Flow monitoring is an essential part of any industrial process. For this reason, most industries have been doing all that it takes to adopt some of the most reliable and efficient ways to control these processes.
This has been possible through the use of various flow meters and switches. For instance, most flow switches in the market today tend to feature the thermal dispersion technology.
Thermal dispersion technology is presently being used to monitor the flow of gases and liquids in very many pipelines. This technology is based on the fact that the amount of heat carried away from the heated probe increases as the velocity of the fluid increases.
There are quite a number of switches which feature this technology. In these switches, the existence of a greater difference in temperature indicate no-flow condition. This temperature difference decreases with an increase in the rate of flow. The most commonly used heated element is the resistance temperature detector (RTD). The rate at which at which it dissipates heat is directly proportional to the rate of fluid flow.
The signal obtained from the RTD is converted to an electrical signal by the electronic conditioning circuit. The electrical signal is later used to adjust other set point circuits. Most of these devices are rugged and reliable as compared to the traditional switches. They are also durable and other problems such as clogging are completely eliminated. Switches which feature this technology can monitor all types of fluids and have a fast response to changes in the flow velocity.
Uses of flow switches
Monitor flow rate; from the temperature which is detected by the RTD, it is much easier to determine the flow rate of the fluid. This is for the reason that the amount of heat being dissipated is proportional to the rate of flow.
Trigger alarm; assuming that the flow rate of a fluid was set not to go past a given level, any increase past the set point will be detected by the switch which will then switch on the alarm for necessary action to be taken. This can also protect other devices such as pumps from being overloaded.
Eases industrial control process; most switches which features the thermal dispersion technology can easily be integrated with the control systems such as the PLCs. This is because the temperature from the RTD is converted to an electrical signal by the signal conditioner. Remember, it’s easier to deal with electrical signal when it comes to any control process.
Wide range of applications; these switches, more so the liquid types, can be used for very many applications which include liquid lines, swimming pool and sea water. As long as the liquid doesn’t react with the material from which the switch was constructed from. Therefore, material consideration is important while choosing these switches. It will also dictate its life span.
There are very many switches available in the market today with some having the optical display systems. Therefore, it important to define the application which the switch is intended for to acquire the right one.
Flow Switch Information
Flow switches can be classified as either air or liquid flow switches. They are used to monitor the flow of liquids, air or steam in various industrial applications where they send signals to other devices such as pumps to either turn off or on depending on the situation at hand.
They also used in heating, hydraulic and air conditioning systems.
These switches come with a wide range of technology which enables them to guarantee high quality performance, resistant to corrosion, flexible actuation settings and a wide range of designs amongst others.
Air flow switches
They provide one of the cheapest ways to detect the loss or change in air velocity. They are preferred for applications which require high to low velocities. Both the loss and the change in velocity can be as a result of a number of factors which include dirty or clogged filter, an overload of the fan’s motor, loose fan wheel or close fan or damper inlet.
The vane actuated switches
In these switches, as air flow through the pipeline, the vane is forced to swing in the direction which the wind blows. This forces the sleeve to move in the upward direction making the magnet to pull in which and actuates the switch.
Disc actuated switches
In this case, the fluid flowing through the valve pushes the disc up and down which as a result lowers and raises the magnetic sleeve respectively. This in makes the magnet to actuate the switch. The principle is almost the same as that of the vane actuated switch.
Liquid flow switches
They are used for a wide range of applications which include electrically operated equipment such motors or signal lights. They can be used to start or stop an electrical equipment. They are mostly used for general purpose applications. They are also used to detect flow in very many industrial applications such an event there’s a failure of flow in certain pipes it can raise an alarm.
There are a number of factors to consider while installing these types of switches in a pipeline. To ensure that the switch operates as required, the arrow on the casing must always point towards the direction of flow. The angle at which the switch should be mounted on the pipeline is also important.
This will depend on the switch’s manual. A filter must also be installed up stream to prevent foreign particles from entering the pipe. They must also be mounted in regions where there’s little or no turbulence i.e. they should be installed away from bends or elbows.
With the advancement in technology, there those flow switches have incorporated thermal technology. In such switches, a no-flow condition is indicated by high temperature conditions while the presence of flow is indicated by low temperatures.
Such types of switched have resistance temperature detectors (RTDs). This is from the simple fact that when there is flow, the RTDs are cooled by the moving fluid. They’ve so far gained popularity in most industrial applications. They have electronic circuits which converts the temperature difference from the RTDs into an electrical signal which is used in various control processes. This makes it easier to be incorporated in various control systems.
FLOW METER SELECTION GUIDE
A number of flow meters installed in most industries do not function satisfactorily due to inaccurate choice of flow measurement instruments.
The variation in meter technology such as the magnetic and ultrasonic technologies just to mention a few, have brought a lot of challenges to most technicians. They find it difficult to select those with high level of performance and accuracy.
Again, each meter has its merits and demerits which must be scrutinized before buying and consequently installing. It’s nearly impossible to come across a perfect one, however, the merits must outweigh the demerits.
It is important to know the following before buying or installing a meter:
Task the instrument should perform; basically, in flow measurement, the following parameters can be examined: instantaneous velocity, mass and volume flow rate. Choosing the right instrument such as the coriolis meter to determine the mass flow rate will be a noble idea. There also situations where accuracy of the rate of flow is given more priority. Thus, going for those which will be precise is crucial. Remember, different meters have different levels of accuracy.
