Across many industrial and academic applications, it’s necessary to accurately measure liquid flow. Depending on the application, accurate measurements can be the difference between making a profit or loss – or inaccurate/incomplete measurements can cause disastrous consequences.
The usual procedure is to infer the flow-rate by measuring the velocity or kinetic energy of the fluid. The velocity is dependent on the pressure-drop gradient across the pipe or conduit. Since the cross-sectional area is constant, the flow rate is proportional to the velocity:
Q = V x A
Q = liquid flow through the pipe
V = average velocity of the flow
A = cross-sectional area of the pipe
The flow rate (and velocity profile) in a pipe can be determined by the liquid's viscosity and density, and the friction of the liquid in contact with the pipe (which is determined by pipe diameter.)
Positive-displacement flowmeters are pushed along by the liquid, which divide the liquid into increments; measuring this incremental motion mechanically or electronically gives the flowrate.
Physicists often measure the properties of a fluid flow with a dimensionless parameter: the Reynolds Number. It is defined as the ratio of the liquid's inertial forces to its drag forces.
Figure 1. Laminar and turbulent flow are two types normally encountered in liquid flow Measurement operations. Most applications involve turbulent flow, with R values above 3000. Viscous liquids usually exhibit laminar flow, with R values below 2000. The transition zone between the two levels may be either laminar or turbulent.
The equation is:
R = 3160 x Q x Gt/(D x µ)
R = Reynolds number
Q = liquid's flow rate, gpm
Gt = liquid's specific gravity
D = inside pipe diameter, in.
µ = liquid's viscosity, cp
The flow rate and the specific gravity are parameters relating to the inertial forces in the flow, and the pipe diameter and viscosity determine the drag forces. In most liquid applications, the cross-section and pipe diameter will stay the same. For slow or viscous fluids, R is low, and the liquid flows in smooth layers. The flow has a parabolic profile – flow is fastest in the middle and slowest towards the edges where the pipe edges constrain the flow. This type of flow is called laminar flow and is typically seen when R < 2000.
In most cases you’ll measure semi-stable or turbulent flow, with R > 3000. Turbulent flow occurs when the velocity and inertial forces are large compared to the drag. The result is turbulent eddies with an approximately uniform average profile.
Numerous types of flowmeters are available for closed-piping systems. In general, the equipment can be classified as differential pressure, positive displacement, velocity, and mass meters. Differential pressure devices (also known as head meters) include orifices, venturi tubes, flow tubes, flow nozzles, pitot tubes, elbow-tap meters, target meters, and variable-area meters.
Positive displacement meters include piston, oval-gear, nutating-disk, and rotary-vane types. Velocity meters consist of turbine, vortex shedding, electromagnetic, and sonic designs. Mass meters include Coriolis and thermal types. The measurement of liquid flows in open channels generally involves weirs and flumes.
Many different kinds of flowmeter are available today; here is an incomplete list:
- Differential Pressure Meters:
- Flow tubes
- Flow Nozzles
- Pitot tubes
- Elbow meters
- Target meters
- Positive-Displacement Meters:
- Reciprocating piston meters
- Oval-gear meters
- Nutating-disk meters
- Rotary-vane meters
- Velocity Meters
- Turbine meters
- Vortex meters
- Ultrasonic flowmeters
- Mass Flowmeters
- Coriolis meters
- Thermal-type mass flowmeters
- Open Channel Meters
SELECTING A FLOWMETER
Experts claim that more than ¾ of installed flowmeters in industry are not performing satisfactorily. Almost all of these problems arise because the wrong instrument was selected in the first place.
The precise purpose of the flowmeter is the most important consideration. Some questions you may wish to consider:
- Is the measurement for process control (where repeatability is the major concern), or for accounting or custody transfer (where high accuracy is important)?
- Is local indication or a remote signal required?
- If a remote output is required, is it to be a proportional signal, or a contact closure to start or stop another device?
- Is the liquid viscous, clean, or a slurry?
- Is it electrically conductive?
