Rotameters For Flow Measurement - Selecting the Right One

Industrial Rotameter Flow Meters
Courtesy King Instrument

Applied extensively in industrial process measurement and control, a rotameter is an instrument that uses a float of given density to establish, for any measurable flow rate, an equilibrium position within the fluid stream where the force of the flowing fluid equals the force of gravity. Let's break that down a little. A rotameter has a tapered tube with a float inside. As the measured fluid flows upward through the tube, it pushes the float upward along the length of the tube. As the float rises in the tube, the cross sectional area of the tube increases and more fluid can bypass around the float. At some point, the upward force of the fluid flow acting on the float will balance with the downward force of gravity. The position of the float along the length of the tube correlates with a certain flow rate when certain properties of the fluid are known. Flow rate scale graduations on the tube can be read by the operator.

Rotameters are very specific to each flow measurement application. It's important that you know your fluid properties, ambient conditions, connection and readability specifications. 

Start with these selection parameters:

• Desired flow rate range
• Fluid specific gravity
• Ambient temperature
• Operating and maximum pressure
• Line size
• Connection type
• Connection orientation
• With or without a valve
• Material requirements to accommodate fluid
• Scale units of measure. Smallest scale divisions needed.

For each application, it's advisable to work closely with a sales engineer to gather all the needed information and coordinate the product selection process.

Here are some things to consider for potential rotameter applications:

• Simple design and operation provide a modest cost solution.
• No external power is required for operation. Inherent fluid properties and gravity are used to measure flow rate.
• Clear glass used for the measuring tube is highly resistant to thermal shock and corrosion.
• Instrument orientation must be vertical, with fluid flowing upward.
• Scale graduations are accurate for a given substance at a given temperature, making the devices application specific.
• Operation of the rotameter may be impacted by changes in the viscosity of the fluid. Consult with a product and application specialist to explore your application.
• Direct flow indication provides resolution that may not be as good as some other flow measurement methods.
• Visual reading of the scale is subject to uncertainty due to float oscillation, parallax, and location on the scale.
• Make sure the fluid turbidity, or another fluid characteristic will not obscure the visibility of the float.

Consult with a product specialist about your flow measurement application. A combination of your process knowledge and their product expertise will produce the best solution.

Introduction to Rotameters

Rotameter
(courtesy of
King Instrument)

A rotameter is one particular type of variable area flowmeter that measures flow by varying the cross-sectional area a fluid or gas travels through in a closed tube.

Advantages

• Requires no external power.
• Is a simple device that can be easily manufactured out of inexpensive materials.
• Linear scale.
• The clear glass tube is resistant to thermal shock and chemical reaction.

Disadvantages

• Must be mounted vertically, with designated top and bottom, and with the fluid flowing from bottom to top.
• Graduations on a given rotameter are only accurate for a given substance at a given temperature. Separate rotameters must be used for fluids with different densities and viscosities, or multiple scales on the same rotameter must be used.
• Resolution is relatively poor and gets worse near the bottom of the scale.
• Oscillations of the float and parallax lend to reduced accuracy.
• Difficult to automate - primarily a manual / visual device

For more information on rotameters, contact:

M.S. Jacobs and Associates, Inc.
810 Noblestown Road
Pittsburgh, Pa 15205
Toll free: 800 348 0089
Fax: 412-279-4810
Email: msjacobs@msjacobs.com
www.msjacobs.com

Steps to Installing a Rotameter

Rotameter

A rotameter, also known as a variable area flowmeter, is a device that measures the flow rate of liquid or gas in a closed tube. It measures flow rate by allowing the cross-sectional area the fluid travels through, to vary, causing a measurable effect. They are a cost-effective flow measuring device that provides excellent repeatability, requires no external power, can be made from a wide variety of materials, and may be designed for high pressure and high temperature applications.

The following are helpful, general guidelines in the proper installation of rotameters:

1. Inspect meter for damage that may have occurred during shipping. Report any damage to the container to the freight carrier immediately. 
2. Make sure your pressure, temperature, fluid and other requirements are compatible with the meter and components (including o-rings). 
3. Select a suitable location for installation to prevent excess stress on the meter which may result from: 
• Misaligned pipe. 
• The weight of related plumbing. 
• “Water Hammer” which is most likely to occur when flow is suddenly stopped as with quick closing solenoid operated valves. (If necessary, a surge chamber should be installed. This will also be useful in pressure start-up situations.) 
• Thermal expansion of liquid in a stagnated or valve isolated system. 
• Instantaneous pressurization which will stress the meter and could result in tube failure. note: In closed thermal transfer or cooling systems, install the meter in the cool side of the line to minimize meter expansion and contraction and possible fluid leaks at the threaded connections. 
4. Handle the meter carefully during installation. 
• Use an appropriate amount of teflon tape on external pipe threads before making connections. Do not use paste or stick type thread sealing products. 
• Over tightening of plastic connections may result in fitting damage. 
5. Install the meter vertically with the inlet port at the bottom. 
6. Meters with stainless steel fittings will support several feet of pipe as long as significant vibration or stress resulting from misaligned pipe are not factors. 
7. Meters with plastic fittings must be installed so that fittings are not made to support any part of the associated plumbing. In addition, meter frame should be fastened to bulkhead, panel or column. 
8. Meters used in gas service should have suitable valves plumbed in at the inlet and outlet of the meter. These valves should be no more than 1-1/2 pipe diameters from the meter ports. The valve at the outlet should be used to create back pressure as required to prevent float bounce. It should be set initially and then left alone. The inlet valve should be used for throttling purposes. Depending on the installation, valves may not be essential, but they are most useful in many installations. Remember: To get a correct reading of flow in gas service, it is necessary to know the pressure right at the outlet of the meter (before the valve). 
9. Pressure and temperature maximums must never be exceeded.