Types of Flow Meters
Flow meters measure the rate at which mass moves through a pipe or tube. The rate can be expressed as the volume moved per unit time such as gallons per minute or the quantity of mass moved per unit time such as kilograms per second. The flow measured is usually considered a fluid, either liquid or gas. It is important to measure flow for any application so that you know how much mass is transported through the system. Some application examples include:
- Measuring the flow of water on the suction side of a pump to ensure the proper amount of fluid is entering the system
- Measuring the flow of sludge on the discharge side of a pump to determine the pumping efficiency
- Measuring the flow of polymer entering a holding tank to determine how much time it will take to fill the tank
- Measuring the flow of pressurized air through a backwash line so that the proper amount of pressure is applied
There are a variety of flow meters that utilize different principles of physics to measure fluid dynamics. For example, there are mechanical, pressure-based, thermal and electromagnetic types of flow meters. Each type of flow meter is useful for a specific set of applications. This article discusses three mechanical types of flow meters: turbine, positive displacement and variable area.
Turbine Flow Meter
The main component of turbine flow meter is a multi-bladed rotor, much like that of a turbine engine. The turbine is placed within the flow line such that the flow of fluid imparts energy onto the blades and causes the blades to rotate. The blade movement is usually detected by a magnetic sensor, which sends a pulse signal to an attached transmitter. The transmitter then interprets the magnetic pulses and outputs a digital reading on a field indicator or sends the information to a control system or computer.
There are generally four connection types for turbine flow meters:
- AN/MS flare connection
- NPT connection
- ANSI flare connection
- Tri-clover connection
Turbine flow meters perform well when installed along pipelines that transport clean fluids with steady flow and low viscosity. This makes them a reliable choice for fuel, natural gas, water or chemical lines. They can also handle high temperature and high pressure fluids, and are useful for mixing and batching applications.
Cavitation can be a problem with turbine flow meters but can be prevented by maintaining a minimum pressure through the turbine. It is important to measure the pressure drop across the flow meter and compare the manufacturer’s recommended minimum value to prevent entrapment of air bubbles within the instrument.
Positive Displacement Flow Meter
This type of flow meter typically consists of at least one gear within a fixed cavity. The gear rotates as fluid enters the cavity, and the movement of the gear is measured by a sensor. There is a direct relationship between the rotation of the gear teeth and the volume of fluid inside the cavity. The sensor is often a magnetic pick-up that sends a signal to a digital output indicator or remote flow control system.
Positive displacement flow meters work well for low viscosity applications such as hydraulics and automotive systems and high viscosity applications such as petrochemicals and oil blending. They also work well for smaller pipelines and tubes with lower flow rates.
Each gear component typically has a bearing to support the imparted load. To protect the bearing from avoidable wear and damage, a filter is recommended upstream of the flow meter to remove potentially damaging particulate matter.
Variable Area Flow Meter
Variable area flow meters are often referred to as “rotameters.” The basic design consists of a vertical, tapered tube with a float inside. Fluid enters the bottom of the tube and pushes upward on the float. The position of the float indicates the volume of fluid flowing through the tube. The tube has indicator marks that provide a direct reading of the flow. Some designs allow for horizontal installation, where the float moves to the left or right depending on the direction of flow.
This type of flow meter works for clean fluids such as water, oil and hydraulic fluid. Since it provides a continuous direct reading, there are no additional sensors or connections to transmitters.
Variable area flow meters are typically made of borosilicate glass, acrylic safety shields and end gasket seals. The end gaskets are usually rubber O-rings that can be removed to clean the interior of the flow meter.
Calibrating Your Flow Meter
Flow meters typically require little maintenance, but they do require calibration to ensure that the measurements are accurate. The National Institute of Standards and Technology sets minimum standards to ensure that manufacturers calibrate their instruments before distributing them to users. Most flow meters undergo NIST traceable calibration at the manufacturer. This method ensures that the quality of measurements adhere to NIST standards where documentation and uncertainties can be traced back directly to NIST through an unbroken chain of calibrations.
Once on site, users are then responsible for calibrating their own instruments during a regularly scheduled maintenance program or by hiring a professional to perform a calibration service.
When to Use Flow Computers
For large systems or those with multiple types of flow meters, flow computers provide a convenient, automatic way to collect flow data from multiple sources, calculate corrected volume and mass flows and relay alarms or help operators make decisions real-time. Plus, they can receive additional sensor information such as temperature and pressure.
Flow computers can receive both digital and analog input, totalize flows during specified time frames or batches and linearize data to ensure that the flow meters are operating within their defined range. They can be connected to control panels and computers to alarm operators when changes occur, when the flow exceeds a defined minimum or maximum or when the flow meter is exceeding its operational limitations.
Accurate Flow Measurement Is Essential
Flow is one of the most important parameters in any fluid transport system and should be not taken for granted. Flow meters must be maintained in optimal condition and calibrated regularly to ensure their accurate performance. For further information, consult your flow meter dealer or flow mater calibration service team to make sure your instruments are protected.