- What is its specific gravity or density?
- What flow rates are involved in the application?
- What are the processes' operating temperatures and pressures?
- Accuracy (see glossary), range, linearity, repeatability, and piping requirements must also be considered.
Understanding the limitations of the flowmeter you’re selecting, as well as the advantages and disadvantages of the instrument is important. Try to match your expectations to the capabilities of the instrument. Suppliers will be keen to ensure your satisfaction by helping you choose the appropriate device: many will give you questionnaires, checklists, and specification sheets designed to help you compare the many devices available.
Technological improvements of different flowmeter types is important; don’t assume that, just because a flowmeter has historically been considered the most effective for a certain application, that this is still the case. For example, software now replaces many of the laborious calculations that were once necessary, saving hours.
Price will also be a consideration; there’s a wide range. Rotameters are often a cheap solution, with prices of around $100. Mass flowmeters are the most expensive; usually more than $3500. However, the total cost for installation for the system may be more than any one device; an orifice plate may cost only about $50, but add in the transmitter and it’s already another $500. The more complicated the design, the more likely installation, operation, and maintenance costs are to be important. That said, "overbuying" is not uncommon; take a good amount of time to select the appropriate flowmeter for the application.
WORKING WITH FLOWMETERS
Suppliers will often provide their own installation services, but again around ¾ of users choose to do their own installations. If you take this route, you must be careful not to make some mistakes – and there are key errors that many installers make, such as failing to allow sufficient upstream and downstream piping for the flowmeter.
All flowmeters will be reasonably capable of handling unstable velocity in the pipe, but proper configurations of the pipe will be required, providing a normal flow pattern. Your measurements will be inaccurate if you don’t ensure this is the case. In many cases, particularly with orifice plates, users will accidentally install flowmeters backwards. Electrical components will often be made intrinsically safe by suppliers, but it’s an important consideration.
An example of a concern is magnetic interference from devices in most industrial locations; power lines, relays, solenoids, transformers, motors, and generators all contribute. Luckily, if you select an appropriate flowmeter and install according to manufacturer’s guidelines, you should be safe from these interference effects.
All flowmeters require an initial calibration; usually the manufacturer will do this as standard, but properly qualified users can do the calibration as well. Depending on the results and performance you get from your flowmeter, you may need to recalibrate over time. This is especially likely in the case where the liquids used are abrasive, erosive, or corrosive.
This may cause the portions of sensitive parts of the device, like individual turbine blades, to erode over time. Of course, the application needs to be considered; if accuracy is crucial and the application critical then regular recalibration is strongly recommended, or at the very least check the applications. Some flowmeters require special equipment for calibration; in that case consult the manufacturer for the use of specialist equipment.
The life expectancy and maintenance requirements can vary, but of course, selecting the correct instrument for the application, for the flow rate, for the liquid and so on will always increase life expectancy; inappropriate devices will fail sooner. Having many moving parts may increase the maintenance and upkeep requirements, but no flowmeter can last forever. Such devices with many moving parts should be inspected internally, particularly if the application involves dirty or viscous liquids; in this case the installation of filters ahead of crucial moving parts can help. Those with fewer moving parts that rely on ultrasonic or electromagnetic meters may develop problems with the wiring and electronics; periodically remove and check the pressure sensors and secondary sensors that are used in such devices.
Differential pressure flowmeters have both primary and secondary elements; the connections between these including piping and valves can cause lots of potential regions where maintenance is necessary. Impulse lines, for example, can get plugged up or corroded over time, requiring cleaning or replacement. Many issues can be alleviated by ensuring that the secondary element of the flowmeter is located in an appropriate place to begin with.
For viscous liquids coating of the pipe can occur, which may cause problems. If the coating is insulating, the operation of magnetic flowmeters will ultimately be impaired if the electrodes are insulated from the liquid. Cleaning away the coating on a regular basis should solve this. Ultrasonic flowmeters rely on refraction angles, but these can drift over time – make sure that you calibrate such instruments regularly.
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.