Cheng Fluid Systems, Inc.
United States
ph: +1(650) 559-9292
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Chemical Engineering: Improve Flowmeter Accuracy in Cramped Quarters.
Obtaining accurate pipeline flow measurement requires that the flow directly before the meter be uniform, streamlined, and stable. In most pipeline applications, meter piping configurations are cramped and do not allow enough room for the required straight length of pipe, to properly install a meter and have it measure accurately. The problem of not having enough space to correctly install a meter close to an elbow, can now be solved by using a CRV®. The CRV® is positioned at the inlet side of an elbow, and the CRV® will produce a slight swirl to the fluid (liquid or gas) keeping the flow uniform as it makes the 90 degree turn through the elbow and exits. All of the elbow induced flow turbulence, vibration, separation, and noise, that is also accompanied with reverse and accelerated flows, will be eliminated. Any type of meter can now be placed as close as one pipe diameter away from the outlet of an elbow, and the meter will measure accurately, being assured of a uniform flow of fluid.
The National Institute of Standards and Technology (NIST), the U.S. Army CECOM-South and Cheng Fluid Systems (CFS) have completed a Cooperative Research and Development Agreement (CRDA) to establish the effects of the CRV® on the flow in an elbow for flow metering applications.
The results on the single long radius elbow showed that at ONLY 2 diameters from the exit plane and on the centerline, velocities were within 5% of the slug flow value, in both vertical and horizontal planes, were for the CRV®-RCRV® (Reverse CRV®) elbow. The unabated elbow velocities in the vertical plane were at least 15% of the slug flow value at the centerline. This result has special significance for accuracy improvement in systems with hotwire anemometers & other centerline calibrated flowmeters without long meter-runs (30 diameters). At the other positions away from the exit plane, the unabated elbow had a pair of strong counter-rotating vortices which were eliminated by the CRV®-RCRV® elbow.
This, again, will improve accuracy in orifice plate type flow meters. Overall turbulence was reduced by about one-third and approached the Laufer-line values (a theoretical projected best value for a smooth pipe) at between 10 to 20 diameters and were below it thereafter. The unabated elbow turbulence values were still above the Laufer-line at over 20 diameters. The data, as compared with the traditional tube bundle, essentially shows that the effectiveness has been improved by an order of magnitude. These tests took place on a flow of Re=100,000. This result will be extremely useful for flow meters mounted less than 10 diameters away from an elbow.
Additional test were performed for CRV® design in close-coupled, out-of plane and long radius elbows.
Flow Meter Problems: | CRV® Benefits: |
Inaccurate Flow Measurements | Accurate Flow Measurements |
Long Straight Pipe Meter Runs Required | More Compact Pipe Layout |
Lack of Space for Proper Installation | Reduced Piping Run & Costs |
Flow Metering devices cannot provide accurate measurement of the flow rate through a pipe when flow entering the measurement device is distorted. Consequently, flow meter manufactures and a number of independent organizations such as ASME, AGA and ANSI/API recommend that flow meters not be installed near and downstream of elbows.
With the CRV® one can reduce the straight pipe run length for an orifice plate flow meter preceded by elbows as seen below.
For example, when an orifice meter with a Beta ration of 0.7 is used and the flow meter is preceded by two elbows in one plane, figure a shows that the American Gas Association (AGA) recommends a minimum of 19 pipe diameters of straight pipe be used between the last elbow and the flow meter. Conventional after-elbow-straightening vanes figure b, only decrease the recommended straight pipe length between the last elbow and the flow meter to 12 pipe diameters. However, the use of CRV® upstream of the elbow in figure c, decreases the recommended straight pipe length between the last elbow and the flow meter to 2 diameters.
In places where less than the recommended straight pipe meter run exists, installation of a CRV® will result in more accurate measurements. For example, dramatic improvements were witnessed in a 10”-diameter flow meter application where the installation of a CRV® decreased the meter error from 30% to less than 5%.
In new Installations, the CRV® allows the close coupling of flow meter to elbows, which results in space savings and reducing straight-run piping costs.
EXAMPLE INSTALLATIONS:
REFINERY AIR FLOW MEASUREMENT
Several 18” diameter CRV® units are installed in a refinery ahead of the flow metering instruments. This type of installation resulted in the saving of 25 diameters of straight pipe run, or 38 feet of 18” pipe. In plants where only a short meter run was provided, installation of CRV® flow conditioner will result in more accurate measurements.
CHEMICAL PLANT AIR FLOW MEASUREMENT
A southwest chemical plant had a flow meter located in an 18" header. Two separate 10" pipes were attached to the header. Flow meters were mounted in each of the 10" pipes about 6 diameters downstream of the elbows. The fluid was air and the meters were hot wire anemometers. Comparing the flow rates as measured by the upstream meter with the sum of the readings of the two downstream meters resulted in an indicated difference of about 30% in total flow. A CRV® was installed between the two downstream meters and elbows. The subsequent comparison of total flow were within 5% of each other.
Our flow conditioners are custom made to specifically address each piping issue offering the greatest achievable flow correction.
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Cheng Fluid Systems, Inc.
United States
ph: +1(650) 559-9292
sales