Rotameters a.k.a Variable Area Flow Meters

Rotameter
(Yokogawa)

Rotameters have diverse industrial processing applications that range from simple to sophisticated. The devices are easy to install, require no electrical connection, and provide direct flow rate reading. They provide fail-safe flow rate in different situations. 

The post will highlight the workings of rotameters, also known as a variable area flow meter.

Rotameters: An Overview 

Invented by German inventor Karl Kueppers in 1908, rotameters measure the volumetric flow rate of liquids and gases. The device was so named because it functions by rotating in a tube. 

Important elements of a rotameter include the tube and the float. 

The tube is fixed vertically and liquid is fed from the bottom. The fluid is fed from the bottom that travels upward and exits from the top. The float remains at the bottom when no liquid is present and rises upward when fluid enters the tube. 

The float inside the tube moves in proportion to the rate of fluid flow and the area between the tube wall and the float. When the float moves upward, the area increases while the differential pressure decreases. A stable position is reached when the upward force exerted by the fluid is equal to the weight of the float. A scale mounted on the tube records the flow rate of the liquid. 

Every float position shows a specific flow rate for the viscosity and density of a liquid. The device helps in determining the flow rate by matching the position of the float to a scale on the rotameter. The flow can be adjusted manually using a built-in valve. 

Types of Rotameters 

Rotameters can be categorized by the type of tube. A sharp metering edge is located on the float from where the reading is observed by a scale on the tube. The connections and end fittings of the various types of rotameters are different. 

Glass Rotameter

Glass rotameters
(Yokogawa)

The basic glass rotameters consist of borosilicate glass tube while the float is made of either glass, plastic, or stainless steel. The most common combination is a glass tube and metal float. This is suitable for a measure the flow rate of liquid of low to medium temperature and pressure. 

Flow rate is determined by the spring and piston combination of the rotameter. The materials and fittings should be chosen as per the temperature and pressure of the liquid. 

Metal Rotameter

Metal tube rotameters are another type that is suitable for temperatures and pressures beyond the glass tubes. They are generally manufactured of stainless steel, aluminum, and brass. The piston position is determined by the mechanical and magnetic followers that can be read from the outside of
the tube. 

Metal rotameter
(Yokogawa)

The meters are generally used for steam applications where glass tube rotameters are not suitable. They are also suitable in situations where other forces would damage the glass metering tubes. 

Industrial Applications of Rotameters

The use of rotameter extends across different industries. The device is extensively used in industries since its an economical way to measure a range of flow rates in almost any conditions. 

Sample Process Analysis 

Rotameters are widely used in sample process analysis. A device with 4 to 20 mA output is used to measure the flow of a sample system. Monitoring the flow rate ensures that the system does not become plugged that could result in big problems. The device can indicate when the flow starts to drop so that the maintenance crew can address the problem before it results in major damage. 

Transport of Liquid Natural Gas 

LNG must be re-gasified during transport to prevent system overpressure that could cause major damage to the storage system. Rotameter can measure the flow rate of vented vapor even at very low temperature. They are ideal for determining the LNG flow rate during transport due to being low cost and providing an accurate reading. 

Rotating Equipment 

Rotameters are also used in measuring the flow of liquid in large rotating equipment. They are used to measure the flow rate of dry gas, coolants, lubricant fluids to ensure safe operations. The device helps in monitoring of coolant flows a lube oil. The flow can be measured continuously due to the use of 4— 20 mA transmitting rotameters.  

Benefits of Rotameters

Rotameters, or variable area flow meters are cost-effective for use in different industrial applications. The price and low maintenance of the device contribute to significant savings for industrial concerns. 

Another advantage of rotameters is that no external power is required. They are mechanical device and no external power source is required to measure the flow rate. This makes it possible for the device to be used in remote and hazardous areas where installing an external power supply can be costly. 

The design of the rotameter allows not the only the measurement of fluid flow but also determine the quality of the liquid. The crew members can see whether the fluid is dirty thereby requiring a change of filter. They can also know whether bubbles are present in the liquid and also whether the liquid is of the correct color. 

