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 Section 2.
VORTEX SHEDDING METER
1. INTRODUCTION. A vortex shedding meter consists of a bluff body to develop
vortices, and an electronic sensing device to monitor the number and rate at
which vortices are shed. Vortex shedding meters have a wide turndown ratio
and temperature range. This type meter is commonly used to measure liquids,
gases, and steam. Vortex shedding meters are available in full-bore or
insertion models.
1.1 Operating Principles. Vortex shedding is the natural effect that occurs
when a gas or liquid flows around an obstruction or bluff body (Figure 6-6).
When flow encounters a bluff body on its downstream course, it separates from
the surface of the bluff body, leaving a highly turbulent wake that takes the
form of a vortex. Each vortex grows and then becomes detached or shed from
the bluff body. These shed vortices travel downstream in a fixed, predictable
pattern. The number of vortices shed from the strut per unit time is
proportional to fluid flow rate. This vortex frequency is detected with a
sensor, which transmits a signal to a totalizer or other metering device.
2. METER DESIGNS. All vortex shedding meter designs consist of two main
components, the bluff body and the sensing device. There are many different
bluff body configurations. In some instances multiple struts are incorporated
into the design.
2.1 Bluff Bodies. Figure 6-6 illustrates three different strut
configurations. Though the shape differs, actual dimensions of the bluff body
are determined by the relationship between the diameter of the pipe, the
viscosity of the fluid, and the flow rate. The strut must have nonstreamlined
edges so that vortex formation can occur.
2.2 Sensors. There are four types of sensors commonly used to detect
vortices developed by the bluff body and shed into the downstream flow:
strain gauge, magnetic pickup, ultrasonic detector , and piezoelectric element.
3. VORTEX SHEDDING METER CONFIGURATION. There are two vortex shedding meter
configurations, full-bore and insertion. The use of one or the other is
dependent on whether the metering is to be permanent or periodic and whether
or not the pipeline can be shut down for installation.
3.1 Full-Bore Meters. The full-bore vortex meter , which is the same diameter
as the pipelines is permanently mounted between pipeline flanges. If
permanent metering is planned at the time of initial pipeline construction or
during pipeline retrofit, the full-bore meter is usually installed. The
full-bore meter has fittings for signal amplification and transmission to
recorders and dataloggers.
3.2 Insertion Meters. Insertion meters are small, vortex-shedding devices
designed to be inserted through fittings and valves permanently affixed to the
system. They are used for obtaining either long term or periodic flow
information. There are two types of insertion meters, fixed and hot tap.
6-10
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