells and Pumps for Greenhouses
Pump Performance Parameters
There are several parameters depending on impeller design, diameter, RPM etc., characterizing a pump. In this section the most important pump parameters will be discussed as well as examples in how to read pump curves.
Pump Curve Terminology
Before discussing specific details, it helps to understand typical terms associated with pump curves:
- Head Head is the net work done on a unit weight of water by the pump impeller. It is the amount of energy added to the water between the suction and discharge sides of the pump. Pump head is measured as pressure difference between the discharge and suction sides of the pump.
- Total Dynamic Head The total dynamic head (TDH) of a pump is the sum of the total static head, the pressure head, the friction head, and the velocity head. An explanation of these terms is given below and graphically shown in Figure 13.10.
- Total Static Head The total static head is the total vertical distance the pump must lift the water. When pumping from a well, it would be the distance from the pumping water level in the well to the ground surface plus the vertical distance the water is lifted from the ground surface to the discharge point.
- Static Lift The static lift is the vertical distance between the centerline of the pump and the elevation of the water source when the pump is not operating. If the water elevation of the source is below the pump elevation, the static lift is positive. If the pump is located at the elevation below the water surface elevation, the static lift is negative.
- Static Discharge The static discharge head is a measure of the elevation difference between the centerline of the pump or top of the discharge pipe and the final point of use. When pumps discharge directly into canals a short distance from the pump at the same elevation, the static discharge head is zero.
- Pressure Head Some irrigation systems require pressure to operate. The range of this pressure varies among systems. High-pressure systems, such as sprinkler systems, may require large operating pressures (up to 100 psi).
- Friction Head Friction head is the energy loss or pressure decrease due to friction when water flows through pipe networks. The velocity of the water has a significant effect on friction loss.
- Velocity Head Flowing water represents energy, and work that must be done by the pump to impart motion to the water. The work required to impart movement to the water is similar in effect to friction.
- Brake Horsepower The pump performance curve will give information on the brake horsepower (BHP) required to operate a pump (horsepower required at the pump shaft) at a given point on the performance curve. Like the head-capacity curve, there is a brake horsepower curve for each different impeller trim.
- Pump Efficiency Pump efficiency is the percent of power input to the pump shaft (the brake power) that is transferred to the water. Since there are losses in the pump, the efficiency of the pump is less than 100 percent and the amount of energy required to run the pump is greater than the actual energy transferred to the water.
- Impeller The moving element in a pump that drives the liquid.
- Impeller Trims Impeller trims or impeller diameter is measured in either inches or millimeters. Pump performance curves generally show performance for various impeller diameters or trims.
- Pump Capacity The capacity of a pump is the amount of water pumped per unit time. Capacity is also frequently called discharge or flow rate (Q).
- Net Positive Suction Head Net positive suction head (NPSH) is a term used to describe the pumps ability to develop a suction lift.
- Suction Lift Suction lift exists for pumps that are placed above the level of the water source. It is defined as the vertical distance in feet from the surface of the water source (during pump operation) to the centerline of the pump.
Pump Power Requirements
The power added to water as it moves through a pump can be calculated with the following formula:
Effect of Speed Change on Pump Performance
The performance of a pump varies with the speed at which the impeller rotates. Theoretically, varying the pump speed will result in changes in flow rate, TDH and BHP according to the following formulas:
Manufacturers use a pump selection chart to describe the performance characteristics of a family of pumps. This type of chart, shown in figure 13.11, is useful in selecting the appropriate pump size for a particular application. The pump designation numbers refer to the pump inlet size, the pump outlet size, and the impeller size, respectively. There is significant overlap among these various pump sizes, which is attributable to the availability of different impeller sizes within a particular pump size. Once a pump has been selected as roughly meeting the needs of the system, the specific performance curve for that pump must be evaluated. Often, a given pump casing can accommodate impellers of several different sizes, with each impeller having a separate, unique performance curve.
Reading a Pump Curve
When the desired flow rate and TDH are known, these curves are used to select a pump. The pump curve shows that a pump will operate over a wide range of conditions. However, it will operate at peak efficiency only in a narrow range of flow rate and TDH.
Changing Pump Speed
In addition, suppose this pump is connected to a diesel engine. By varying the RPM of the engine we can vary the flow rate, the TDH and the BHP requirements of this pump.
Pump efficiency is the ratio of the liquid horsepower delivered by the pump and the brake horsepower delivered to the pump shaft. When selecting a pump, a key concern is optimizing pumping efficiency. Net Positive Suction HeadIt is good practice to examine several performance charts at different speeds to see if one model satisfies the requirements more efficiently than another. Whenever possible the lowest pump speed should be selected, as this will save wear and tear on the rotating parts.
Net Positive Suction Head
Suction lift exists for pumps that are placed above the level of the water source (See Figure 13.10). It is defined as the vertical distance in feet from the surface of the water source (during pump operation) to the centerline of the pump. When a pump is operating the rotation of the impeller reduces the water pressure at the impeller’s inlet and causes a vacuum (pressure less than atmospheric pressure) to form. The difference between the atmospheric pressure and the pressure at the impeller inlet causes water to flow into the intake pipe. Even in the best of circumstances (including a near perfect vacuum with cold water at sea level elevation), the maximum water column (i.e., suction lift) that can be forced by atmospheric pressure never exceeds about 33.9 feet in height, but the practical limits are less. As elevation, water temperature, and pipe friction increase, the height of the water column that can be forced drops. The maximum column of water that can be created in a pipe under a given set of conditions is known as net positive suction head (NPSH).
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