VORTEX SHEDDING METER

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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. 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.

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.

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. 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.

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.

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.

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.

FIGURE 6-6. Vortex Shedding Meter

Fixed insertion meters are used on systems where the pressure can be relieved during installation and removal of the meter. Hot tap insertion meters are designed for use where system monitoring is necessary without interrupting line pressure. The meter is inserted through a full-port ball or gate valve. Hot tap meters require probe length clearance above the mounting flange and valve to allow full retraction and removal of the meter.

RECOMMENDED APPLICATIONS: Vortex shedding meters are designed for use on clean gas and liquid systems, but they are commonly used on dirty gases and dirty or corrosive liquids which decrease meter life expectancy.

LIMITATIONS. The recommended applications for vortex shedding meters require the consideration of the following limitations:

• Inaccurate at low fluid velocities compared to turbine meters.
• Space availability above the flange for insertion meters.
• Alignment and location are critical.
• Working pressure limit of 1,500 psi.
• Common temperature limit of 560°F.
• Pipe size of 1- to 8-inch diameter.

INSTALLATION: The location of a vortex shedding meter in a system is important. Whenever possible, it is preferable to locate the primary element in a horizontal line. To ensure accurate flow measurement, fluid must enter the primary element with a fully-developed velocity profile, free from swirls or vortices. Such a condition is best achieved by use of adequate lengths of straight pipe, both preceding and following the primary element. Straightening vanes can preclude the need for long lengths of straight pipe. The minimum recommended lengths of piping are shown in Figure 6-7. The configuration in Figure 6-7 that corresponds closest to the actual piping arrangement for the meter location should be used to determine required lengths of straight pipe on the inlet and outlet. These lengths are those necessary to limit errors due to piping configurations to less than ±0.5%. If these minimum distances are not observed, the flow equations and resultant flow calculations may result In inaccurate data. For specific applications, refer to manufacturer’s installation criteria,

ACCURACY AND RELIABILITY: Vortex shedding meters have an accuracy of ±1.0% when calibrated, with a turndown capability up to 10:1 dependent upon pipe size and fluid properties. The lack of moving parts results in reduced maintenance and makes this type of measuring device a very reliable means of flow measurement.

FIGURE 6-7. Minimum Straight Length Piping for Vortex Shedding Meters

MAINTENANCE: At least once a year vortex shedding meters should be Inspected for possible damage from dirty or viscous fluids. Check upstream filters and replace if necessary. Devices for sensing and transmitting pressure and temperature values to the flow processor should be inspected and calibrated at least once per year.

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