Water Well Journal
Issue link: http://read.dmtmag.com/i/643500
Although propeller meters are versa- tile and can be placed in numerous ap- plications, they are also quite limited in accuracy when placed in undesirable locations or applications. They are especially prone to inaccuracy when the proper upstream and downstream straight run of pipe diameter distances are not provided. This is particularly a sensitive problem with cable-driven me- ters. This is due to the flow disruption and disturbance caused by the possible changes of flow rotation as the water approaches the propeller element. This occurrence is most pronounced when pumps, ells, valves, or other similar flow-disturbing elements are placed too close to the inlet of the flowmeter. Most manufacturers of propeller flowmeters recommend a straight and unimpeded distance of no less than five pipe diameters in front of the meter and at least two pipe diameters downstream of the meter be placed. When using a flanged meter, this is generally provided as well as straightening vanes. In addition to the register head usu- ally placed directly on the top of the meter, various electronic methods are usually available to also transmit the in- stantaneous flow rate and totalized flow to SCADA devices and remote loca- tions. End connections run the gamut and include threaded pipe for sizes of 2–4 inches, flanged or Victaulic connec- tions for sizes of 2–48 inches, and plain end for all sizes. This type of meter is also available with saddle and weld-on connections to enable fitting inside most pipe types and sizes. Spinning from the propeller is generally transmitted to the register head through one of two meth- ods: a cable drive or a right angle gear. Each method has inherent advantages and disadvantages. However, the cable drive enables replacement of the cable from the top of the meter without re- moving from service. A primary disad- vantage of propeller flowmeters is their inaccuracy of registering well below a specific low flow value. This value varies between sizes, but a good rule of thumb is not to allow the flow velocity to fall below 2 feet per second. Annubars The last method of flow measure- ment is by definition not a mechanical type of meter. But the principle follows a basic hydraulic principle, it's been around for decades, and is still used in many applications—particularly those for process and wastewater. A primary advantage to the annubar method over other methods is the lack of any interfering elements into the flow stream. This is particularly important when the flow stream might carry suspended solids that could impinge on the interference. The annubar method works on the principle of differential pressure occur- ring in a flowing pipeline between two points. It is comparable to the pitot tube on an airplane measuring the speed of the plane by measuring the airflow pass- ing over a pitot port. As a liquid passes by a port or slit, it creates a pressure in the port as the liq- uid then travels downstream of the port and the resulting lower pressure is inter- preted as a differential pressure. This difference, when factored for the pipe diameter, is then evaluated in velocity or the flow rate. This principle is also used on some of the electronic flow measur- ing devices we will discuss next month. Earlier methods of annubars used a graduated scale to read this differential pressure that was then related to the flow rate, while differential pressure switches were often used to control pumps or other devices. New methods of annubars use electronic means to factor these values and provide a flow readout in the values desired. Annubars are available in wafer styles to fit between pipe flanges as well as direct insertion into or on the top of pipelines through pre-drilled ports. An example of a typical pipeline mounting using a wafer annubar assembly be- tween flanges is shown in Figure 4. Another advantage of annubars is their ability to work on various fluids as well as other liquids such as steam and gas. Although they are not frequently used today, partially due to the recent influx of electronic flow measurement devices, the annubar method of flow determination remains an accurate and reliable method of flow measurement and should be considered as a viable instrument in the proper application. We will conclude this topic next month with an overview on the electro- mechanical and ultrasonic types of flowmeters. Until then, work safe and smart. WWJ Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 35 years experience in the water well business, specializing in engineer- ing and business management. He can be reached at epbpe@juno.com. 60 March 2016 WWJ DACUM Codes To help meet your professional needs, this article covers skills and competen- cies found in DACUM charts for drillers and pump installers. PI refers to the pumps chart. The letter and number immediately following is the skill on the chart covered by the article. This article covers: PIA-4, PIC-9, PIE-18, PIF-4 More information on DACUM and the charts are available at www.NGWA.org. Figure 4. Pipeline mounting using a wafer annubar assembly. Learn How to Engineer Success for Your Business Engineering Your Business: A series of articles serving as a guide to the ground-water business is a compilation of works from long time Water Well Journal columnist Ed Butts, PE, CPI. The hardback book features 37 complete "Engineering Your Business" columns and the figures and tables that originally ran with the articles. Visit NGWA's Online Bookstore at www.NGWA.org for more information. waterwelljournal.com ENGINEERING from page 59