Selecting a Flowmeter for Oil and Wastewater Applications

Both gases and liquids can be used with OMEGA flow products, with many different uses including processing chemicals, detecting filters and leaks, processing pulp and paper, and the handling of petroleum, oil, and wastewater.

What Type of Flowmeter Is Best?

There are many different flowmeters available, varying in style and technologies used, which each suit a specific application. By writing a flow specification outlining the use of the meter, the engineers can select a suitable technology. It is important that the critical specifications are known as different uses require different meters. One example is that Coriolis meters do not react rapidly enough for injection flow, whereas Turbine flowmeters do not function in thick slurries.

Considerations Before Selecting a Flowmeter

Satisfactory selection of a flowmeter requires a coherent understanding of the application requirements. Therefore, it is important that a suitable amount of time is spent evaluating the type of fluid and the requirements for the installation in a step-by-step and systematic process.

First Steps to Choosing the Right Flowmeter

Initially, there are various factors which need to be considered including:

• Type of fluid to be measured
• Requirements of volume or mass flow measurements
• Nature and viscosity of fluid
• The maximum and minimum pressure, temperature and flowrate
• Liquid compatibility with the flowmeter
• Pipe size
• Requirements for a local display, electronic signal outputs or specialized communications protocols (including RS232/RS485, Ethernet, HART, MODBUS)

Choosing the Right Flowmeter Type

Coriolis

The exceptional accuracy and versatility when measuring difficult flow applications make the Coriolis meters superior to alternatives. These meters allow the detection of all liquids in addition to dense gases. They are particularly useful when the assessment of numerous factors is required, including mass and volume flow, density and temperature. This device functions by utilizing the inertia formed by liquid flowing through an oscillating cylinder, which acts to rotate the tube in proportion to mass flowrate.

Ultrasonic Fowmeters

Non-invasive devices which can be used in applications which could impair other sensors are called Ultrasonic flowmeters. The operation of this flowmeter uses the basis of the frequency shift effect. This effect, also known as the doppler effect, uses the assumption that an ultrasonic signal will be reflected by suspended particles or gas bubbles within the flowing liquid. The alternate transit time method relies upon the minor alterations in time taken for an ultrasonic wave to migrate.

Magnetic Fowmeters

Electrically conductive fluids can be measured using electromagnetic meters. When the pressure drops, it is equivalent across the meter as there are no moving components or hindrances to the flow of liquid. These devices utilize Faraday’s law of electromagnetic induction, which stipulates that a conductor moving through a magnetic field will induce a voltage. The fluid serves as the conductor, whereas the magnetic field is generated by charged loops located outside the flow pipe.

Mass Flowmeters

A type of device which is mainly utilized in measurement of the mass flow rates of gases is the thermal-type mass flow meter. This device is very useful in measuring low flow in milliliters per minute and in testing for leaks due to a minor reliance on pressure, density and fluid viscosity. This technique can use two different methods to determine the true mass flow rate of gases. The first type is the differential pressure transducer and temperature sensor. Alternatively, the second technique uses heated sensing components and thermodynamic heat conduction.

Vortex Meters

A class of meters which can assess the flow of steam, gas and liquids at immense temperatures are the vortex meters. Due to their low susceptibility in alternate environment conditions and their minimal corrosion, these meters can withstand extreme temperatures whilst producing reliable results. Vortex shedding is the process which arises when a fluid streams around an object. This phenomenon is utilized by vortex meters. The abundance of vortex shedding can be directly attributed to the velocity of fluid which moves through the meter, thereby enabling the flow to be assessed by measuring the prevalence of vortex shedding.

Variable Area Flowmeters

The most commonly used flowmeter for the measurement of gas and fluid flow is the variable area flowmeter. This device consists of a float which is attached to a tube, which diminishes in size to create a point. This device is so popular as it is very inexpensive, with a large range, linear output and low pressure drop, therefore making it ideal in the application of flow measurement.

Positive Displacement Flowmeters

When no linear tube is available, positive displacement flowmeters can be used to measure both low and high viscosity fluids. When this device is used, the liquids are split into precisely measured increments and moved on. This is particularly valuable in use when a basic mechanical meter system is required.

Turbine Flow Transmitter

One very precise method of measuring clean fluid is using turbine meters. These devices demand that there is at least a tube diameter of ten inches within the inlet and a minimum of five inches on the outlet. The structure of this device is composed of a multi-bladed rotor which is mounted internally within the tube and horizontally to the stream of fluid. When the liquid flows past the rotor, it pushes the blades and therefore causes it to rotate. The speed of the rotor movement is directly proportional to the rate of liquid flow. Therefore, this speed can then be measured by several different methods, including: photoelectric cells, gears or magnetic pick-up. Overall this method is very useful, particularly with low-viscosity fluids.

Paddle Wheel Sensors

Paddle wheel sensors are a very cost-effective method for measuring the flow of both fluid and water, which makes them extremely popular. These are often offered with various flow fittings and insertion styles which allows customization. A minimal diameter of pipe is required, which consists of 10 inches of straight pipe on the inlet and 5 inches on the outlet. Before use, the compatibility of the chemical should be established. There are various outputs available, including Sine wine and Square wave. Transmitters for both built-in systems and panel mounting are also available.

Spring and Piston Flowmeters

Other types of meters which are useful in applications of both gas and fluid measurement include spring and piston flowmeters. These meters have the benefit of being able to be mounted in many positions. Specific gravities determine the scales, with 0.84 used for oil meters and 1.0 used for water meters. Due to both their minimal design and the simplicity in setting up transition of electrical signals, these meters have become a very affordable rival to the variable area flowmeters.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.