Variable area flowmeter-FAQ

Introduction

A Variable area flowmeter is an instrument for measuring the flow of liquid and gasses in pipelines. It has a vertical tube through which the fluid flows and a float that can move vertically. At Instruthink, we manufacture all types of flow meters for air, gas, and other liquids.

In that guide, we will discuss FAQs regarding Variable areaS flowmeter installation and working. Hopefully, you will get the answers that you are looking for. In addition, that information will help you while working with a variable area flowmeter.

What is a variable area flowmeter?

The Variable area flowmeter measures the volumetric flow of liquids and gases. It has a tube and float. The response to floating rate changes is linear, with a standard flow range of 10-to-1. In the laboratory Variable area flowmeter, you can achieve greater flexibility by using correlation equations.

Variable area flowmeter is popular because it has a linear scale, relatively long measurement range, better flexibility, and a low-pressure drop. It consists of a vertical tube through which the fluid flows. The diameter of the vertical tube increases from bottom to top.

 

What are the three primary forces of fluid on a Variable area flowmeter?

Variable area flowmeter operates at the constant delta pressure and the area change of flowrate. Therefore, with the increase in the flow rate, the area will increase. The most popular design of Variable area flowmeter is cone and float type, also known as rotameter. It is a tapered tube with a self-centering float that moves upward by flow and moves down by gravity.

Three main forces are acting on the float. They include buoyancy A, the weight G, and the flow force S. The buoyancy is dependent on the density of the float, whereas the weight G is dependent on the mass of the float. Finally, the flow force S depends on the flow change you achieve a new equilibrium.

What is the basic design of a Variable area flowmeter?

The technology of the Variable area flowmeter follows the variable area principle. In this principle, the flow causes a rise of float in the tube.  That leads to enhancing the area for the passage of fluid in the tube. It consists of measurement elements; float, meter tube, flow rate scale, fittings, threading for mounting in the pipeline, and O rings for seals.

There are float stops for restricting the float movements and a protecting housing that surrounds the meter tube. The measurement range of the Variable area flowmeter is 1:12:15 that corresponds to 8 to 10%.

What is the Working principle of variable area flowmeter?

Variable area flowmeter is the instrument for flow measurement of liquids and gases in pipelines. As flow rises, the float moves to a higher position. It results in increasing the area for the passage of fluid. You can balance the flow rate by the float’s weight that remains constant of the flow rate.

If the flow increases, so do the float. The height of the float is directly proportional to the flow rate. When measuring the flow of liquids, the float rises by the buoyancy and velocity head of the fluid. When measuring gases, the float response to velocity head only.  When the upward force by the fluids equals the downward gravitational fluid exerted by the weight of the float, the float reaches a steady position.

 

How to use a Variable area flowmeter?

Variable area flowmeter measures the flow of liquid and gases such as chemicals, industrial gases, and others. You should avoid using dirty, opaque fluid in the Variable area flowmeter. They can block the view of the float that makes the flowmeter inoperable.

It can be applied to clean liquids and gasses that do not coat the measuring tube. The fluid should be clean so that you can see the float. They are available in sizes up to 3 inches approximately with materials of construction. Variable area flowmeters provide a cost-effective measurement of liquid or gas flows such as mineral processing, pulp, paper, mining, etc.

What are the applications of a Variable area flowmeter?

Variable area flowmeters are widely used for various gas and liquid applications. Some of the applications are following:

• For measuring water and gas flow in plants or labs
• For chemical line and instrument airlines
• Monitoring hydraulic oils
• For process applications for fluids such as water, de-ionized water, compressed air, chemicals, plants, lubrication oils, etc.
• They are also used as internal parts for other instruments and systems to adjust flow rates.

What are the cautions for Variable area flowmeter?

The installation of a Variable area flowmeter should be in vertical orientation because of gravity. Some Variable area flowmeters require an upward flow of fluids which is not suitable for many applications where the flow rate is dependent on gravity.

In glass metering tubes of Variable area flowmeter, you must take precautions while operating the flow meter as it can create a safety hazard if the glass metering tube breaks. It is a more concerning issue if the dangerous fluid is present in the flowmeter. Make sure to install these flowmeters in areas where glass cannot be damaged.

How to choose the suitable Variable area flowmeter for your application?

For choosing the Variable area flowmeter, you have to know the specifications of each flowmeter so that you can select the one accordingly. The variable area flowmeter has the glass tube indicating type. The metal float is made of stainless steel to provide corrosion resistance. The glass tube and stainless steel combinations are available.

For higher pressures and temperatures, metal tubes are the best option. They have materials of aluminum, brass, or stainless steel. You can use them where there is a risk of damage to metering tubes. Plastic tubes are popular because of their lower costs and high impact strength. They have polycarbonate, with metal or plastic fittings material.

How do you read a variable area flowmeter?

