Flow Meter

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How to Pick the Right Flow Meter 
 

A good flow metre should be chosen based on a clear understanding of what the application needs. So, it's important to take the time to fully evaluate the process fluid and the whole installation. Creating application requirements specifications should be done in a methodical, step-by-step way.

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First Steps 

The first step in choosing a flow sensor is to decide if the flow rate information should be continuous or totalized and if this information is needed locally or remotely. If the transmission is happening from far away, should it be analogue, digital, or shared? And, if it is shared, how often (at least) must the data be updated? Once these questions are answered, the properties and flow characteristics of the process fluid and the piping that will hold the flow metre should be evaluated.

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Selection criteria for a flowmeter

Flow meters are used in many different areas. In order to narrow down the choice for your own requirements, it is advisable to first clarify the measuring conditions.

• Should the flowmeter be permanently installed at one point or should it be applied for the measurements at different points?

• Is the material gaseous, liquid or bulk?

• Does the material flow through a completely filled pipe cross-section?

• If so, may the flowmeter protrude into the pipe cross-section?

• Which flow velocities should be measured?

• What accuracy is expected from the flowmeter?

• Should the measurement be carried out during a running process and should the process be influenced?

• Should the measured values be stored or transmitted?

• Should the flow meter be calibrated and adjustable?

• Below, some flow meters are briefly described according to their operating principle.

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Fluids and How They Move:

The fluid and its pressure temperature, allowable pressure drop, density (or specific gravity), conductivity, viscosity (Newtonian or not? ), and vapour pressure at maximum operating temperature are listed, along with an idea of how these properties might change or interact. Also, all safety or toxicity information should be given, along with detailed information about the fluid's composition, bubbles, solids (rough or soft, particle size, fibres), the tendency to coat, and light transmission qualities (opaque, translucent, or transparent?).

Ranges of Temperature and Pressure

In addition to the normal operating values, the expected minimum and maximum pressure and temperature should be given. Whether flow can go in the opposite direction, if it doesn't always fill the pipe, if slug flow (air-solids-liquid) can happen, if aeration or pulsation is likely, if sudden temperature changes can happen, or if cleaning and maintenance require special precautions, these facts should also be stated.

The Piping and Installation Area 
The following information should be given about the pipes and where the flow metre is to be put: for the piping, its direction (avoid downward flow in liquid applications), size, material, schedule, flange-pressure rating, accessibility, upstream or downstream turns, valves, regulators, and available straight-pipe run lengths.

The specifying engineer needs to know if there are vibrations or magnetic fields in the area or if they could be there. They also need to know if there is electric or pneumatic power if the area is classified for explosion hazards, and if there are any other special requirements, like meeting sanitary or clean-in-place (CIP) regulations.

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Flow Rates and Preciseness 

The next step is to figure out the required metre range by figuring out the minimum and maximum flows (in mass or volume) that will be measured. After that, the required accuracy of the flow measurement is figured out. Most of the time, accuracy is given as a percentage of the actual reading, a percentage of the calibrated span, or a percentage of the full scale. The requirements for accuracy should be stated separately for the minimum, normal, and maximum flow rates. If you don't know about these requirements, your metre may not work well over its entire range.

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Accuracy versus repeatability 

Absolute accuracy is very important when products are sold or bought based on a metre reading. In some situations, absolute accuracy may be less important than being able to do it over and over again. So, it's best to figure out the accuracy and repeatability needs of each application separately and include both in the specifications.

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When the accuracy of a flow metre is given in % CS or % FS units, the absolute error will go up as the measured flow rate goes down. If the metre error is given in % AR, the error is the same whether the flow is high or low. Because full scale (FS) is always bigger than calibrated span (CS), a sensor with a certain FS performance will always have a bigger error than one with the same CS performance. So, to compare all bids fairly, it's best to change all quoted error statements into the same per cent AR units.

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The user should also compare installations by looking at the total error of the loop. For example, the error of an orifice plate is given in percent AR, while the error of the d/p cell that goes with it is given in percent CS or per cent FS. In the same way, a Coriolis meter's error is the sum of two errors, one given in % AR and the other in % FS. To figure out the total error, you take the square root of the sum of the squares of the errors of each component at the flow rates you want.

