Flow Meter

What is a flow meter?

Flow meter (or flow transmitter, flow sensor, flow indicator) is an instrument used to measure the linear, nonlinear, mass or volumetric flow of a liquid or gas. When selecting a flow meter, intangible factors such as the familiarity of plant personnel at a particular plant site, their calibration and maintenance experience, spare parts availability, and the average time between failures history should be considered. It is also recommended that installation costs be calculated only after these steps have been taken.

How to choose a flowmeter?

Before choosing a flow sensor, there are 9 questions that need to be answered.

1. What is the fluid being measured?
2. Do you need rate measurement and/or totalization?
3. If the fluid is not water, what is the viscosity of the fluid?
4. Do you need a local display on the flow meter or do you need an electronic signal output?
5. What are the minimum and maximum flow rates?
6. What are the minimum and maximum process pressures?
7. What are the minimum and maximum process temperatures?
8. Is the fluid chemically compatible with the flowmeter’s wetted parts?
9. If this is a process application, what is the pipe size?

The first step in selecting the right flow meter

First step in selecting a flow sensor is to determine whether the flow information should be continuous or cumulative, and whether the information is required locally or remotely. If remote, should the transmission be analog, digital, or shared? And, if shared, what is the required (minimum) data update frequency? Once these questions have been answered, the properties and flow characteristics of the process fluid as well as of the pipeline housing the flow meter should be evaluated.

Fluid and flow characteristics

Lists the fluid and its given pressure, temperature, allowable pressure drop, density (or specific gravity), conductivity, viscosity (is it in Newtons?) and vapor pressure at the maximum operating temperature, and how these properties may indicate changes or interactions. In addition, all safety or toxicity information should be provided, as well as detailed data on fluid composition, presence of air bubbles, solids (abrasive or soft, particle size, fibers), coating tendencies and translucent quality (opaque, translucent) or transparent?) .

Pressure and temperature range

When selecting a flow meter, the expected minimum and maximum pressure and temperature values should be given in addition to the normal operating values. The fact whether the flow will be reversed, whether it does not always fill the pipe, whether agglomerated flows (air-solid-liquid) will form, whether inflation or pulsation is possible, whether sudden changes in temperature will occur or whether special precautions are required for maintenance during cleaning and purging should also be stated.

Piping and installation area

Flow rate and accuracy

The next step is to determine the required meter range by determining the minimum and maximum flow rate (mass or volume) that will be measured. After that, the required accuracy of the flow measurement is determined. Typically, accuracy is specified as a percentage of actual reading (AR), a percentage of calibration range (CS), or a percentage of full scale (FS) units. Accuracy requirements should be stated separately at minimum, normal, and maximum flow rates. Unless you understand these requirements, the performance of your meter may not be acceptable over its entire range.

In applications where products are sold or purchased based on meter readings, absolute accuracy is critical. In other applications, repeatability may be more important than absolute accuracy. Therefore, it is recommended that the accuracy and repeatability requirements for each application be determined separately and that both be stated in the specification.

When the accuracy of a flow meter is expressed in % CS or % FS units, the absolute error will rise as the measured flow rate decreases. If the meter error is expressed in % AR, the absolute error remains constant at high or low flow rates. Because the full scale range (FS) is always larger than the calibration range (CS), a sensor with % FS performance will always have a larger error than a sensor with the same % CS specification. Therefore, to fairly compare all bids, it is recommended that all quoted misstatements be converted to the same % AR units.

In carefully prepared flow meter specifications, all accuracy statements are converted to uniform % AR units, and these % AR requirements are specified for minimum, normal, and maximum flow rates, respectively. All flowmeter specifications and bids should clearly state the accuracy and repeatability of the flowmeter at minimum, normal, and maximum flow rates.

Accuracy and repeatability

If acceptable metrology performance can be obtained from two different meter classes and one has no moving parts, choose the one with no moving parts. Moving parts are a potential source of problems, not only for the obvious reasons of wear, lubrication and sensitivity to coatings, but also because moving parts require clearance space, which can sometimes introduce “slop” in the measured flow. Even with well-maintained and well-calibrated instruments, this unmeasured flow can vary with fluid viscosity and temperature. Temperature changes can also alter the internal dimensions of the meter and require compensation.

In addition, if the same performance can be obtained from a full flow meter and a point sensor, a flow meter is usually recommended. Because point sensors do not view the full flow rate, they can only be read accurately if they are inserted to a depth where the flow rate is the average of the flow rate distribution in the pipe. Even if this is carefully determined during calibration, it is unlikely to remain constant because the velocity profile will vary with flow rate, viscosity, temperature and other factors.

Mass or volume units

Before specifying a flow meter, it is also recommended to determine if flow information expressed in mass or volume units is more useful. Volumetric flow is not very meaningful when measuring the flow of compressible materials, unless the density (and sometimes viscosity) is constant. When measuring the velocity (volumetric flow) of incompressible liquids, the presence of suspended air bubbles can cause errors; therefore, air and gas must be removed before the fluid reaches the meter. In other velocity sensors, pipe lining may cause problems (ultrasonic) or the meter may stop working if the Reynolds number is too low (in vortex flowmeters, RD > 20,000 is required).

Given these considerations, mass flow meters are insensitive to changes in density, pressure and viscosity and are unaffected by changes in Reynolds number and should be kept in mind. Also underutilized in the chemical industry are various flumes that measure the flow of partially full tubes and can pass large floating or settleable solids.

Mass flow or volume flow?

So you want to measure the flow rate? The answer seems to be to buy a flow meter. Defining fluid flow as the amount of fluid flowing through a given location seems simple enough – any flow meter will suffice. However, consider the following equation describing the flow of fluid in a pipe.

Q = A xv

Q is the flow rate, A is the cross-sectional area of the pipe, and v is the average fluid velocity in the pipe. Putting this equation into practice, the flow rate of fluid through a pipe with a cross-sectional area of 1 square meter at an average velocity of 1 meter per second is 1 cubic meter per second. Note that Q is the volume per unit of time, so Q is usually expressed as a “volumetric” flow rate. Now consider the following equation.

W = rho x Q

where W is the flow rate (read again) and rho is the fluid density. Putting this equation into action, when a fluid with a density of 1 kg/m3 is flowing at 1 cubic meter per second, the flow rate will be 1 kg/sec. (You can do the same for the commonly used “pounds”. No need to elaborate – assume pounds is the unit of mass.) Note that W is the mass per unit of time, so W is usually expressed as “mass” “flow rate. Now – which flow rate do you want to measure? Not sure? In some applications, the volumetric flow rate needs to be measured.

Consider filling the tank. Volumetric flow may make sense to avoid overflowing a tank that can be added with different density liquids. (Again, level transmitters and high level switches/shutoffs may not be required for flow meters.) Consider controlling the flow of fluid into a process that can only accept a limited volume per unit of time. Volumetric flow measurement seems to apply.

In other processes, mass flow is important. Consider a chemical reaction that requires the reaction of substances A, B and C. What is of interest is the number of molecules present (their mass), not their volume. Similarly, when buying and selling products (regulatory transfers), mass is important, not quantity.

How much maintenance does a flow meter require?

Many factors can affect maintenance requirements and the expected life of a flow meter. The main factor, of course, is matching the correct instrument to the specific application. Poorly selected devices will always cause problems early on. Flowmeters without moving parts usually require less attention than devices with moving parts. But all flowmeters will eventually require some kind of maintenance.