Q KePx

Pressure (kPa)

Figure 5.35. Discharge-pressure curves for typical types of emitters.

Pressure (kPa)

Figure 5.35. Discharge-pressure curves for typical types of emitters.

where q is the emitter flow rate (L h-1), Ke is the proportionality factor that characterizes the emitter dimensions, P is the operating pressure (kPa), and x is the emitter discharge exponent, which characterizes the flow regime. The coefficients Ke and x are determined by plotting q versus P on a log-log plot. The slope of the straight line is x, and the intercept at P = 1 is Ke:

Actual discharge/pressure relationships may differ significantly from those given by the manufacturers [125]. Reliable information on x and Ke is often available from laboratories where irrigation equipment is tested.

Low values of x (low sensitivity to pressure variations) allow the use of long laterals or small lateral diameters. In addition, the performance of drippers laid along steep slopes is improved. As x approaches zero (pressure-compensating drippers), the discharge varies little with variations in pressure. Good pressure-compensating emitters should have x values below 0.1 over the expected pressure range. For laminar flow emitters, x is close to 1; therefore, the variations in operating pressure should be held within about ±5% of the desired average. For turbulent-Sow emitters, x = 0^5, and the pressure-head variation should be within about ±10% of the desired average.

Sensitivity to Temperature

Emitter flow will vary with temperature if the flow cross sections vary with the thermal expansion and contraction of the emitter material. With long-path, laminar-flow emitters, flow also varies with the viscosity of the water, which changes with temperature. Temperature effects can be important because the temperature of water flowing slowly through polyethylene laterals lying in the sun can increase substantially (>20°C) from the head to the tail end. Manufacturers should give information on the temperature effects on emitter flows. Laminar-flow emitters should not be used where temperatures vary through the system. Information on the temperature discharge ratio (TDR), relating the emitter discharge at high temperature to the standard emitter discharge at 20°C, is given by Keller and Bliesner [11].

Sensitivity to Clogging

Two critical parameters affecting emitter clogging susceptibility are the minimum flow-passage dimension and the velocity of the water through the passage. The relation between the passage cross section and the susceptibility to clogging is very sensitive (<0J mm), sensitive (0.7-1.5 mm), and relatively insensitive (>L5 mm) for continuously flushing emitters. Keller and Bliesner [11] give information on minimum flow-passage dimension for main types of emitters. For microsprinklers in Florida citrus orchards, Boman [126] found that plugging decreased about 50% when orifice diameter was increased about 30%. Velocities of water through the emitter passage ranging from 4 to 6 ms-1 generally result in reduced clogging.

The manufacturer's recommendations for filtration also give an indication of the emitter's sensivity to clogging. The greater the sensivity, the finer the recommended filtration. The following classifications and filter size (^m) recommendations used in France [98] are

Coefficient of Manufacturing Variation

The coefficient of manufacturing variation for an emitter, Cv, is used as a measure of the anticipated variations in discharge for new emitters. The value of Cv should be available from the manufacturer. It also might be available from independent testing laboratories and can be measured from the discharge data of a sample set of at least 50 emitters operated at a reference pressure:

where q1, q2, ..., qn are individual emitter discharge rates (L h-1), n is the number of emitters in the sample, and qa is the average emitter discharge rate for the sample (L h-1). Manufacturing variability can be classified in accordance with Table 5.11. Significant differences between Cv values given by the manufacturers and those obtained in independent tests often occur [125]. Many emitters are available with Cv in the range of 0.03 to 0.05.

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