Q: I have a question about the selecting a flowmeter. I’m an irrigation student, and I want to select a flowmeter for an irrigation water pipe. Could you explain to me the exact meaning of "dirty" liquid?
Please refer me to some sources about irrigation water measurement, particularly covering when irrigation water has air bubbles or reverse flow, or if the pipe is made of plastic materials.
F. Nayebloie
[email protected]
A1: Cost is usually an overriding concern in irrigation applications. The cash value of irrigation water is usually too low to justify spending extra money to reduce flow data uncertainty. Properly installed impulse lines make simple or average pitot tubes practical. They’re widely used and less expensive than magnetic flowmeters. The installation should clear out any plugging without shutting down the system. Also, it’s common to install pitot tubes with valves and packing to allow removal and replacement while the pipeline is operating.
In a few applications, we’ve specified a small flow of filtered water to purge impulse lines, where the flowing fluid is very dirty. Instrumentation selection should be based on the value of the data as increased accuracy is often more expensive than justified. Simplicity and robustness of the system are valuable even if it’s difficult to put a definite monetary value on them.
Cullen Langford
process control consultant
[email protected]
A2: Irrigation has always been an important part of water use in agriculture. Today, as climate change causes drought in some part of the world, measuring water flow and distribution among users is gaining importance. My handbook describes all flowmeters in detail, but for this answer I’ll focus on those commonly used in water distribution, irrigation and wastewater measurement. Some of these flowmeters can detect the flow in pipelines, while others only operate in open channels.
In pipeline applications, it’s important that the pipe be full and clean on the inside. Meters can be located at low points if the water is clean or there are vertical up-flow locations with straight runs, as specified. It's also desirable for the sensor to have no moving parts and have smooth inner surfaces that are free of restrictions.
Magnetic, ultrasonic, propeller, venturi tube and orifice meters are all used in irrigation applications, though orifices are used only on clean-water applications. They can be installed in metal or plastic pipes (except for clamp-on ultrasonic units). Except for some propeller type designs, they can’t measure reverse flow and because they’re volumetric flowmeters, and the presence of air bubbles will add to the passing volume of water. Therefore, these meters will read high. If only a local readout of totalized flow is required, you can use an inline venturi tube with a small and inexpensive totalizing displacement flowmeter installed in a bypass line between its high- and low-pressure taps. If properly calibrated, this setup can display totalized flow without requiring an electric power supply. In remote areas without a power grid, battery-stored, solar electricity is often used as the energy source for wireless data transmission.
In Figure 1, the trajectory measurement method (1A) is the least expensive. It’s also the least accurate. It doesn’t require a flowmeter; it only requires a full pipeline and the measurement of the distances A, B and D. The formulas for converting these measurements into flows in gallons per minute (gpm) can be found at https://extension.okstate.edu/fact-sheets/irrigation-water-flow-measurement.html
Pitot tubes (1B) can only be used for clean water. They’re low cost and suitable for any pipeline size above 2 inches. In addition, they only require low, permanent pressure drop, and can be inserted while the system is under pressure. Their disadvantages include low accuracy (usually under 2% full scale) and low rangeability (3:1). In addition, they’re only suitable for high-velocity flows (Reynolds number above 20,000) and can require frequent cleaning or purging.
Propeller meters (1C) are second in popularity to magnetic flowmeters in irrigation applications. They’re available in full-bore and insertion-style models for up to 24-inch pipes. Insertion-style units are ideal for larger lines. They cost less than magmeters but have moving parts, so they’re subject to degradation with time, and may need bearing replacements. Propeller flowmeters are less accurate at low flow rates (5-10% of full flow) and shouldn’t be operated at velocities exceeding the specified velocities because premature bearing wear can occur. Their blade movements are usually detected magnetically, with each blade generating a pulse that’s compatible with EtherMeter and most AMR systems.
Measurement of larger open flows usually involves either the measurement of velocity or level. You can design your own flow measurement by selecting an approximately 50-foot long, straight and constant cross ditch, and measure the time it takes for a float to travel over to it (2A in Figure 2). Because water travels faster on the surface, the actual flow rate is usually about 60% of the measured. While this method is rather inaccurate, it’s free and reasonably repeatable.
Also in Figure 2, weirs (2B) can be installed in open channels to measure flows without upper limits. Their limitations include low accuracy (10%) and the requirement for considerable difference in head between the upstream and downstream water surfaces. Such a requirement is often either unavailable in flat-grade ditches or undesirable. Also, it’s frequently necessary to construct a pool or stilling area above the weir to slow water velocity. Weir installations in earthen ditches can be particularly troublesome because the stilling area in the ditch above the weir frequently “silts in,” while excessive erosion may also occur downstream of the weir. For rectangular weirs. flow is calculated by:
Q = 3.33 (L-0.2H)H1.5
Where:
Q = flow in cubic feet per second
L = crest length, which is the length in feet of the bottom of the rectangular opening
H = head, which is the difference in feet between the upstream level and the bottom of a rectangular weir
Parshall flumes (2C) operate with less difference in elevation between upstream and downstream water surfaces than weirs, and don’t require large upstream stilling basins. Also, their higher flow velocity tends to flush away silt deposits. Similar to weirs, measuring upstream level indicates flow, which is proportional to H1.6, with H being level. Rangeability is high at a throat section of 8 feet, and they can measure flow rates from, 2,000-62,000 gpm. In addition, they can be made of plastic or concrete.
Béla Lipták
[email protected]