All accuracy statements in well-written flow metre specifications are converted into uniform % AR units, and the % AR requirements for minimum, normal, and maximum flows are listed separately. All flow metre specs and bids should make it clear how accurate and repeatable the metre is at minimum, normal, and maximum flows.

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Range of Reynolds numbers (Re or RD) that can be used by different types of flow metres. One of the first steps in choosing the right flow metre is figuring out both the minimum and maximum Reynolds numbers for the application. The best time to calculate RD is when flow and density are at their highest and viscosity is at its lowest. On the other hand, you can get the lowest RD by using the lowest flow and density and the highest viscosity.

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If you can get good meeting performance from two different kinds of flow metres but only one of them has moving parts, choose the one without moving parts. Moving parts can cause problems, not only because they wear out, need to be oiled, and are sensitive to coatings, but also because they need clearance spaces that can cause the flow being measured to "slip." Even if the metres are well-maintained and calibrated, this unmeasured flow changes when the viscosity and temperature of the fluid change. Temperature changes also affect how big the inside of the metre is, so it needs to be adjusted.

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Also, if a full flow metre and a point sensor both give the same results, it is usually better to use the full flow metre. Because point sensors don't look at the whole flow, they can only give accurate readings if they are placed at a depth where the flow speed is the average of the speed profile across the pipe. Even if this point is carefully chosen at the time of calibration, it is unlikely to stay the same since velocity profiles change with flow rate, viscosity, temperature, and other factors.

If all other factors are the same but one design loses less pressure, it is best to choose that one. Part of the reason is that the pressure loss will have to be paid for by higher pump or compressor operating costs over the life of the plant. Another reason is that a pressure drop can be caused by any blockage in the flow path, and any blockage in a pipe can lead to material buildup, plugging, or cavitation.

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Mass or volume units

Before specifying a flow metre, it is also a good idea to figure out if the flow information will be more useful in mass or volumetric units. When measuring the flow of materials that can be compressed, the volumetric flow doesn't tell us much unless the density (and sometimes also the viscosity) stays the same. When the speed (volumetric flow) of non-compressible liquids is measured, bubbles in the liquid will throw off the results. To avoid this, air and gas must be taken out of the liquid before it gets to the metre. In other velocity sensors, pipe liners can cause problems (ultrasonic), or the metre may stop working if the Reynolds number is too low (in vortex-shedding metres, RD > 20,000 is needed).

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Mass flow metres, which are not affected by changes in Reynolds number and are not affected by changes in density, pressure, or viscosity, should be kept in mind. In the chemical industry, the different flumes that can measure flow in pipes that are only partially full and can pass large floating or settling solids are also not used enough.

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How to Care for a Flow Meter 

Several things affect how often flow metres need to be fixed and how long they last. The most important thing, of course, is to use the right tool for the job. Poorly chosen devices will always cause problems early on. Flow metres that don't have moving parts usually need less care than those that do. But flow metres all need maintenance at some point.

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In differential pressure flowmeters, connecting the primary elements to the secondary elements requires a lot of piping, valves, and fittings. This means that maintenance may need to be done often on these installations. Impulse lines can get blocked or corroded, so they need to be cleaned or replaced. And measurement mistakes can happen if the second element is not put in the right place. Moving the element can cost a lot of money.

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Flowmeters with moving parts need to be checked inside every so often, especially if the liquid being measured is dirty or thick. By putting filters in front of these units, you can reduce buildup and wear. Instruments with no obstructions, like ultrasonic or electromagnetic metres, may have problems with the electronic parts of their secondary elements. Pressure sensors on secondary elements should be taken out and checked on a regular basis.

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Instruments that don't have any obstructions, like magnetic or ultrasonic units, could also have trouble in situations where coatings might happen. Magnetic flowmeters won't work as well if the coating is insulating and keeps the electrodes from touching the liquid. Regular cleaning will keep this from happening. With ultrasonic flowmeters, the refraction angles can change, and if the coating soaks up sonic energy, the metre will stop working.

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