The most important class of flow meter design is based on the fluid that is either accelerated or retarded at the measuring section by reducing the flow area and causing a change in the kinetic energy. This change is measured by recording the pressure difference produced. Fluid flow in volume base as the measuring unit of m3/hr i.e. volume of fluid passes out in an hour.
This classification of measuring fluid flow:
Differential Pressure Flow Measurement
DP flow measurement systems detect pressure differences between two points in a pipe. They're commonly used in power plants, oil refineries, and chemical processing.
When designing a DP flow measurement system, it's essential to select a suitable location with minimal turbulence and straight pipe runs. The pressure tap configuration and location should also be carefully chosen to minimize errors. Additionally, the fluid properties, such as density and viscosity, should be considered when selecting the DP flow measurement technology. Proper calibration and maintenance are also crucial to ensure accurate measurements.
Velocity-Based Flow Measurement
By detecting the speed of fluids using techniques like Doppler shift or vortex shedding. They're used in various applications, including wind tunnels, ocean current monitoring, and propulsion systems.
Positive Displacement
The volume of fluid passing through by counting the number of times a rotor or piston displaces a fixed volume of fluid. They're ideal for applications requiring high accuracy, such as fuel dispensing, batching systems, and life support systems.
Electromagnetic
Using electromagnetic fields to detect the flow of conductive fluids. They're commonly used in chemical processing, water treatment, and ballast water management.
Acoustic
This flow measurement systems use high-frequency sound waves to detect fluid flow. They're used in various applications, including gas flow measurement, liquid flow measurement, and custody transfer.
Mass
It measure the mass flow rate and density of fluids. They're commonly used in applications requiring high accuracy, such as custody transfer, viscous fluid measurement, and aerospace.
Turbine-Based
This systems detect fluid flow by measuring turbine rotation. They're used in various applications, including fuel flow measurement, gas flow measurement, and propulsion systems.
Vortex-Based Flow Measurement
By measuring vortices created by fluid flowing around a bluff body. They're used in various applications, including steam flow measurement, gas flow measurement, and chemical processing.
Measuring Devices
Pitot tube:
A small element of fluid is brought to rest at an orifice situated at right angles to the direction of flow. The flow rate is then obtained from the difference between the impact and the static pressure. With this instrument, the velocity of the fluid is measured directly by a small filament of fluid.
Orifice meter:
Fluid is accelerated at a sudden constriction (the orifice) and the pressure developed is measured. This is a relatively cheap and reliable instrument although the overall pressure drop is high because most of the kinetic energy of the fluid at the orifice is wasted.
Venturi meter:
Fluid is gradually accelerated to a throat and gradually retarded as the flow channel is expanded to the pipe size. A high proportion of the kinetic energy is thus recovered but the instrument is expensive and bulky.
Nozzle:
It allows the fluid to flow with increasing velocity gradually until to the end of the throat of the instrument but expansion to pipe diameter is sudden as with an orifice. This is an expensive instrument because of the accuracy required over the inlet section.
Notch or weir:
Fluid flows over the weir so that its kinetic energy is measured by determining the head of the fluid flowing above the weir. This instrument is used in open-channel flow and extensively in tray towers where the height of the weir is adjusted to provide the necessary liquid depth for a given flow. Each of these devices will now be considered in more detail together with some less common and special purpose meters.
