How Flowmeters Work

Overview of Flow Measurement

Flowmeters are devices that measure the amount of liquid, gas or vapor that passes through them. Some flowmeters measure flow as the amount of fluid passing through the flowmeter during a time period (such as 100 liters per minute). Other flowmeters measure the totalized amount of fluid that has passed through the flowmeter (such as 100 liters).

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Flowmeters consist of a primary device, transducer and transmitter. The transducer senses the fluid that passes through the primary device. The transmitter produces a usable flow signal from the raw transducer signal. These components are often combined, so the actual flowmeter may be one or more physical devices.

Flow measurement can be described by

Q = A · v, which means that the volume of fluid passing through a flowmeter is equal to the cross-sectional area of the pipe (A) times the average velocity of the fluid (v); and

W = r · Q, which means that the mass flow of fluid passing through a flowmeter (A) is equal to the fluid density (r) times the volume of the fluid (Q).

Volumetric flowmeters directly measure the volume of fluid (Q) passing through the flowmeter. The only flowmeter technology that measures volume directly is the positive displacement flowmeter.

Velocity flowmeters utilize techniques that measure the velocity (v) of the flowing stream to determine the volumetric flow. Examples of flowmeter technologies that measure velocity include magnetic, turbine, ultrasonic, and vortex shedding and fluidic flowmeters.

Mass flowmeters utilize techniques that measure the mass flow (W) of the flowing stream. Examples of flowmeter technologies that measure mass flow include Coriolis mass and thermal flowmeters.

Inferential flowmeters do not measure volume, velocity or mass, but rather measure flow by inferring its value from other measured parameters. Examples of flowmeter technologies that measure inferentially include differential pressure, target and variable area flowmeters.

Flow computers are often used to compensate flow measurements for actual process conditions, such as pressure, temperature, viscosity, and composition.

Additional flowmeter technologies include flowmeters that measure liquid flowing in an open channel, and insertion flowmeters that measure flow at one location in a pipe and use this measurement to infer the flow in the entire pipe. Insertion flow measurement systems often use a flow computer to compensate for hydraulic effects.

Installation Cautions for Flowmeters

In liquid service, be sure that the flowmeter is installed such that it remains full of liquid, because gas/vapor in the flowmeter can alter its geometry and adversely affect accuracy.

In gas/vapor service, be sure that the flowmeter is installed such that the flowmeter remains full of gas/vapor, because liquid in the flowmeter can alter its geometry and adversely affect accuracy.

Disturbances located upstream (and sometimes downstream) of the flowmeter, such as pipe elbows and control valves, can adversely affect measurement accuracy, because the flowmeter may not be able to accurately measure disturbed flow streams. Be sure to locate control valves downstream of the flowmeter so their flow disturbances are not introduced directly into the flowmeter (as they would be if located upstream). Also, be sure to properly design the upstream and downstream piping with sufficient straight run to remove disturbances that can affect measurement accuracy.

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Be especially careful when flow is two-phase, such as liquid/gas flow and liquid/solid flow, because these flows can adversely affect the accuracy of many flowmeters. Be careful because some flowmeters can become plugged and stop working in liquid/solid flow streams.

Application Cautions for Flowmeters

Each type of flowmeter has its own specific applications and installation constraints. There is no "one size fits all" flowmeter. The way to select the right flowmeter is to use the application as your guide, not the technology. Many of these technologies will all work well on many applications. If you start with the application, you can select the technology you wish to use based on accuracy, cost, durability and reliability, rather than trying to make the technology you chose fit the application you have.

Measuring the flow of liquids is a critical need in many industrial applications. In some operations, the ability to conduct accurate flow measurements is so important that it can make the difference between making a profit or taking a loss. In other cases, inaccurate flow measurements &#; or failure to take measurements &#; can cause serious (or even disastrous) results.

