A flow meter is a mechanical tool used to measure linear, nonlinear, mass, or volumetric flow rate of liquid and gas.
In selecting an ideal flow meter, you should consider particular intangible factors like the familiarity of plant personnel, their experience in calibration, participation in maintenance, availability in spare parts, mean time between failure history, and so forth. More than that, it is recommended to compute the installation cost only after taking these steps.
Among the most common flow assessment mistakes is doing the sequence in a reverse manner. There are instances where less expensive devices are bought rather than choosing a flow sensor that will perform adequately. Surprisingly, those inexpensive purchases can be the most costly. Keep on reading this article to understand the essential factors to consider to choose the right flow meter.
1st Step: Determining and Evaluating
The first step is to determine the flow rate information, like should it be continuous or totalized? Is the information required locally or remotely? If remotely, should it be in analog, digital, or shared? And if shared, what is the minimum necessary data-update frequency? Once these have been determined, you can evaluate the following:
➢ Fluid and Flow Characteristics
The fluid and flow characteristics should be analyzed crucially. You need to check and evaluate the following features:
- admissible pressure drop
- given vapor pressure at maximum operating temperature
- fluid’s composition
- presence of bubbles
- presence of solids
- tendency to coat
- light transmission qualities
➢ Temperature and Pressure Ranges
The expected maximum and minimum temperature and pressure values should also be considered to the average operating values when opting for flow meters.
- whether the flow can reverse
- whether it can develop slug flow
- whether there’s pulsation or aeration
- whether it does not fill the pipe regularly
- whether a sudden change in temperature can occur or
- whether distinct precautions are required during maintenance
➢ Piping and Installation Area
When it comes to piping, the material, direction, size, schedule, and accessibility are needed to be looked at. Along with up or downstream turns, flange-pressure rating, regulators, valves like high-performance butterfly valves, and available straight-pipe run lengths. For installation, the specifying engineer should know:
- if magnetic fields or vibration are present, or likely
- if pneumatic or electric power is available if the place is classified for explosion hazards or
- if there are other exclusive requirements, like compliance with sterilization-in-place (SIP) or clean-in-place (CIP) procedures.
2nd Step: Flow Rate Accuracy
The next step is to see the required meter range through classifying the maximum and minimum flows that will be measured. Only then will you determine the precise required flow measurement.
Generally, the accuracy is specified in percentage of actual reading (AR), calibrated span (CS), or full scale (FS). The precision requirements should be separately quantified at minimum, average, and maximum flow rates. Unless you are aware of these requirements, your flow meter’s operation may not be acceptable over its full limit.
In applications where devices are sold or purchased based on a meter reading, absolute accuracy is essential. Yet, there may be instances where repeatability may be more critical than absolute accuracy. Hence, it is advisable to establish individual requirements for accuracy and repeatability and state both in the specifications.
Flow Meter Accuracy
If the flow meter accuracy is stated in percentage CS or percentage FS units, its absolute error as the measured flow rate drops. But if meter error is indicated in percentage AR, the error in absolute terms stays the same at low or high flows. Since FS is always larger in quantity compared to CS, a sensor with percentage FS performance will always have more significant error than the one with a similar percentage CS specification. Thus, to correlate all bids fairly, it is advisable to convert all error statements into the same percentage AR units.
In well-equipped flow sensor specifications, all precision statements are converted into uniform percentage AR units, and these percentage AR requirements are separately specified for maximum, average, and minimum flows. All flow meter specifications and bids should state both the repeatability and accuracy at maximum, average, and minimal flows.
Step 3: Accuracy vs. Repeatability
If acceptable metering performance can be achieved from two different flow meter and one of them has no moving parts, better select the latter. Moving parts can be a potential source of problems since they require clearance spaces that can sometimes show slippage into the flow that is being measured. Know that even well maintained and calibrated meters, the unmeasured flow differs with changes in fluid temperature and viscosity.
Also, if you can get the same performance from both point sensors and full flow meter, it is advisable to use the latter. This is because point sensors do not detect full flow; they read precisely only if the sensors are inserted to the extent where the flow velocity is the average of the velocity profile over the pipe.
Step 4: Choosing Volumetric or Mass Unit
It is recommended to know if the flow information will be more useful if shown in volumetric or mass units. In measuring the flow of compressible components, volumetric flow is not very meaningful, unless the density – and sometimes viscosity – is constant.
When the velocity of incompressible liquids is measured in volumetric flow, the indication of suspended bubbles will cause an error. Therefore, gas and air should be removed before the fluid reaches the meter. In other velocity meters, pipe liners can cause ultrasonic issues, or the meter may halt functioning if the Reynolds number is too low – however, mass flow meters are not affected by changes in Reynolds number.
To sum it up, choose a flow meter that is suitable for your plant operation. It is imperative to determine the fluid and flow characteristics, temperature and pressure ranges, as well as piping and area of installation for your evaluation. Flow rate accuracy and units to be used should also be regarded. If you are still undecided and having a hard time in decision-making, it would be best to seek help from a reliable flow rate manufacturer.