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Orifice Meter Objectives: Flow Profiles

The main objective when using an orifice type meter is to maintain the correct flow profile. Initially, the original coefficients of discharge were determined under laboratory-specific flow conditions. Any altered condition that changes the flowing profile from the original profile must not be allowed or error will result. In the diagram below three profiles are displayed:

bulletThe center (red) profile represents the laboratory condition where virtually no error occurs. Manufacturers of orifice meter tubes must follow strict guidelines established by the AGA, ISO and other organizations. Some states ratify these standards which means that gas or liquid contracts within those states are governed by the state laws whether or not these are referenced in the contract.
bulletThe left (blue) profile indicates gas steamlines moving much faster toward the center of the flow stream than at the outer edge. This causes gas to rapidly move through the orifice creating less differential pressure than the laboratory condition resulting in an understatement of gas or liquid flow.

There are many causes for this problem. Contaminates of most any kind can create drag on the surface of the pipe wall, much like debris in the edge of a stream of water. Oil, water, emulsion, dirt, iron sulfide, salt, sand, paraffin, rust or pipe scale, large amounts of glycol are often found in gas flow streams.

bulletThe right (yellow) profile is very blunt, indicating stream lines flowing at the outer edge of the flow stream are traveling almost as fast as the center streamlines. This condition causes excess differential pressure resulting in an overstatement of gas or liquid flow.

Causes of blunt profile: For gas flow, very thin layers of liquid such as glycol, coat the inside of the pipe wall creating an ultra smooth surface. If the fluid is thick enough to cause ripples as the gas passes by then the effect is canceled to some degree.



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