In the Variable area flowmeter, the tube is mounted vertically with a small end at the bottom. The fluid is put at the bottom, passes through the fluid, and exits at the top. The float rests at the bottom when there is no flow. As soon as the flow enters, the float begins to rise. The float is dependent on the fluid flow rate and annular area between the float and the tube wall.

As float increases, the annular area also increases. As the annular area opening increases, the differential pressure across the float decreases. As soon as the upward force exerted by the fluid equals the weight of the fluid, the float reaches a stable position. The float position depends on the fluid’s density and viscosity.

What are the measuring tubes of the Variable area flowmeter?

There are different measuring tubes of Variable area flowmeters. In addition, the material of these measuring tubes varies.

• The plastic tube of the material allows reading the flow directly if the fluid is transparent. If it is not transparent, you could not see the float and do the readings.
• The other material is the glass tube. Like a plastic tube, it is possible to read the flow rate if the fluid is transparent.
• The third one is the metal tube. In it, you cannot make the visual reading of the flow. Instead, the float’s position is transmitted to the magnetic coupling inside the indicator box to make the visual reading of the flow.

What are the different types of variable area flowmeters?

Generally, there are two main types of Variable area flowmeters. They include:

• Rotameters and
• Piston type meter

Rotameters

Rotameters are the most common and widely used Variable area flowmeters. In that device, the rise and fall of the float in the tube measures the flow rate. They are known as gravity-type flowmeters because they measure the downward force of gravity and the upward pressure of flowing fluid. You can measure the flow rate by observing the flow.

Piston type meters

In this meter, you fit a piston inside the sleeve. The pressure of the fluid lifts the piston, uncovering the sleeve and permitting the passage of the flow. These types of Variable area flowmeters are ideal for use in mobile or industrial pneumatic systems.

 

What are the performance characteristics of a Variable area flowmeter?

• Linearity

In a Variable area flowmeter, the flow rate is proportional to the area, so these flowmeters have equal scale increments.

• Differential

The pressure flow across the float in the Variable area flowmeter is constant. However, the differential across the meter will increase at higher flow rates because of the friction.

• Accuracy

The accuracy of a Variable area flowmeter is 2% of full-scale reading. It has high accuracy and repeatability with individual calibration.

• Capacity

Variable area flowmeters are the best choice for the measurement of low flow rates. Its capacities range from 0.5cm3/min of water and 30 std cm3/min of air to over 1200 liters/min of water.

What is the operating pressure for a Variable area flowmeter?

The operating pressure is the pressure that exists in the meter tube of the Variable area flowmeter. This pressure is the same as the downstream pressure of the flowmeter. When measuring gases, you should place the needle valve at the outlet of the flowmeter. It keeps the pressure in the meter tube constant, regardless of backpressure variations.

The needle valve is immaterial at the inlet and outlet when measuring the liquids. For measuring atmospheric pressure, you should install a needle valve in the inlet of the flowmeter. For measuring gases, you should consider the location of the inlet when determining the operating pressure in the Variable area flowmeter. Because of the density changes of the compressed gas, the flowrate values also change.

What are the benefits of a Variable area flowmeter?

One of the main advantages of a Variable area flowmeter is its relatively low cost and easy installation. In addition, it has a simple design which makes the Variable area flowmeter maintenance-free and long operating life. Another advantage of this flowmeter is the flexibility in handling a different range of chemicals.

When using a Variable area flowmeter with a built-in valve, you can monitor the flow rate of fluid and control it by opening and closing the valve. It has a readable design that makes Variable area flowmeters a better option to monitor the wide range of liquid, emulsions, and fluids. Also, they can withstand various applications in automotive, oil and gas, mining and discrete manufacturing, etc.

What are the Disadvantages of a Variable area flowmeter?

Apart from several advantages that a Variable area flowmeter possesses, it has few disadvantages as well. One potential disadvantage of the flowmeter occurs when pressure and fluid temperature contradict calibration temperature and pressure. The temperature and pressure variations cause gas to contract that change its density and viscosity. So the calibration cannot work because of these variations.

When operating a Variable area flowmeter, the temperature and pressure keep on fluctuating. That results in accuracy degrading. In addition, the conventional formulas in the Variable area flowmeter do not consider viscosity, which leads to significant errors.

What is the effect of pressure deviation on a Variable area flowmeter?

The effect of pressure on the Variable area flowmeter is very significant. You can calculate the flow rate by reading the pressure loss across a pipe. If you keep all the parameters constant, the increase in outlet pressure in the flow rate drops off. The pressure change affects the gas’s viscosity and density, and the calculated rate will deviate from the calibrated flow rate.

The gas flow rate also changes with the change in gas temperature. Therefore, while operating the Variable area flowmeter, you must be aware of the difference between operating conditions for gas flows and the calibration condition.

 

What is the effect of viscosity on the Variable area flowmeter?