With most liquid flow measurement instruments, the flow rate is determined inferentially by measuring the liquid&#;s velocity or the change in kinetic energy. Velocity depends on the pressure differential that is forcing the liquid through a pipe or conduit. Because the pipe&#;s cross-sectional area is known and remains constant, the average velocity is an indication of the flow rate. The basic relationship for determining the liquid&#;s flow rate in such cases is:

Q = V x A

Where

Q = Liquid flow through the pipe

V = Average velocity of the flow

A = Cross-sectional area of the pipe

Other factors that affect liquid flow rate include the liquid&#;s viscosity and density, as well as the friction of the liquid in contact with the pipe.

What is a Flow Meter?

A flow meter (or a flow sensor) is type of flow instrument that is used to indicate the amount of liquid, gas, or vapor moving through a pipe or conduit by measuring linear, non-linear, mass, or volumetric flow rates. Since flow control is often essential, measuring the flow of liquids and gasses is a critical need for many industrial applications &#; and there are many different types of flow meters that can be utilized depending on the nature of the application.

When choosing a flow meter, one should consider such intangible factors as familiarity of plant personnel, their experience with calibration and maintenance, spare parts availability, and meant time between failure history, etc., at the particular plant site. It is also recommended that the cost of the installation be computed only after taking these steps. One of the most common flow measurement mistakes is the reversal of this sequence: instead of selecting a sensor which will perform properly, an attempt is made to justify the use of a device because it is less expensive. Those &#;inexpensive&#; purchases can be the costliest installations.

How to Choose a Flow Meter

The basis of good flow meter selection is a clear understanding of the requirements of the particular application. Therefore, time should be invested in fully evaluating the nature of the process fluid and of the overall installation. The development of specifications that state the application requirements should be a systematic, step-by-step process.

Initial Steps

The first step in the flow sensor selection process is to determine if the flowrate information should be continuous or totalized, and whether this information is needed locally or remotely. If remotely, should the transmission be analog, digital, or shared? And, if shared, what is the required (minimum) data-update frequency? Once these questions are answered, an evaluation of the properties and flow characteristics of the process fluid, and of the piping that will accommodate the flow meter, should take place (Table 1).

Fluid and Flow Characteristics

The fluid and its pressure temperature, allowable pressure drop, density (or specific gravity), conductivity, viscosity (Newtonian or not?), and vapor pressure at maximum operating temperature are listed, together with an indication of how these properties might vary or interact. In addition, all safety or toxicity information should be provided, together with detailed data on the fluid&#;s composition, presence of bubbles, solids (abrasive or soft, size of particles, fibers), tendency to coat, and light transmission qualities (opaque, translucent, or transparent?).

Pressure and Temperature Ranges

Expected minimum and maximum pressure and temperature values should be given in addition to the normal operating values. Whether flow can reverse, whether it does not always fill the pipe, whether slug flow can develop (air-solids-liquid), whether aeration or pulsation is likely, whether sudden temperature changes can occur, or whether special precautions are needed during cleaning and maintenance, these facts, too, should be stated.

Piping and Installation Area

Concerning the piping and the area where the flow meter is to be located, the following information should be specified: For the piping, its direction (avoid downward flow in liquid applications), size, material, schedule, flange-pressure rating, accessibility, up or downstream turns, valves, regulators, and available straight-pipe run lengths.

In connection with the area, the specifying engineer must know if vibration or magnetic fields are present or possible, if electric or pneumatic power is available, if the area is classified for explosion hazards, or if there are other special requirements such as compliance with sanitary or clean-in-place (CIP) regulations.

Flow Rates and Accuracy

The next step is to determine the required meter range by identifying minimum and maximum flows (mass or volumetric) that will be measured. After that, the required flow measurement accuracy is determined. Typically, accuracy is specified in percentage of actual reading (AR), in percentage of calibrated span (CS), or in percentage of full scale (FS) units. The accuracy requirements should be separately stated at minimum, normal, and maximum flowrates. Unless you know these requirements, your meter&#;s performance may not be acceptable over its full range.

