Technical Data Sheet 00816-0100-3250, Rev GD October 2015 Rosemount 8800 Vortex Installation Effects 2 Technical Data Sheet 00816-0100-3250, Rev GD Vortex Flowmeter October 2015 Flow 1.1Introduction The Rosemount 8800 Vortex Meter provides methods for maintaining accuracy in less than ideal installations. In designing the 8800, Rosemount tested the meter for three separate types of installation effects: Process fluid temperature variation Process piping inside diameter Upstream and downstream disturbances As a result of this testing, compensation factors are included in the vortex meter software; this allows the output of the vortex meter to be adjusted for the actual process temperature and process piping being used. Data is presented in this paper to demonstrate the effectiveness of the design in limiting the errors resulting from piping disturbances. For upstream disturbances caused by pipe elbows, contractions, expansions, etc., Rosemount has conducted extensive research in a flow lab to determine the effect that these have on the meter output. These tests are the basis for the recommended 35 upstream piping diameters. While this is optimal, it is not always possible in the real world of plant design and layout. Therefore, the data presented in this paper outlines the effects of different upstream and downstream piping conditions on the vortex meter. 1.1.1Temperature effects on K-factor The vortex meter is fundamentally a velocity measuring device. As fluid flows past the shedder bar, vortices are shed in direct proportion to the fluid velocity. If the process temperature is different than the reference calibration temperature, the meter bore diameter will change slightly. As a result, the velocity across the shedder bar will also change slightly. For example; an elevated process temperature will cause an increase in the bore diameter, which in turn will cause a decrease in the velocity across the shedder bar. Using the Reference K-factor and the value for Process Temperature input by the user, the Rosemount 8800 automatically calculates for the effect of temperature on the meter by creating what is called the Compensated K-factor. The Compensated K-factor is then used as the basis for all flow calculations. 1.1.2Pipe ID effects on K-factor All Rosemount 8800 Vortex Meters are calibrated in schedule 40 pipe. From extensive testing done in piping with different inside diameters/schedules, Rosemount has observed there is a small K-factor shift for changes in process pipe ID (inside diameter). This is due to the slight change in velocity at the inlet to the meter. These changes have been programmed in to the 8800 electronics and will be corrected for automatically when the user supplied pipe ID is other than schedule 40. 3 Technical Data Sheet 00816-0100-3250, Rev GD Vortex Flowmeter October 2015 Flow 1.1.3Upstream and downstream piping configurations The number of possible upstream and downstream piping configurations is infinite. Therefore, it is not possible to have software automatically calculate a correction factor for changes in upstream piping. Fortunately, in almost all cases, elbows, reducers, etc. cause less than a 0.5% shift in the meter output. In many cases, this small effect is not a large enough shift to cause the reading to be outside of the accuracy specification of the meter. The shifts caused by upstream piping configurations are basically due to the changes in the inlet velocity profile caused by upstream disturbances. For example, as a fluid flows around an elbow, a swirl component is added to the flow. Because the factory calibration is done in a fully-devel- oped pipe flow, the swirl component caused by the elbow will cause a shift in the vortex meter output. Given a long enough distance between an elbow and the meter, the viscous forces in the fluid will overcome the inertia of the swirl and cause the velocity profile to become fully-devel- oped. There rarely is sufficient length in actual process piping installations for this to occur. Even though the flow profile may not be fully-developed, testing indicates that the Rosemount vortex meter can be located within 35 pipe diameters of the elbow with minimal effect on the accuracy or repeatability of the meter. Although the upstream disturbance may cause a shift in the K-factor, the repeatability of the vortex meter is normally not affected. For example, a meter 20 pipe diameters downstream of a double elbow will be as repeatable as a meter in a straight pipe. Testing also indicates that while the K-factor is affected by upstream piping, the linearity of the meter remains within design specifications. In many applications, this means that no adjustment for piping configuration will be necessary even when the minimum recommended installation lengths of upstream and downstream piping cannot be used. On the following pages are drawings illustrating various installation configurations. Extensive testing has been performed in a flow