Abstract

The effects of tube bundle straightening vane location on orifice meter C and axial velocity profile have been investigated for nitrogen flow at a Reynolds number of 9*10 in a long (45 D), 4 inch diameter (100 mm) meter tube downstream of a 90 deg. elbow. A 19 tube bundle straightening vane was modified to allow changing the vane to orifice distance. (x/D), without reassembling the meter tube. Results are presented for C variation for B=0.75 and 0.40, and for axial velocity profile as a function of x/D from 5 to 42.5.

Introduction

The Buckingham and Stolz orifice coefficient equations, as well as the new orifice equation were derived from orifice metering experiments with long straight lengths of meter tube upstream and downstream of the orifice. When long upstream lengths are used, it is usually assumed that the axial velocity profile upstream of the orifice plate is "fully developed" and the profile upstream of the orifice plate is "fully developed" and the flow is swirl-free. The fully developed velocity profile assumption implies that the measured value of C would not change if the meter tube was lengthened further. For fully developed turbulent pipe flow, the velocity profile is axisymmetric and the profile shape is a function of only the meter tube Reynolds number and the pipe wall roughness.

However, standard pipe fittings such as fees, elbows, and valves upstream of the meter tube can produce axial velocity profile distortion, increased turbulence levels and a significant swirl velocity component at the orifice location. Depending upon the orifice diameter ratio, B, and the type and amount of velocity profile distortion, the value of C may also be affected profile distortion, the value of C may also be affected Evidently, the C value can also be changed simply by flowing through a flow conditioner even when long upstream lengths of pipe are used. pipe are used. The work described in this paper is part of a research program to minimize flow meter installation effects that can affect adversely the accuracy of orifice and turbine gas flow metering installations. To facilitate testing, a "sliding vane" technique was developed to change the location of a standard 19 tube bundle rapidly during a test. This technique makes it possible to measure the effects of several different tube bundle locations during a single test, from as close as x/D=5 to as far as permitted by the upstream fitting (elbow, tee or valve).

Recently, axial gas flow velocity profiles were measured in the meter tube using a hot-film anemometer. This paper presents the results obtained by traversing a hot-film probe in the vertical and horizontal directions through the upstream orifice flange taps for a 45 D long meter tube downstream of a 90 deg. elbow during steady flow at a Reynolds number of approximately 9*10.

TESTS AND ANALYSIS
MRF Low Pressure Loop

All tests were performed in the Low Pressure Loop (LPL) of the GRI Meter Research Facility (MRF) shown in Figure 1. The test section is comprised of a 100 mm diameter three segment meter tube with orifice flanges and flange taps. The two upstream tube segments are 19.5 D and 25.5 D long, and the downstream tube segment is 5 D long. The meter tube average surface roughness, Ra, (for a 2.5mm cutoff) measured upstream of the orifice flange is approximately 110 uinch (2.8 um).

For "baseline" C calibration tests both upstream tube segments are used giving a total length of 45 D. The meter tube is attached to a 500 mm diameter stagnation bottle, either directly (as indicated in Figure 1) or with an oversized 6 inch (150 mm) diameter Sprenkle flow conditioner inserted between the meter tube and the stagnation bottle. For this paper, only results obtained with the meter tube attached directly to the stagnation bottle are presented Gas flow enters the stagnation bottle through an inlet located 12 inches (300 mm) above and at a 90 deg. angle to the outlet leading to the meter tube.

Sliding Vane Apparatus

For the sliding vane tests, the stagnation bottle was-replaced by a 90 deg long radius elbow, modified to accept the sliding vane apparatus. Figure 2 is a sketch of the sliding vane apparatus. The meter tube is connected to the downstream end of either a 90 deg. elbow or a tee (as shown in the figure). While data has been taken for both an upstream elbow and an upstream tee, this paper presents only the results for the upstream elbow. presents only the results for the upstream elbow. A standard "in-line" 19 tube bundle straightening vane was modified to allow it to slide freely through the entire length of ice meter tube. The location of the tube bundle in the meter tube is adjusted by a 1/8 inch (3 mm) diameter wire cable and a 5/8 inch (16 mm) steel rod that pass through the upstream 90 deg. elbow. The rod is used to position the tube bundle in the meter tube and is calibrated to indicate the distance from the downstream end of the tube bundle to the orifice.

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