The effect of viscosity on the Variable area flowmeter is significant and acts as a potential disadvantage of the flow meter while measuring liquids. When measuring the viscous fluid such as beverages, fossil fuel, the error occurs due to the viscous nature of these fluids. The rotameter is more affected by viscosity.

The piston Variable area flowmeters have a spring opposed piston. They move with the flow rate and are less sensitive to viscosity. When it comes to the viscous liquid, layers of fluid will build upon the float. This built-up causes this viscous liquid to yield the same buoyant force as the less viscous fluids. The larger the viscosity, the higher the error. It would be best if you made a calibration for a higher viscous fluid.

How to avoid an error for high viscous liquid in a Variable area flowmeter?

When you are operating a Variable area flowmeter, you cannot measure higher viscous liquid. Therefore, in this flowmeter, only water-like liquids can run and be measured correctly. But, if you want to measure the high viscous liquid or the thicker liquid than water, a correction factor could be used. You can use it without the need to calibrate the flowmeter.

If you have made a deviation plan, first discuss your deviation from calibration fluid and conditions with your manufacturer.

What is meant by calibration of a Variable area flowmeter?

The calibration of the Variable area flowmeter is the process of comparing the metering of the flowmeter to the standard scale of measurement to conform to the standards. Calibration is a necessary procedure used by a broad range of industries. Calibration requires high accuracy measurement with zero error.

Variable area flowmeter is calibrating by comparing and adjusting the metering with the given and predefined standard. The widely used calibration procedures are:

• Master meter calibration
• Gravimetric calibration
• Piston Prover calibration

How to Calibrate Variable area flowmeter?

There are many several ways through which you can calibrate a Variable area flowmeter. When calibrating, it is best to use its specified calibration gas. If you must use a surrogate, it is best to use gravity similar to the gas for using the Variable area flowmeter.

You can do the calibration by applying a correction factor for reading pressure and temperature without subjecting the rotameter to the specified pressure of the flow meter. This procedure is popular for rotameter calibration.

When calibrating the Variable area flowmeter using the primary piston prover, correction must be applied to the flow measurements to measure the difference between the actual temperature, pressure, and gas against the Variable area flowmeter’s specified temperature, pressure, and gas requirements.

How to install a Variable area flowmeter?

When installing a Variable area flowmeter, ensure that the meter is suitable for liquids with high flowability free of solids and gasses with linear flow behavior and sufficient inlet pressure.

There are two ways of installation: vertical or horizontal. For horizontal installation, the indicator unit should not be pointing upward so that the water does not enter the device. But, again, avoid a corrosive atmosphere, and if this is not possible, then make sure to install the ventilation.

What are the cautions for measuring liquids and gases in a Variable area flowmeter?

When you measure liquids, make sure there are no gas bubbles in them. It would help if you avoided activating solenoid valves to prevent the measurement flow from rocketing upwards. Water hammers should be avoided to prevent any damages.

When you are using gas measurements in a Variable area flowmeter, increase the operating pressure gradually. Keep changing the operating pressure through a setting valve. This will prevent the measuring body from being exposed to any flow hammer. Also, it will avoid damaging the measuring element.

What are the flow corrections of the Variable area flowmeter?

Variable area flowmeters have scales for specific media at specific conditions. These flowmeters are calibrated for air for gas use and water for liquid at standard atmospheric conditions of 70 °F  and 14.7 PSIA. These are the standard conditions for using the Variable area flowmeter. If you use flowmeters other than these standards, you cannot get accurate flow readings, and you must make corrections.

Gases and liquids have different densities based on pressure and temperature. The pressure is not atmospheric for air, so corrections must be made to get a standard flow rate.

What are the economic considerations of Variable area flowmeter?

Variable area flowmeters have a competitive pricing and cost-effective flowmeter. In recent years, their usage and popularity have increased because of the simple structure and low pricing. However, as several manufacturers enter the market, the prices of Variable area flowmeters remain steady or even decreased.

The flow meters have reasonable prices and advanced technology. As a result, several industries are using Variable area flowmeters. These include food and beverage, car wash, agriculture, and industrial water treatment.

What are the other factors to consider in the Variable area flowmeter?

If you are a user of a Variable area flowmeter, you must check the chemical compatibility of the flowmeter along with the compatibility charts. It’s better to perform your testing as manufacturers provide kits to their users for compatibility testing.

The gravity of the liquid in the flow meter determines the accuracy and reliability of the Variable area flowmeter. Viscosity is another essential factor to consider as it determines the degree to which the fluid resists flow under an applied force.

Conclusion

A Variable area flowmeter is an excellent and cost-effective tool for measuring liquids and gases. You can purchase this flowmeter from Instruthinkas we are the best manufacturers of flow meters in the market. We ensure in providing high-quality services to our customers with high-quality and high-performance flowmeters. As a result, our customers trust us when they need accurate and repeatable measuring solutions for their company. If you have any questions about the Variable area flowmeter, you can contact our team at sales@instruthink.com or directly call us on +44 7700 307784.