Rotameter Flow Meter: Design, Working Principle, Applications, Advantages and Limitations

🔗Types of fluid flow meters

What is Rotameter?

In various industries and processes, the accurate measurement of the flow rate of liquids, gases, and steam is crucial. A widely used device for this purpose is the rotameter, also known as a variable-area flowmeter. This flowmeter measures the volumetric flow rate of fluid in a closed tube and operates based on the principle of differential pressure. Engineers and technicians in the chemical, petrochemical, and process control industries need to understand the basic operation and principles of the rotameter.

This article discusses the design and operating principles of the rotameter, as well as the applications and limitations of the rotameter. This will provide a comprehensive understanding of this device, which is essential for professionals in the process control industry.

Design and Operation of Rotameter

A rotameter typically consists of a tapered glass or plastic tube, a float, and an indicating scale. The tapered tube, with a wider opening at the bottom and a narrower opening at the top. The rotameter tube is installed vertically in a pipeline, with the flow entering from the bottom. The float is typically made of stainless steel, sapphire, or similar dense material and is free to move vertically within the tube in response to the flow rate. Guidewire is used for the movement of the float. Notches are provided at the float's upper periphery, so it rotates slowly and maintains a central position in the tube. The float used in the rotameter must have a higher density than the fluid being measured; otherwise, it will float to the top even if there is no flow. 

Forces Acting on Float of Rotameter

The position of the float is determined by the balance of all forces upon it, including gravity, buoyancy, viscous drag, and differential pressure. Gravity or float's weight acts as a downward force. The buoyancy force is caused by the fluid displacement by the suspended float. In contrast, the viscous force is caused by the average shear on the surface of the float. The viscous force depends on the fluid's velocity, viscosity, and the float's geometry.

Operating Principle of Rotameter

The basic operating principle of the rotameter is Bernoulli's effect. According to Bernoulli's principle, as the velocity of a fluid increases, its pressure decreases simultaneously. In a rotameter, the tapered tube provides a varying cross-sectional area for the fluid flow. When the flow increases, the float will rise, thus increase in the annular area between float and tube.

The discharge of the rotameter is given by (Flowrate equation of rotameter)

Here 

C_d = Co-efficient of discharge

A_annular = Annular area between float and tube

V_float = Volume of float

ρ_float = Density of float material

ρ_fluid = Density of fluid

A_fluid = Maximum cross-sectional area of the fluid.

Except for annular area between the float and tube, everything else is constant for a rotameter. So, discharge is proportional to the annular area. The annular area is a function of float height. So, the flow rate scale can be directly engraved on the tube for a particular float.

How Does Rotameter Work?

The float is free to move vertically within the tube and is affected by the rate of fluid flow moving through the assembly. The vertical position of the float within the tube indicates the flow rate, with the float rising as the flow rate increases and falling as the flow rate decreases. The indicating scale, located on the side of the tube, provides a visual indication of the flow rate. In some cases, the rotameter can also be equipped with flow switches and transmitters for remote observation of flow.

Classification of Rotameters

Rotameters can be classified into several types based on their design and operation. Some of the common types of rotameters include:

Glass Tube Rotameter: This is the most common type of rotameter, where the flow is measured by observing the position of a float inside a transparent glass tube.

Metal Tube Rotameter: This type of rotameter is similar to the glass tube rotameter, but the tube is made of metal instead of glass. Metal tube rotameters are typically used in high-pressure or temperature applications. In a glass tube rotameter, the scale is directly imprinted on the tube; in the case of a metal tube, it is not possible. Other means of scale or indication are used, such as magnetic rotameter.

Magnetic Rotameter and Digital Rotameter: This type of rotameter uses a magnetic float inside a metal or glass tube. The magnetic float interacts with a magnetic field generated by an external source, and the flow rate can be determined based on the float's position in the magnetic field. Once the measurement is converted to an electrical signal, it can be shown on a digital display or can transmit over a long distance.

Applications of Rotameter

Rotameters are relatively insensitive to piping configuration and are usually self-cleaning, making them an ideal choice for many applications. Rotameters have a wide range of applications in various industries, including:

1. Water Treatment: Rotameters are commonly used to measure water flow rate in treatment plants, where accurate flow measurement is crucial for effective treatment.
2. Chemical Processing: In chemical processing, rotameters are used to measure the flow rate of chemicals and ensure consistent and accurate dosing of chemicals. Rotameter is used for high-pressure flow on offshore oil platforms.
3. HVAC: Rotameters are used in heating, ventilation, and air conditioning (HVAC) systems to measure the flow rate of air or gas and ensure proper ventilation.

Advantages of Rotameter

Rotameters offer several advantages, making them a popular choice for flow measurement in various industries. Here are some of the key advantages:

1. Simple Design: The simple design of rotameters makes them easy to use and maintain. They have minimal moving parts and are relatively insensitive to piping configurations.
2. Accurate Measurement: Rotameters provide accurate measurements of the flow rate, with inaccuracies ranging from 0.5% to 5% of the flow rate. They also have a turndown ratio of approximately 10:1, which means they can measure a wide range of flow rates.
3. Versatility: Rotameters can measure a wide range of fluids and gases, liquids with viscosities less than 3cP and steam.
4. Cost-Effective: Compared to other flow measurement devices, rotameters are relatively low-cost, making them a cost-effective solution for many industries.
5. Easy to Read: The visual indication of the flow rate provided by rotameters makes them easy to read and interpret. The scale is approximately linear.
6. Self-cleaning: The self-cleaning nature of the rotameter ensures that foreign materials are removed as the float moves up due to the velocity of the fluid
7. No power required: A rotameter operates without the need for external power or fuel, relying solely on the natural properties of the fluid and gravity to determine the flow rate.
8. Highly resistant clear glass is used, which is capable of withstanding thermal shock and chemical reactions.

Limitations and Disadvantages of Rotameter

1. Rotameter is typically limited to use in pipes with diameters less than 8 cm.
2. They are not suitable for highly viscous liquids or abrasive fluids. 
3. Rotameters can be sensitive to changes in fluid temperature and pressure.
4. Since it utilises the buoyancy force, the rotameter must be placed in the vertical position with fluid flow in upward direction.
5. Rotameters are not as precise as venturi meters or orifice meters.
6. Resolution of the rotameter is poor because it uses direct flow indication. Oscillation of float and parallax error could lead to even more inaccuracies in the measurement.
7. Adapting rotameters for machine reading can be challenging.