Fluid flow measurement technique

1.    Introduction

Fluid flow measurement is classified into several types since each type necessitates careful consideration of variables such as accuracy requirements, cost considerations, and the use of flow information to achieve the desired end results. The nature of the fluid to be measured must be considered when deciding on the best form of meter to measure a particular flow. The characteristics of the flow are also essential. The best flow measurement is needed in custody transfer metering so that the two parties to the transaction are handled fairly. This chapter discusses the factors to consider when selecting a meter to measure fluids in various circumstances.

2. Flow Metering Principals

a. Differential Pressure Flow meters

b. Velocity Flowmeters

c. Positive Displacement Flow meters

d. Mass Flow meters

e. For Open Channel Flowmeters - weirs, flumes, submerged orifices, current meters, acoustic flow meters and more

3. Differential pressure flow meter

The  flow is determined in a differential pressure drop device by measuring the pressure drop over an obstruction inserted in the flow. The differential pressure flow meter is founded on the Bernoulli Equation, which states that the pressure drop and subsequently measured signal are proportional to the square flow speed.

dp = ρ v2 / 2

where

dp = pressure difference (Pa, psi)

ρ = density of fluid (kg/m3, slugs/ft3)

v = flow velocity (m/s, in/s)

Common types of differential pressure flow meters are:

·       Orifice plate

Fluid flow is evaluated using an orifice plate by measuring the pressure differential between the upstream and downstream sides of a partially obstructed pipe. The flow-obstructing plate provides an exactly measured obstruction, narrowing the pipe and forcing the flowing fluid to constrict.

The orifice plates are simple, inexpensive, and can be supplied in almost any material and for almost any application. For aperture plates, the Turn Down Ratio is less than 5:1. At modest flow rates, their accuracy is poor. A good orifice plate with a sharp edge on the upstream side is required for high precision. Wear will decrease precision.

In industrial uses, flow nozzles are frequently used as measuring elements for air. The flow nozzle is relatively simple and Inexpensive, and it is available in a variety of materials for a wide range of uses. The orifice plate can be used to measure the Turn Down Ratio and accuracy.

·  Critical (Choked) Flow Nozzle - Sonic Nozzle

When a gas is accelerated through a nozzle, its velocity increases while its pressure and density drop. The highest velocity is attained at the throat, the smallest area, where it exceeds Mach 1 or sonic speed. At this juncture, increasing the flow by lowering the downstream pressure is not an option. The movement is obstructed. Many control systems use this scenario to keep the constant, accurate, and repeatable gas flow.

· Venturi meter

Because of its simplicity and dependability, the Venturi tube flowmeter is frequently used in applications requiring greater Turn Down Ratios or lower pressure drops than an orifice plate can provide.

The fluid flowrate in the Venturi Tube is measured by reducing the cross-sectional flow area in the flow path, resulting in a pressure differential. Following the constricted region, the fluid is routed through a pressure recovery exit section, which recovers up to 80% of the differential pressure generated at the constricted area.

The Venturi Tube flowrate can be reduced to about 10% of its full-scale range with appropriate instrumentation and flow calibration. This results in a 10:1 Turn Down Rate.

· Rotameter

The rotameter is made up of a vertically aligned glass (or plastic) tube with a larger end at the top and a free-moving metering float within the tube. The float rises in the tube due to fluid flow because the upward pressure differential and buoyancy of the fluid overcome the impact of gravity.

The float rises until the annular area between the float and tube grows large enough to enable a state of dynamic equilibrium between the upward differential pressure and buoyancy factors and the downward gravity factors.

The movement rate is indicated by the height of the float. The tube can be calibrated and graded in flow units of choice.

The rotameter meter has a TurnDown Ratio of up to 12:1. The precision may vary.

4.Velocity flow meter

·        Calorimetric Flowmeter

The calorimetric principle for measuring fluid flow is based on two temperature sensors that are in direct contact with the fluid but thermally insulated from each other.

One of the two sensors is continuously heated, and the cooling effect of the flowing fluid is used to measure the flowrate. There is a constant temperature difference between the two temperature sensors in a stationary (no flow) fluid state. When the fluid flow rises, heat energy is drawn from the heated sensor, and the temperature difference between the sensors decreases. The reduction is proportionate to the fluid flow rate. Due to the thermal conductivity of the fluid, response times will differ. In general, lesser thermal conductivity necessitates higher velocity for appropriate measurement.

·      Pitot tube

The pitot tube is one of the most common (and least expensive) methods of measuring fluid flow, particularly in air uses such as ventilation and HVAC systems and is even used in aeroplanes for speed measurement.

The pitot tube detects fluid flow velocity by converting the flow's kinetic energy to potential energy. The pitot tube can only be used for point measurements. The dynamic pressure can be measured across the velocity profile using the "annubar," or multi-orifice pitot probe, and the annubar gets an averaging effect.

5. Mass flow meter

Mass meters measure the mass flow rate directly.

·    Thermocouple Flowmeter

The thermal mass flowmeter is not affected by density, pressure, or friction. Thermal metres make use of a heated sensing element that is isolated from the fluid flow route and transfers heat from the sensing element. The temperature differential is calculated to mass flow and is directly proportional to the conducted heat. The precision of the thermal mass flow device is determined by the process's calibration reliability and variations in temperature, pressure, flow rate, heat capacity, and viscosity.

·  Flowmeter with Coriolis Rotation

Direct mass estimation distinguishes Coriolis flowmeters from other technologies. Changes in pressure, temperature, viscosity, and density have no effect on mass determination. Coriolis flowmeters are capable of measuring liquids, slurries, and vapours.

6. Open channel flowmeters

The height of the liquid as it passes over an obstruction in the channel, such as a flume or weir, is a common way of measuring flow through an open channel.

7. Reference

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