| From: McCrometer News <news@mccrometer.com> Subject: McCrometer Flow Measurement News Vol. 2 No. 07 From: McCrometer, "The Flow Measurement Specialists" ........ http://www.mccrometer.com <http://www.mccrometer.com> ........ ******************************************* This newsletter is e-mailed by McCrometer monthly to subscribers worldwide. It consists of flow measurement information of interest for professionals in this field. For More information on our complete line of flowmeters go to: http://www.mccrometer.com ******************************************* Ask The Experts... Turbulence Revisited Last month, we looked at the role of turbulence in flow measurement, focusing in particular on the requirement for straight pipe runs. As a number of our readers pointed out, the problem caused by short pipe runs is better described by the term "irregular flow profile" rather then "turbulent flow". To clarify this point, we will explain here the difference between the two terms and where they fit in making accurate flow measurements. Strictly speaking, "turbulent" flow is necessary in order to have a good flow measurement. It is normally defined as a flow with a Reynolds Number above 4000, characterized by eddies and swirls in the flow stream. A Reynolds Number of 2000 or less describes "laminar flow", when the flow moves in a nice linear stream parallel to the pipe wall, while flows with Reynold Numbers between 2000 and 4000 are described as transitional flows. The problem for flowmeters comes therefore not from turbulence but rather from irregular flow profiles. A "regular" flow velocity profile is one where the flow in the center of the pipe is moving slightly faster relative to the rest of the flow, while flow at the pipe walls is moving slightly slower relative to the rest of the flow. If the flow is measured where the velocity is faster, the meter will over-indicate. On the other hand, if it is measured where the velocity is slower, the meter will under-indicate. This phenomenon is compensated for in most meters. However, a problem occurs when a disturbance upstream of the meter puts the faster portion of the flow in a different place than where the meter expects it. Obstructions such as pipe elbows and valves tend to cause this "irregular" or asymmetric velocity profile, and lead to inaccurate meter readings. Over a long straight pipe run, this irregularity tends to dissipate. Flow conditioners can help by flattening the velocity profile so that it is the same across the entire cross-section of pipe. But flow conditioners are expensive, both in terms of cost and the energy from the flow they consume. This is where the V-Cone flowmeter comes in. It is unique in acting as its own flow conditioner, creating a flatter velocity profile and therefore doing away with the need for long straight runs to measure flow effectively. To find out more about the V-Cone flowmeter or other products in McCrometer's line of flowmeters, please contact your nearest McCrometer flowmeter expert at 951-652-6811 or visit our web-site www.mccrometer.com <http://www.mccrometer.com> . ******************************************* Distributed by McCrometer "The Flow Measurement Specialists" |