Table of Fluid Flow Meters
| Flow Meter Type | Measurement Principle | Range | Situation Applied | Widely Used | Less Maintenance | Perfect Analysis |
|---|---|---|---|---|---|---|
| Differential Pressure (DP) | Measures pressure difference | 0.1-100,000 GPM | High-pressure applications, clean fluids | Oil & Gas, Power Generation | No | Good for simple applications |
| Velocity | Measures fluid velocity | 0.1-100 ft/s | Low-viscosity fluids, small pipe sizes | Chemical Processing, Water Treatment | Yes | Suitable for variable flow rates |
| Positive Displacement (PD) | Measures volume of fluid passing through | 0.01-100,000 GPM | High-accuracy applications, viscous fluids | Chemical Processing, Food & Beverage | No | Ideal for batching and filling applications |
| Magnetic | Measures voltage generated by magnetic field | 0.1-100,000 GPM | Conductive fluids, corrosive environments | Water Treatment, Chemical Processing | Yes | Non-invasive, low maintenance |
| Ultrasonic | Measures time difference between ultrasonic signals | 0.1-100,000 GPM | Clean fluids, small pipe sizes | " " | Yes | Non-invasive, easy to install |
| Coriolis | Measures changes in frequency of vibrating tube | 0.1-100,000 GPM | High-accuracy applications, viscous fluids | Chemical Processing, Oil & Gas | No | Ideal for high-accuracy, low-flow applications |
| Turbine | Measures rotation of turbine | 0.1-100,000 GPM | Clean fluids, small pipe sizes | " " | Yes | Suitable for variable flow rates |
| Vortex | Measures vortices created by fluid flowing around bluff body | 0.1-100,000 GPM | Clean fluids, small pipe sizes | Chemical Processing, Water Treatment | Yes | Non-invasive, low maintenance |
| Flow Meter Type | Calculation Formulas |
|---|---|
| Differential Pressure (DP) | ΔP = ρ * g * h; Q = Cd * √(2 * ΔP / ρ) |
| Velocity | v = ΔP / (ρ * g); Q = v * A |
| Positive Displacement (PD) | Q = V / t; V = π * (d²) / 4 * L |
| Magnetic | E = -N * dΦ/dt; Q = E / (K * B) |
| Ultrasonic | Δt = 2 * L / c; Q = (π * d²) / 4 * v |
| Coriolis | Δf = (2 * m * v) / (L * ρ); Q = Δf * K |
| Turbine | Q = (π * d²) / 4 * v * N; N = f * 60 |
| Vortex | St = (f * d) / v; Q = (π * d²) / 4 * v |
Variables Used:
- ΔP: Pressure difference (Pa)
- ρ: Fluid density (kg/m³)
- g: Acceleration due to gravity (m/s²)
- h: Height of fluid column (m)
- Q: Volumetric flow rate (m³/s)
- C_d: Discharge coefficient
- v: Fluid velocity (m/s)
- A: Cross-sectional area of pipe (m²)
- V: Volume of fluid passing through (m³)
- t: Time (s)
- d: Diameter of pipe (m)
- L: Length of pipe (m)
- E: Voltage generated by magnetic field (V)
- N: Number of turns of coil
- Φ: Magnetic flux (Wb)
- K: Constant of proportionality
- B: Magnetic field strength (T)
- Δt: Time difference between ultrasonic signals (s)
- c: Speed of sound in fluid (m/s)
- m: Mass of fluid (kg)
- f: Frequency of vibrating tube (Hz)
- St: Strouhal number
- N: Number of revolutions per minute (rpm)
- f: Frequency of turbine rotation (Hz)
- ΔP: Pressure difference (Pa)
- ρ: Fluid density (kg/m³)
- g: Acceleration due to gravity (m/s²)
- h: Height of fluid column (m)
- Q: Volumetric flow rate (m³/s)
- C_d: Discharge coefficient
- v: Fluid velocity (m/s)
- A: Cross-sectional area of pipe (m²)
- V: Volume of fluid passing through (m³)
- t: Time (s)
- d: Diameter of pipe (m)
- L: Length of pipe (m)
- E: Voltage generated by magnetic field (V)
- N: Number of turns of coil
- Φ: Magnetic flux (Wb)
- K: Constant of proportionality
- B: Magnetic field strength (T)
- Δt: Time difference between ultrasonic signals (s)
- c: Speed of sound in fluid (m/s)
- m: Mass of fluid (kg)
- f: Frequency of vibrating tube (Hz)
- St: Strouhal number
- N: Number of revolutions per minute (rpm)
- f: Frequency of turbine rotation (Hz)