Rotameters can be installed along with other flow measuring devices to ensure accurate readings. The device can continuously determine an accurate flow rate. They are simple to install and easy to maintain. Just connect the process line to rotameter’s inlet and outlet pipe. That’s why they are commonly used in many industries where it’s critical to accurately measure the flow rate of the liquid. 

Contact Power Specialties, Inc. with any questions regarding the use of rotameters. Call them them at (816) 353-6550 or visit https://powerspecialties.com.

Basics of Rotameters (Variable Area Flowmeters)

Rotameter (Yokogawa)

A rotameter is a flow measuring device that belongs to a group of instruments called variable area flowmeters. For decades variable area flowmeters have become established in industrial measurement technology with an economical, mature measurement principle.

The variable area flowmeter is an instrument for metering the flowrate of liquids and gases in a pipeline. It includes a vertically oriented conical tube, whose diameter is larger at the top than at the bottom, through which the fluid flows upward and in which a vertically moving float is positioned. The height of the float in the tube increases as the flowrate increases in such a manner that the resistance to the flow is always balanced by the weight of the float and remains constant regardless of the flowrate. The height of the float in the tube is a measure of the flowrate.

The value of the flowrate can be read from a scale. Variable area flowmeters are the most cost effective solution for almost all applications involving the measurement of industrial process liquids, gases or steam. They meet the application requirements by featuring a wide range of design varieties and sizes.

Rotameters offer long life and high reproducibility and are excellent mechanical back-up meters because no external power supply is needed. For more information on rotameters (variable area flowmeters) visit http://www.powerspecialties.com or call Power Specialties at (816) 353-6550.

The video below does a great job introducing the various types and uses for rotameters.

The Bubbler Method of Liquid Level Measurement

Bubbler Level Measurement
(courtesy of Yokogawa)

Remotely utilized measurements of tank liquid level are common throughout the fluid processing industries. Various means of inferential or direct measurement are available, each with its own set of performance, maintenance, and cost attributes that may make it the preferred choice for a particular application.

Determining liquid level using a hydrostatic pressure measurement is a simple, easy to implement strategy for delivering a continuous level signal to the process operating and control system.

There can be situations where mounting the measuring gear near the bottom of a tank or other vessel may be impractical or undesirable. A pressure transmitter mounted low on a tank may expose it to damage from plant traffic or other physical hazards. It is also possible that the tank may not have a suitable fitting located low enough to provide the needed measuring range. Having a potential leakage point at a fitting low on the tank may also be undesirable. Another, and certainly not final, scenario would be an application involving a corrosive liquid which must not come in contact with the pressure sensor.

The bubbler method of liquid level measurement employs an arrangement that overcomes all of the previously mentioned impediments. It can utilize connections at the top of the tank, above the liquid level. The way in which the method works will keep the pressure sensor out of contact with the process liquid, providing isolation from potential corrosive effects.

The apparatus for level measurement using the bubbler method employs a simple dip tube that extends from the pressure sensor or transmitter to nearly the bottom of the tank or vessel. A small amount of purge air or gas continually flows through the dip tube and will bubble out the bottom of the tube. This dip tube arrangement essentially transfers the hydrostatic pressure at the bottom opening of the tube to the pressure transmitter, while the purge gas keeps the liquid from advancing up into the pipe. The bubbler can be used on atmospheric or pressurized tanks with a properly configured pressure or differential pressure transmitter.

Probably the most significant application point of the bubbler method that will distinguish its use from many other level measurement methods is the importance of maintaining air or gas flow in the dip tube. The flow provides isolation of the sensor, but the flow must also be set to a level that will not impact the pressure measurement in the tube itself. If the flow is excessive, back-pressure in the tube can be offset from the level imparted by the tank contents, with the result being an incorrect measurement.

Below is an application note from Yokogawa, showing how their pressure transmitters and rotameters can be used to create the setup. Share your process measurement challenges and requirements with instrumentation experts, combining your own process knowledge with their product application expertise to produce effective solutions.

Rotameters and Bubble Tube Purge for Level Measurement from Power Specialties, Inc.

Yokogawa Variable Area Flowmeter from Power Specialties, Inc.

Yokogawa EJX110A Differential Pressure Tranismitter from Power Specialties, Inc.