Selecting Variable Area Flow Meters for Process Flow Measurement

Many industrial processes involve the movement of fluid product components, either liquid or gaseous, through pipes. Because processing is about control, an input to the facility's management, measurement, or data logging centers will answer the query, "How much is going through that pipe?" 

In the industrial process measurement and control industry, there are several ways for quantifying flow, each with unique characteristics that may be useful under certain operating situations. All flow measuring methods are indirect because the actual measurement is of a character influenced predictably by a change in the media flow. Flow measurement is a critical component that, when paired with other fluid properties, is used to calculate the total mass of a fluid passing through the measurement site. 

The variable area meter, commonly known as a rotameter and VA meter, is a tried and true flow measuring device that operates by creating an equilibrium between an upward force created by fluid motion and a downward force, gravity. A tapering glass or metal tube encases a specifically shaped float, also known as a shaped weight, in the device. VA meters must be positioned vertically, with the media flowing upward from the bottom, so that the gravity force required for functioning is correctly aligned with the flow direction. As fluid flows upward via the precisely tapered tube, drag is created on the float enclosed within the tube, pulling it upward. As the float rises, the open space between the float and the tube wall expands, causing the fluid velocity and drag force to decrease. The flow velocity within the tube will cause the weight to climb for any given flow volume until the drag force generated by the flow reaches equilibrium with the countervailing power of gravity on the float. The tube and float are well designed, allowing for an immediate indication of flow volume.

Variable area flowmeters have the following operating characteristics: 

• There is no need for external power or fuel for functioning. 
• Vertical installation is required, with flow arriving at the bottom. 
• Meters are calibrated to a given chemical and temperature. 
• The operation is stable and has a modest pressure drop. 
• For operation, constant gravity is required. 
• Flow rate can be read locally using a meter or a scale inscribed on the tube. 
• The visibility of the float through the medium is required for unit flow readings using glass tubes. 
• For industrial flow metering equipment, accuracy is relatively low. 
• Inexpensive upkeep, simple construction, and low comparative cost.

Brooks Instrument, a world-renowned maker of flow, pressure, and level measurement equipment, created a brief paper highlighting the parameters to consider when specifying a VA meter and how each aspect influences the unit's performance. The description is realistic and straightforward to grasp. It is suggested reading for all process stakeholders who want to improve their flow measuring skills.

For more information about variable area flow meters contact Miller Energy. Call 800-631-5454 or visit https://millerenergy.com.

Factors For Selecting a Variable Area Flow Meter (Rotameter)

Three of many configurations of
variable area flow meters.
Courtesy Brooks Instrument

Industrial processes have many instances where fluid product components, liquid or gaseous, are moving within pipes. Processing is about control, so it follows that an input to the control, measurement, or data logging centers of the facility will answer the question, "How much is flowing through that pipe?".

There are numerous methods employed for quantifying flow in the industrial process measurement and control field, each with particular attributes that may be considered advantageous under  certain operating conditions. All flow measurement methods are indirect, as their actual measurement is of a property that is impacted in a predictable manner by a change in the flow. Flow measurement is an essential element that, combined with other fluid attributes, is used to calculate the total mass of a fluid that has traversed the measurement point.

One time tested method of measuring flow is the variable area meter, also called a rotameter.

Operation of the variable area meter (also referred to as a VA meter) is based upon creating an equilibrium between an upward force, produced by the fluid motion, and a downward force, gravity. The device includes a tapered glass or metal tube that encases a specially shaped float, often referred to as a shaped weight. VA meters must be installed vertically, with the media flowing from the bottom upward, so that the gravitational force necessary for operation is properly aligned with the flow direction. As fluid flows upward through the specially tapered tube, it creates drag on the float contained within the tube, lifting it upward. As the float rises, the free area between the float and the tube wall increases, causing a reduction in the fluid velocity and drag force. For any given flow volume, the flow velocity within the tube will cause the weight to rise until the drag force created by the flow reaches equilibrium with the countervailing force of gravity on the float. Proper design of the tube and the float allow for direct indication of flow volume.

Some of the attributes of variable area meters include:

• No external power or fuel required for operation
• Must be installed vertically, with flow entering bottom
• Meters are characterized to a specific substance, at a specific temperature
• Operation is stable, with low pressure drop
• Requires constant gravity for operation
• Direct local readout of flow rate with meter or scale imprinted on tube
• Glass tube based unit flow readings require visibility of float through the medium
• Accuracy is comparatively low for an industrial flow measurement device
• Generally low maintenance, simple construction, low comparative cost

Brooks Instrument, a world renowned manufacturer of flow, pressure, and level measurement instruments, has produced a concise and compact white paper that summarizes the factors to consider when specifying a VA meter, as well as how each factor impacts operation of the unit. The description is practical and easily understood. It is recommended reading for all process stakeholders to build their flow measurement knowledge.

I have included the paper below. Browse the paper. Contact the flow instrumentation specialists to discuss your application requirements and challenges. Combining your process know-how with their product application knowledge will produce a good solution.

Selecting Variable Area Flow Meters for Process Measurement from Miller Energy, Inc.