Accuracy vs Repeatability

In applications where products are sold or purchased on the basis of a meter reading, absolute accuracy is critical. In other applications, repeatability may be more important than absolute accuracy. Therefore, it is advisable to establish separately the accuracy and repeatability requirements of each application and to state both in the specifications.

When a flow meter&#;s accuracy is state in % CS or % FS units, its absolute error will rise as the measured flow rate drops. If meter error is stated in % AR, the error in absolute terms stays the same at high or low flows. Because full scale (FS) is always a larger quantity than the calibrated span (CS), a sensor with a % FS performance will always have a larger error than one with the same % CS specification. Therefore, in order to compare all bids fairly, it is advisable to convert all quoted error statements into the same % AR units.

It is also recommended that the user compare installations on the basis of the total error of the loop. For example, the inaccuracy of an orifice plate is stated in % AR, while the error of the associated d/p cell is in % CS or % FS. Similarly, the inaccuracy of a Coriolis meter is the sum of two errors, one given in % AR, the other as a % FS value. Total inaccuracy is calculated by taking the root of the sum of the squares of the component inaccuracies at the desired flow rates.

In well-prepared flow meter specifications, all accuracy statements are converted into uniform % AR units and these % AR requirements are specified separately for minimum, normal, and maximum flows. All flow meter specifications and bids should clearly state both the accuracy and the repeatability of the meter at minimum, normal, and maximum flows.

Table 1 provides data on the range of Reynolds numbers (Re or RD) within which the various flow meter designs can operate. In selecting the right flow meter, one of the first steps is to determine both the minimum and the maximum Reynolds numbers for the application. Maximum RD is obtained by making the calculation when flow and density are at their maximum and viscosity at its minimum. Conversely, the minimum RD is obtained by using minimum flow and density and maximum viscosity.

If acceptable meeting performance can be obtained from two different flow meter categories and one has no moving parts, select the one without moving parts. Moving parts are a potential source of problems, not only for the obvious reasons of wear, lubrication, and sensitivity to coating, but also because moving parts require clearance spaces that sometimes introduce &#;slippage&#; into the flow being measured. Even with well-maintained and calibrated meters, this unmeasured flow varies with changes in fluid viscosity and temperature. Changes in temperature also change the internal dimensions of the meter and require compensation.

Furthermore, if one can obtain the same performance from both a full flow meter and a point sensor, it is generally advisable to use the flow meter. Because the point sensors do not look at the full flow, they read accurately only if they are inserted to a depth where the flow velocity is the average of the velocity profile across the pipe. Even if this point is carefully determined at the time of calibration, it is not likely to remain unaltered, since velocity profiles change with flowrate, viscosity, temperature, and other factors.

If all other considerations are the same, but one design offers less pressure loss, it is advisable to select that design. Part of the reason is that the pressure loss will have to be paid for in higher pump or compressor operating costs over the life of the plant. Another reason is that a pressure drop is caused by any restriction in the flow path, and wherever a pipe is restricted becomes a potential site for material build-up, plugging, or cavitation.

Mass or Volumetric Units

Before specifying a flow meter, it is also advisable to determine whether the flow information will be more useful if presented in mass or volumetric units. When measuring the flow of compressible materials, volumetric flow is not very meaningful unless density (and sometimes also viscosity) is constant. When the velocity (volumetric flow) of incompressible liquids is measured, the presence of suspended bubbles will cause error, therefore, air and gas must be removed before the fluid reaches the meter. In other velocity sensors, pipe liners can cause problems (ultrasonic), or the meter may stop functioning if the Reynolds number is too low (in vortex shedding meters RD > 20,000 is required).

In view of these considerations, mass flow meters, which are insensitive to density, pressure, and viscosity variations and are not affected by changes in the Reynolds number, should be kept in mind. Also underutilized in the chemical industry are the various flumes that can measure flow in partially full pipes and can pass large floating or settleable solids.

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Maintaining a Flow Meter