Rotameter

Rotameter

Rotameter is simple industrial flow meter that measure the flow rate of liquid or gas in a closed tube.

Rotameters are popular because they have linear scales, a relatively large measurement range, low pressure drop, and are simple to install and maintain.

Rotameters are a subset of meters called variable area flow meters that measure the flow rate by allowing the fluid to travel through a tapered tube where the cross sectional area of the tube gradually becomes greater as the fluid travels through the tube.

The flow rate inside the rotameter is measured using a float that is lifted by the fluid flow based on the buoyancy and velocity of the fluid opposing gravity pulling the float down.  For gasses the float responds to the velocity alone, buoyancy is negligible.

The float moves up and down inside the rotameter’s tapered tube proportionally to the flow rate of the fluid.  It reaches a constant position once the fluid and gravitational forces have equalized.

Changes in the flow rate cause rotameter’s float to change position inside the tube.  Since the float position is based on gravity it is important that all rotameters be mounted vertically and oriented with the widest end of the taper at the top.

It is also important to remember that if there is no flow the float will sink to the bottom of the rotameter due to its own weight.

The operator reads the flow from a graduated scale on the side of the rotameter, which has been calibrated to a specific fluid with a known specific gravity.

Rotameters can be calibrated for other fluids by understanding the basic operating principles.  Rotameter accuracy is determined by the accuracy of the pressure, temperature, and flow control during the initial calibration.

Any change in the density and weight of the float will have impacts on the rotameter’s flow reading.  Additionally any changes that would affect the fluid such as pressure or temperature will also have an affect on the rotameter’s accuracy.  Given this, rotameters should be calibrated yearly to correct for any changes in the system that may have occurred.

There are several advantages to a rotameter over a more complicated flow meter including:

Rotameter Selection

• What are the minimum and maximum flow rate for the flow meter?
• What are the minimum and maximum process temperature?
• What is the size of the pipe?
• Would you like a direct reading rotameter or is a look-up table acceptable?
• What accuracy do you need?
• Do you require a valve to regulate the flow?
• Will there be back pressure?
• What is the maximum process pressure?

advantages

• Simple to install and maintain
• The cost of the rotameter is low.
• It provides a linear scale.
• It has good accuracy for low and medium flow rates.
• The pressure loss is nearly constant and small.
• Usability for corrosive fluid.
• Rotameters can be installed in areas with no power since they only require the properties of the fluid and gravity to measure flow, so you do not have to be concerned with ensuring that the instrument is explosion proof when installed in areas with flammable fluids or gases.
• Rotameters are simple devices that are mass manufactured out of inexpensive materials keeping investment costs low.
• Pressure loss due to the rotameter is minimal and relatively constant because the area through the tapered tube increases with flow rate.  This results in reduced pumping costs.
• The rotameter’s scale is linear because the measure of flow rate is based on area variation.  This means that the flow rate can be read with the same degree of accuracy throughout the full range.

disadvantages

• When opaque fluid is used, the float may not be visible.
• It has not well in pulsating services.
• Glass tube types subjected to breakage.
• It must be installed in the vertical position only.

Rotameter application

• The rotameter is used in process industries.
• It is used for monitoring gas and water flow in plants or labs.
• It is used for monitoring filtration loading.
• No external power required – suitable for hazardous areas and remote areas where it would be expensive to supply power.

Refrences:

• https://www.brooksinstrument.com/en/blog/why-use-a-rotameter-variable-area-meter-to-measure-flow
• https://www.polytechnichub.com/advantages-disadvantages-applications-rotameter
• http://www.globalw.com/support/rotameter.html
• https://en.wikipedia.org/wiki/Rotameter