GB/T 18688-2023 Agricultural irrigation equipment - Pressure losses in irrigation valves - Test method
1 Scope
This document applies to manually-activated valves only.
This document specifies a test method for determining the pressure loss in agricultural irrigation valves under steady-state conditions when water flows through them. The scope and accuracy of the valve performance specifications presented will assist agricultural irrigation system designers in comparing pressure losses through various types of valves.
The measurement of pressure losses provides a means for determining the relationship between pressure loss and flow rate through the valve.
This document also describes the method of reporting pertinent test data.
No attempt is made to define product use, design or applications.
The test method is suitable for valves with equal inlet and outlet nominal sizes.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1
nominal size
DN
conventional numerical designation used to indicate the size of an irrigation valve
3.2
volume flow rate
flow rate
qV
volume of water flowing through the valve per unit time
3.3
pressure loss
Δp
difference in pressure due to water flow between two specified points in a system or in part of a system
3.4
piping pressure loss
Δpp
pressure loss in the upstream and downstream portions of the test bench piping between the pressure taps, but excluding the pressure loss in the valve tested (see 5.4.4)
3.5
bench pressure loss
Δpb
head loss between the pressure taps upstream and downstream from the measurement area without the device being tested
3.6
valve pressure loss
Δpv
pressure loss in the valve tested
3.7
reference velocity
νref
velocity of flow through the valve calculated from the actual flow rate through the valve divided by the reference cross-sectional area of the valve
3.8
steady-state flow
state of flow where the flow rate through a cross-section does not vary with time
3.9
valve flow coefficient
Kv
number equal to the flow rate of water, in cubic metres per hour (m3/h), that will flow through a fully open valve with a one bar pressure loss across the valve
3.10
flow resistance coefficient
ζ
coefficient used in non-dimensional presentation of valve loss
4 Test installation
4.1 Permissible deviation of measuring devices
The permissible deviation of the reading indicated on the measuring devices from the actual value shall be as follows:
flow rate: ± 2 %
differential and actual pressure: ± 2 %
temperature: ± 1 °C
The measuring devices shall be calibrated according to the existing calibration rules in the country performing the test.
4.2 Test equipment
4.2.1 Piping
Upstream and downstream piping shall be the same diameter as that of the test valve connection. The lengths of the straight, uniform-bore pipe shall be as specified in Figure 1. The inside surface of the piping shall be free of flaking rust, mill scale and irregularities which might cause excessive turbulence.
In that part of the test apparatus shown within the frame, in Figure 1, the order of the fittings/devices shown in the key and the distances between them shall be adhered to, with the exception that the lengths indicated as 2d and 10d shall be understood to be the minimum allowable lengths.
4.2.2 Throttling valve
A downstream throttling valve shall be used to control the flow through the test specimen. There are no restrictions on the size or type of this valve. The throttling valve shall be located downstream of the downstream pressure tap (used for measuring bench pressure).
4.2.3 Flow measuring device
Locate the measuring device at the head of the system.
If an open measuring device (such as a calibrated volumetric tank) is used, it shall be located at the downstream end of the assembly, i.e. downstream of the downstream throttling valve.
The flow-measuring device shall be installed in accordance with the specific installation instructions and, where applicable, shall be installed with the required length of straight piping before and after the device.
The accuracy of the measuring device shall be ± 2 %.
4.2.4 Pressure differential measuring device
Any device capable of measuring pressure differential with acceptable accuracy may be used.
4.2.5 Pressure taps
Pressure taps (see Figure 2) shall be provided on piping for measurement of static pressure, and spaced as shown in Figure 1. The drilling centreline of the taps shall intersect the centreline of the pipe perpendicularly, as shown in Figure 2. The diameter shall depend on the DN of the valve, see Table 1.
The length, l, of the tap bore shall be not less than twice the diameter of the bore. For thin-walled pipes where the wall thickness is less than 2d1 , a boss may be added to the pipe wall where the pressure taps are to be located (see Figure 2).
Pressure taps shall be free of burrs and other irregularities and the inside wall of the piping shall be machine-finished. For pipes of 50 mm diameter and larger, four taps shall be made, situated 90° ± 5° apart on the circumference so that no tap is located on the lowest point of the pipe circumference. For pipe diameters of less than 50 mm, two taps will suffice. All taps, whether two or four in number, shall be connected by a conduit whose bore shall not be less than two pressure-tap cross-sections. The pressure taps shall provide appropriate values of d1 and l, and may be made as illustrated in Figure 2.