Where the instrument is to be installed; are you planning to install the meter in pressurized systems? Is the fluid whose flow rate is to be determined clean or slurry?
Could the environmental conditions such as temperature or electrical interference affect the performance of the meter? Ensure the meter can work optimally without unnecessary interference from either the surrounding environment or the fluid whose flow characteristics is to be determined.
For example, turbine and positive displacement meters can be used to measure flow characteristics of clean and viscous fluids; electromagnetic types ate designed dirty, conductive, clean and viscous fluids while the ultrasonic one are suitable for clean and viscous fluids. Al these must be considered.
Operating range; both the lowest and the highest flow rates must be determined before installation. Therefore, the operating range must not exceed that of the flow meter. This is to ensure that it’s not overwhelmed since this will only lead to inaccurate readings.
Size of the pipe; it will also determine the type to purchase and install. In most industrial applications, most pipes are very large and thus some meters cannot be used.
Calibration; of course all meters are calibrated by the manufacturer. However, after it has been used for some time, it’s important to re-calibrate it to ensure that all the readings obtained are accurate. Re-calibration will depend on the meter’s technology and the nature of the fluid.
Does it give room for upgrade? Most of these instruments are upgraded each year to ensure that they operate optimally, accurately and efficiently. In fact, with the era of automation where the SCADA and the PLCs are used in most industrial processes, it must give room for being incorporated in such systems.
Installing or buying the right flow meter alone isn’t enough. These instruments require periodical maintenance to ensure that they work accurately. Designing a scheduled checkup program especially for those with moving parts is necessary. The accuracy must be maintained within the recommended range. On the other hand, those without moving parts such as the ultrasonic and the electromagnetic types, their electronic components must be checked.
FLOW MEASUREMENT SYSTEMS
Measurement of the rate of flow is essential in very many industrial process such as chemical, steel and public utilities. To ease this process, a number of flow meters have been developed to ensure that all information about any flowing fluid is obtained accurately.
Matter exist in three major states which include solids, liquid and gaseous states. When the term “fluids” is used, basically, we refer to both liquids and gases since they have the ability to flow under the action of deforming forces. This is due to both shear stress and viscosity; the major principles of fluid mechanics. Liquids and gases have their unique properties which must critically be analyzed before opting for any measuring instrument. There are many gas flow meter types to choose from. There’s a bewildering number of meters in the market today and therefore, one has to be careful to get an accurate, reliable and robust instrument.
Types of flow meters
These instruments can be classified depending on the type of information/measurement an individual would wish to obtain from a fluid. These include:
- Determining the velocity of fluid at a point
It’s an essential process when it comes to investigating velocity distribution at a given point or in situations where the velocity profile is to be established before the installation of a permanent meter. To achieve this the following can be used:
Figures 1 and 2 shows how Pitot – static tube is placed in a section whose velocity profile is to be determined. As the fluid flows, at the impact, the fluid is usually brought to rest and thus, kinetic energy is reduced to zero. This makes this section to possess only the pressure energy.
On the other hand, at its static sections, there’s both the kinetic and pressure energy due to the movement of fluid. This in return causes pressure difference which is brought about by the velocity. It’s a convenient way to obtain the difference between total, static and dynamic pressure. It’s not suitable for measuring velocity in turbulent flow.
Hot-wire and film anemometers
This is another efficacious instrument which is also used to determine the velocity of fluid at a point. One good thing about it is that it can be used to obtain the average velocity and turbulence.
- Determining the volume flow rate
They’re very many types of meters which can be used to determine the volume flow rate. These include:
The differential flow meters
They are widely used in most industrial applications for both liquids and gases. It’s usually placed in a pipe after which a differential pressure (D/P) is developed across the constriction. With these instruments, it’s difficult to deduce the volume flow rate from the D/P. The most common differential meters include the nozzle, orifice plate, Dall tube and the Venturi. The images below shows some of the most commonly used differential meters.
The vortex or vortex shedding meters
Principles of Operation
The principle of operation is based on the fact that when a fluid meets an obstruction, its particles separate and tends to move around the obstructing object as it continues to move downstream. This results to an oscillating pressure gradient. With an increase in the frequency of vortices due to increase of flow, a liner relationship between velocity of flow and shedding frequency can be realized.
They are installed along the path of a moving fluid where the fluid forces them for move/rotate. The volume flow is determined by the number of cycles/second. They are mostly used to measure the total volume of the fluid. A good example if the turbine meter.
- Mass flow rate measurement
Determining mass flow rate is crucial for companies such as those which sell crude oil since they use these meters to determine the accurate mass of the commodities they sell. Measuring mass can be done in to major ways; either by inferential or direct method. Determining the mass flow rate requires special instrument called:
Most of these meters were mostly used in industrial application after 1980s. There are very many designs with most of them coming in U-shaped or horseshoe-shaped. They can handle large mass flow rates.
At times, it may seem impossible to obtain fluid measurements using either of the above flow meters and thus, resorting to specialized instruments is necessary. Some these instruments include:
Electromagnetic meter; principle of operation is based on Faraday’s law of electromagnetic induction
Ultrasonic meters; they use sensors which are placed outside a pipe making them more suitable for multiphase flows.
Cross-correlation flow meter; with this instrument, it is assumed that various property of the fluid such as conductivity, velocity, temperature and density changes in a random manner.
There are very many instruments available in the market and thus it’s prudent to consider all principles of fluid mechanics to be sure of the accuracy of the measurements obtained.