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Variable Area Flow meter

Hot:10302013-11-21 12:40:33 From: FTD Automation Instruments Limited - Field Instruments

Variable Area Flow meter

The variable area flowmeter ,often referred to as a rotameter, consists of a vertical, tapered bore tube with the small bore at the lower end, and a float that is allowed to freely move in the fluid. When fluid is passing through the tube, the float's position is in equilibrium with:

  • The dynamic upward force of the fluid.
  • The downward force resulting from the mass of the float.
  • The position of the float, therefore, is an indication of the flowrate.

    In practice, this type of flowmeter will be a mix of:
  • A float selected to provide a certain weight, and chemical resistance to the fluid.

    The most common float material is grade 316 stainless steel, however, other materials such as Hastalloy C, aluminium or PVC are used for specific applications.

    On small flowmeters, the float is simply a ball, but on larger flowmeters special shaped floats are used to improve stability.
  • A tapered tube, which will provide a measuring scale of typically between 40 mm and 250 mm over the design flow range.

    Usually the tube will be made from glass or plastic. However, if failure of the tube could present a hazard, then either a protective shroud may be fitted around the glass, or a metal tube may be used.

    With a transparent tube, flow readings are taken by observation of the float against a scale. For higher temperature applications where the tube material is opaque, a magnetic device is used to indicate the position of the float.

    Because the annular area around the float increases with flow, the differential pressure remains almost constant.
Fig. 4.3.7 - Variable area flowmeter  
Variable area flowmeter

Advantages of variable area flowmeters:

  • Linear output.
  • Turndown is approximately 10:1.
  • Simple and robust.
  • Pressure drop is minimal and fairly constant.

Disadvantages of variable area flowmeters:

  • The tube must be mounted vertically (see Figure 4.3.8).
  • Because readings are usually taken visually, and the float tends to move about, accuracy is only moderate. This is made worst by parallax error at higher flowrates, because the float is some distance away from the scale.
  • Transparent taper tubes limit pressure and temperature.

Typical applications for variable area flowmeters:

  • Metering of gases.
  • Small bore airflow metering - In these applications, the tube is manufactured from glass, with calibrations marked on the outside. Readings are taken visually.
  • Laboratory applications.
  • Rotameters are sometimes used as a flow indicating device rather than a flow measuring device.

 

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