Turbine Flow Meter Working Principle

What is Turbine Flow Meter?

• A turbine flow meter is a device for sensing a volume of fluid flow. As working fluid such as liquid or gas is moved through the turbine housing the freely suspended turbine blades tend to rotate along its axis.
• The rotor velocity and fluid passing through the flow meter is in direct proportion to each other.
• Turbine flow meters act as flow straightener, stabilize the flow and reduces the negative effects of turbulence.
• The turbine flow meter contains a flow tube with end connections and a multi-bladed magnetic freely rotating impeller fixed inside. 
• The shaft supports a rotor that rests on internally mounted supports.

Magnetic Pickup Sensor

• Magnets in Turbine Flow Meters are fixed to turbine blades, and these magnetic pickup sensors generate the pulse signal.
• For a high flow rate, the rotors rotate very fast hence generating a large number of pulses.

• The voltage level and shape of the pulse signal generated depend upon the type 
• The magnetic pickup device used may be a 2-wire sensor to produce AC output
• Hall Effect sensor is the type of electrical pickup sensing device with a 3-wire to produce clean and uniform square wave pulses.

Volumetric Flow Rate

• As we know, the turbine rotor blade turns at a variable speed based on the flow velocity of the fluid.
• Fluid Velocity is measured as a distance of a particle travelling per unit of time. 
• Fluid Velocity plays a significant role in Turbine flow meter operation.
• But a Turbine flow meter is considered as a measuring element of the turbine flow meter.
• The volumetric Flow rate represents the fluid volume passing at a point in a unit time period.
• The typical Volumetric Flow rate is expressed in units

1. Feet per Second (F/S)
2. Meter per Second. (M/S)
3. Gallons per Minute (GPM)
4. Cubic Meters per Second (M³/S)
5. Cubic Feet per Second (Ft³/S)

The K-Factor

• K-factor is a constant value that consists of the character of substance and pipe such as viscosity, temperature, diameter, etc.
• It is a key term to display pulses per unit volume or mass of the medium.
• It is necessary for accurate display.
• The K factor is indicated as a coefficient in the product certificate after calibration is done. 
• The K-Factor and tag number are unique to every Turbine Flow Meter declared by the manufacturer.
• K-Factor is expressed as the total number of pulse signals such as 200 pulses per gallon of fluid.
• The K-Factor is a unique term that defines actual pulses generated over every time unit.
• The K-Factor of the Turbine Flow Meter is determined using a specific set of parameters.

Flow Meter Frequency

• Consider a Turbine flow meter to measure flow in gallons per minute (GPM) with a K-factor of 3 pulses per gallon.
• We know that the frequency measured in Hertz.
• 1 Hertz of frequency is equal to 1 cycle per second.
• But the Turbine Flow Meter generates a frequency in pulses per second expressed in terms of Hertz.
• Let us consider that we have a K factor of 3 pulses per gallon the output frequency is 10 Hertz for a Volumetric Flow rate of 200 GPM

Installation of a Turbine Flow Meter

• Installation of a turbine flow meter requires about 10 pipe diameters on the upstream side of the straight pipe and 5 pipe diameters on the downstream side of the straight pipe.
• The rotor blade must be fixed at the exact center of the turbine flow meter
• The laminar flow is much crucial that requires a straight vane.
• This Turbine flow meter must be used for cleaning and lubricating fluid because suspended particles present damage to the device.

Cautions for Turbine flow meters

• At lower flow rates these turbine flow meters are less accurate.
• These flow meters must be operated above 5 % of the rated flow.
• This Turbine flow meter’s premature bearing may get damaged operated at high velocity.
• Bearing wear causes the flow meter to become inaccurate during non-lubricating measurements.
• If the bearing is being replaced frequently, it increases maintenance costs.
• Dirty and corrosive fluids must be avoided to reduce the probability of wear and damage in the flow meter. 
• Moving parts of turbine flow meters are subjected to degradation with time and use.
• Sudden Transitions from gas flow to liquid flow must be strictly avoided since these might stress the flow meter, degrade its accuracy, and damage the flow meter. 
• Generally, these situations occur during pipe filling under slug flow.
• Two-phase flow conditions cause inaccuracy in the turbine flow meter.

The downsides of using a Turbine flow meter

• The K-Factor is not constant over the entire flow rate
• These Turbine Flow Meters offer low accuracy at very low flow rates.
• Rotor speed is varied with respect to fluid viscosity and may affect the calibration.

Applications of Turbine flow meters

Turbine Flow Meters can be used for 

1. Steam Service.
2. Water & Waste Water Service
3. Oil & Gas Utility,
4. Chemical Service
5. Power, Food & Beverage,
6. Aerospace, & Pharmaceutical,
7. Metals & Mining
8. Pulp & Paper.

Advantages of Turbine flow meters

1. Construction is simple and durable.
2. Suitable for gases and liquids.
3. Low cost.
4. Installation is easy 
5. Operation is easy.
6. Good performance.
7. Wide variety of process connections
8. Low-pressure drop
9. Provides a convenient signal output

Disadvantages of Turbine flow meters

1. Requires clean fluid.
2. Installation should be done carefully to eliminate errors.
3. Cavitation problems.
4. Bearing degradation and bubbles in liquids affects accuracy.
5. Errors caused by viscosity changes.
6. Require frequent calibration checks
7. Sensitive to changes in fluid viscosity

Frequently Asked Questions

What is a turbine flow meter? How and when is it used?

• A turbine flow meter is a device for sensing a volume of fluid flow. 
• As working fluid such as liquid or gas is moved through the turbine housing the freely suspended turbine blades tend to rotate along its axis.
• Turbine flow meters are used for clean liquid with lower viscosity, dry gases, and liquids for high accuracy and precise measurements.

Can liquid meters measure gas and vice versa?

• No gas content in a liquid meter causes the meter to read high causing catastrophic failures.

What is the maximum working pressure for the turbine flow meter?

• 10 Bar

What happens when the flow goes in the reverse direction in the meter?

• These TFM are bi-directional so that the meter will not damage.
• But the counter reads the total flow for both the forward flow and reverse flow.

How do you size meters for certain applications?

Turbine flow meters are sized by volumetric flow rate. However, the main factors are

1. Fluid Type.
2. Viscosity.
3. