Flow Measurement Fundamentals | Questions and Answers

Correct Answer: B. Volume per unit time

Volumetric flow rate is the volume of fluid passing through a given cross-sectional area per unit time. It is commonly represented by Q and is expressed in units such as m³/s, m³/h, L/min, or ft³/min.

Correct Answer: C. m³/s

The SI unit of volumetric flow rate is cubic meters per second (m³/s). It represents the volume of fluid transported through a flow section during one second.

Correct Answer: A. Q = Av

Volumetric flow rate is calculated as Q = A × v, where A is the cross-sectional area and v is the average fluid velocity. Therefore, increasing either pipe area or average velocity increases volumetric flow.

Correct Answer: B. Fluid velocity

The variable v represents the average fluid velocity through the flow area. For a known pipe area, increasing velocity directly increases volumetric flow rate.

Correct Answer: C. Mass per unit time

Mass flow rate is the mass of fluid passing a particular point per unit time. It is represented by ṁ and is commonly expressed in kg/s, kg/h, or lb/h.

Correct Answer: A. kg/s

The SI unit of mass flow rate is kg/s. Unlike volumetric flow, mass flow directly represents the amount of matter transported and is therefore useful when fluid density changes significantly.

Correct Answer: D. Orifice meter

An orifice meter is a differential-pressure flowmeter. The restriction created by the orifice causes a pressure difference, and the measured DP is related to flow velocity and therefore flow rate.

Correct Answer: A. Differential pressure

An orifice plate creates a restriction in the pipeline. As fluid passes through the restriction, its velocity and pressure change, producing a measurable differential pressure that can be related to flow.

Correct Answer: D. It increases

As flow velocity increases through the restriction, the pressure difference across the orifice generally increases. For an idealized DP flow measurement, flow is approximately proportional to the square root of differential pressure.

Correct Answer: B. Square root of DP

For a differential-pressure primary element, the fundamental relationship is approximately Q ∝ √DP. This is why a square-root extraction is normally required when converting a DP transmitter signal into a linear flow indication.

Correct Answer: D. Differential pressure

An orifice flow measurement requires the pressure difference between two locations. Therefore, a DP transmitter is commonly connected across the upstream and downstream pressure tapping points.

Correct Answer: C. Create measurable DP

The purpose of the orifice plate is to create a controlled restriction that produces a measurable pressure differential. The relationship between DP and flow allows the flow rate to be calculated.

Correct Answer: A. Magnetic flowmeter

A magnetic flowmeter has an unobstructed flow tube because there is no primary restriction or moving mechanical element inside the flow path. This results in very low permanent pressure loss.

Correct Answer: B. Magnetic flowmeter

A magnetic flowmeter operates according to Faraday’s law of electromagnetic induction. When a conductive fluid moves through a magnetic field, a voltage is induced across electrodes, and this voltage is proportional to fluid velocity.

Correct Answer: C. Electrical conductivity

A magnetic flowmeter requires the process fluid to have sufficient electrical conductivity so that the induced voltage can be detected by the electrodes. Water, wastewater, and many conductive chemicals are suitable applications.

Correct Answer: D. Hydrocarbon oil

Conventional magnetic flowmeters require conductive fluids. Most hydrocarbon oils have very low electrical conductivity, making them generally unsuitable for standard electromagnetic flow measurement.

Correct Answer: A. Moving conductive fluid

When a conductive fluid moves through the magnetic field generated inside the flowmeter, an electrical voltage is induced. The magnitude of this voltage is proportional to the average fluid velocity.

Correct Answer: B. Measuring electrodes

Magnetic flowmeters use electrodes mounted in the flow tube to detect the voltage induced by the moving conductive fluid. The electrodes must make suitable electrical contact with the process fluid.

Correct Answer: C. To sense induced voltage

The electrodes detect the small voltage generated by the moving conductive fluid. Without electrical contact between the fluid and electrodes, the induced flow signal cannot be measured correctly.

Correct Answer: A. Magnetic flowmeter

Magnetic flowmeters are particularly suitable for conductive liquids, including water, wastewater, acids, alkalis, and many slurries. They have no moving parts and introduce very little permanent pressure loss.

Correct Answer: B. Coriolis meter

A Coriolis flowmeter directly measures mass flow using the Coriolis effect created by fluid movement through vibrating measurement tubes. It does not require a separate density measurement to calculate mass flow.

Correct Answer: A. Density

Many Coriolis flowmeters can also determine fluid density by measuring the natural frequency of their vibrating tubes. Some models can also provide process temperature as an additional measurement.

Correct Answer: D. Vortex shedding

The operating principle of a vortex flowmeter is vortex shedding. The frequency of vortices produced behind the bluff body is detected and converted into a flow measurement.

Correct Answer: A. It generally increases

Vortex shedding frequency increases approximately in proportion to flow velocity within the meter’s specified operating range. Therefore, measuring frequency provides an indirect measurement of flow rate.

Correct Answer: C. Doppler method

The Doppler ultrasonic method determines flow from a frequency shift in ultrasonic signals reflected by particles or bubbles moving with the fluid.

Correct Answer: D. Transit-time method

The transit-time method is commonly used for relatively clean liquids. It compares the travel times of ultrasonic signals moving upstream and downstream through the fluid.

Correct Answer: A. Reflective particles

Doppler ultrasonic flowmeters require sufficient reflective particles, suspended solids, or gas bubbles to scatter the ultrasonic signal. The reflected signal experiences a frequency shift related to fluid velocity.

Correct Answer: B. Clamp-on ultrasonic

Clamp-on ultrasonic flowmeters are mounted externally on the pipe and measure flow through the pipe wall. They do not require penetration into the process, making them useful for temporary measurements and some retrofit applications.

Correct Answer: C. Rotameter

A rotameter is a variable-area flowmeter containing a float inside a tapered tube. The float moves to a position where the upward fluid force balances the effective downward forces acting on it.

Correct Answer: D. Variable area

A rotameter operates on the variable-area principle. As the float rises, the annular flow area increases until the forces acting on the float reach equilibrium at a position corresponding to the flow rate.

Correct Answer: A. It rises

Increasing flow produces greater upward force on the float, causing it to rise. The float position is read against a calibrated scale to determine flow rate.

Correct Answer: C. Turbine flowmeter

A turbine flowmeter contains a rotor that spins as fluid passes through the meter. Rotor speed is related to fluid velocity and therefore flow rate.

Correct Answer: D. Flow velocity

In a turbine flowmeter, the rotor speed is related to the fluid velocity through the meter. The instrument electronics detect rotor rotation and convert it into a flow measurement.

Correct Answer: A. Fluid viscosity

Fluid viscosity can affect the rotor’s mechanical behavior and calibration, particularly at low flow rates. Turbine meters therefore require suitable fluid and operating conditions for accurate measurement.

Correct Answer: B. Venturi tube

A Venturi tube consists of a converging section, throat, and diverging section. Fluid velocity increases through the throat, producing a corresponding reduction in static pressure.

Correct Answer: C. It increases

The throat has a smaller cross-sectional area than the upstream pipe. By continuity, the fluid velocity increases as it passes through the smaller area.

Correct Answer: A. Venturi meter

A Venturi meter generally produces lower permanent pressure loss than an orifice plate because its gradual diffuser allows better pressure recovery downstream.

Correct Answer: B. Orifice plate

Orifice plates are mechanically simple, relatively inexpensive, and easy to replace. Their disadvantages include permanent pressure loss and sensitivity to installation and upstream flow conditions.

Correct Answer: C. Permanent pressure loss

An orifice plate creates a restriction that causes permanent pressure loss. Some of the fluid’s pressure energy is not recovered downstream of the restriction.

Correct Answer: B. Bore-to-pipe diameter

The beta ratio, β, is generally defined as the orifice bore diameter divided by the internal pipe diameter, β = d/D. It is an important parameter in DP flow calculations.

Correct Answer: D. Vortex flowmeter

Vortex flowmeters are widely used for steam measurement because they can handle gases, liquids, and steam and have no moving mechanical parts.

Correct Answer: D. Flow regime of fluid

Reynolds number characterizes the flow regime of a fluid by comparing inertial forces with viscous forces. It is used to determine whether flow is predominantly laminar, transitional, or turbulent. A low Reynolds number indicates that viscous forces dominate and the flow tends to be laminar, while a high Reynolds number indicates that inertial forces dominate and the flow tends to be turbulent. This parameter is widely used in flow measurement and fluid-flow calculations.

Correct Answer: C. Fluid velocity

Reynolds number depends on parameters including fluid density, velocity, characteristic diameter, and viscosity. Fluid velocity is therefore one of its important variables.

Correct Answer: C. It increases

For steady incompressible flow, continuity requires Q = Av. If Q remains constant while area decreases, the average velocity must increase.

Correct Answer: D. Continuity equation

The continuity equation represents conservation of mass in fluid systems. For steady incompressible flow, it gives A₁v₁ = A₂v₂.

Correct Answer: B. Bernoulli principle

The Bernoulli principle relates pressure, velocity, elevation, and fluid energy for idealized steady incompressible flow. It is fundamental to the operation of many differential-pressure flow devices.

Correct Answer: D. Thermal mass flowmeter

Thermal mass flowmeters can provide sensitive measurement of gas mass flow and are widely used for applications such as compressed air, nitrogen, natural gas, and other gases.

Correct Answer: A. Mass flow

A thermal mass flowmeter determines mass flow by measuring heat transfer from a heated sensor to the flowing gas. Its output is related to the mass flow rate rather than simply the volumetric flow.

Correct Answer: B. Heat transfer

Thermal mass meters use heat transfer between a heated sensing element and the flowing fluid. Increasing mass flow generally increases the rate at which heat is carried away from the sensor.

Correct Answer: A. Coriolis flowmeter

Many Coriolis flowmeters can provide mass flow and density from the same measurement principle. Density is derived from the resonant behavior of the vibrating measurement tubes.

Correct Answer: B. To stabilize flow profile

Upstream fittings such as elbows, valves, reducers, and tees can distort the velocity profile. Adequate straight pipe length allows the flow profile to become more stable and improves measurement accuracy.

Correct Answer: C. Flow-profile distortion

An upstream elbow can create swirl and an asymmetric velocity profile. If the flowmeter is sensitive to the velocity profile, this distortion can introduce significant measurement error.

Correct Answer: D. To improve flow profile

Flow conditioners reduce swirl and help establish a more suitable velocity profile before the flow reaches the meter. They can be useful when available straight-run piping is limited.

Correct Answer: B. No moving parts

Ultrasonic flowmeters have no moving mechanical parts, reducing mechanical wear. Many designs can also be installed without cutting into the pipe, especially clamp-on versions.

Correct Answer: D. Trapped fluid volume

Positive displacement flowmeters repeatedly trap and pass known volumes of fluid. The number of displacement cycles per unit time is therefore related directly to volumetric flow.

Correct Answer: B. Have moving parts

Positive displacement meters contain mechanical elements that repeatedly isolate and transport known quantities of fluid. Their moving parts require consideration of wear, maintenance, and fluid cleanliness.

Correct Answer: D. Maximum-to-minimum flow

Flowmeter turndown is the ratio between the maximum specified measurable flow and the minimum specified measurable flow while maintaining the required performance. A larger turndown provides greater usable measurement range.

Correct Answer: A. Meter output at no flow

Zero-flow verification checks the meter’s output when the actual flow is zero. It is useful for detecting zero offsets, installation problems, trapped flow, or other conditions that can cause measurement errors.

Correct Answer: C. It increases

A greater restriction produces a larger pressure difference for a given flow condition. Therefore, increasing the restriction generally increases the DP generated by the primary element.

Correct Answer: C. Coriolis force

Coriolis flowmeters use the Coriolis effect. When fluid flows through vibrating tubes, the resulting Coriolis forces create measurable tube deformation or phase differences that are proportional to mass flow.

Correct Answer: D. Mass flow

A major advantage of a Coriolis meter is its ability to directly measure mass flow rate. The transmitter detects the effect of fluid mass moving through the vibrating tubes.

Correct Answer: B. Coriolis meter

Coriolis flowmeters are widely recognized for high-accuracy direct mass-flow measurement. They are commonly selected where accurate mass flow, density measurement, or both are important.

Correct Answer: C. Vortex flowmeter

A vortex flowmeter places a bluff body in the flow. As fluid passes the body, alternating vortices are shed downstream. The vortex shedding frequency is related to flow velocity.

Correct Answer: D. Measure flow using DP

A flow nozzle is a differential-pressure primary element. It creates a controlled restriction, and the resulting pressure difference is used to determine flow rate.

Correct Answer: A. Flow nozzle

Flow nozzles are mechanically robust and are commonly used in applications involving high-velocity fluids, including steam and other industrial services.

Correct Answer: A. Laminar flow

In laminar flow, fluid particles generally move in relatively smooth, orderly layers with limited mixing between adjacent layers. Viscous forces dominate the flow behavior.

Correct Answer: B. Turbulent flow

Turbulent flow is characterized by irregular fluctuations, eddies, and strong mixing. Industrial flowmeters often have specified Reynolds-number requirements because flow regime affects their performance.

Correct Answer: A. Vapor bubble formation

Cavitation occurs when local liquid pressure falls below the liquid’s vapor pressure, causing vapor bubbles to form. When the bubbles enter higher-pressure regions and collapse, they can cause noise, vibration, erosion, and equipment damage.

Correct Answer: B. Error and equipment damage

Cavitation can produce strong vibration and unstable flow conditions. It may also damage internal components and cause inaccurate or erratic flow measurement.

Correct Answer: C. Vapor remains after pressure drop

Flashing occurs when liquid pressure falls below vapor pressure and vapor forms, but the vapor remains present because downstream pressure does not recover sufficiently to collapse the bubbles.

Correct Answer: D. No density conversion needed

Direct mass-flow measurement eliminates the need to calculate mass flow from volumetric flow and separately measured density. This can simplify measurement where density varies with pressure or temperature.

Correct Answer: C. Pressure not recovered downstream

Permanent pressure loss is the portion of pressure loss that remains after the flow passes through a restriction and downstream pressure recovery has occurred. It represents lost pressure energy in the system.

Correct Answer: D. Density changes with pressure

Steam density changes significantly with pressure and temperature. If a volumetric flow measurement is used, pressure and temperature compensation may be necessary to calculate accurate mass flow.