Pakarab fertilizers limited multan
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| Vortex shedding meter The phenomenon referred to as vortex shedding is the basis for the vortex- shedding meter. When flowing fluid passes an un-streamlined body (referred to as a bluff body or strut) the flow cannot follow the sharp contours of the obstruction. The fluid separates into layers and rolls around the bluff body. The rolling action creates vortices that form on the sides of the body and move downstream. The formation of vortices by the bluff body is a naturally occurring phenomenon caused by the shape of the obstruction in the flow path. The vortices alternately spin clockwise and then counterclockwise. This is the natural way vortices form and is the basis for the meter’s operation. As a vortex forms on one side of the body, a low-pressure area is created. At the same time, the effect of the spinning fluid behind the obstruction starts a vortex on the opposite side. When the vortex on top of the bluff body breaks away from the body, the beginning of anew vortex forms on the bottom. In turn, it also sheds and is followed by another vortex on top. 73 Field Instrumentation GEN Rev) It is the alternate shedding of vortices from one side of the bluff body to the other that forms the basis of the meter’s operation. Remember that pressure decreases when a vortex is formed. When the vortex is shed, pressure increases until the next vortex forms, at which time pressure again decreases. On the opposite side of the bluff body, pressure also increases and decreases due to vortex formation and shedding. The result is a measurable increase and decrease of pressure across the bluff body. One kind of vortex-shedding meter is designed with sensors on opposite sides of the bluff body to detect this change in pressure The vortex-shedding frequency (the time between formation of vortices) is a function of flow velocity. Suppose fluid flows through the meter at a velocity of 5 ft/s and a vortex forms and sheds 100 times each second (shedding frequency is 100 Hz. The sensors mounted on each side of the bluff body detect the change in pressure that accompanies the shedding of a vortex. The output of the pressure sensors is a noisy sine wave. Its frequency is identical to the vortex-shedding frequency 74 Field Instrumentation GEN Rev) Some kind of vortex-shedding flow meters use a piezoelectric element that detects small movements of the strut as vortices are shed. The ultrasonic, thermal, and piezoelectric flow meters use a preamplifier to send a signal to a remote converter. Others include a converter mounted with the flow meter. Download 3.37 Mb. Share with your friends:
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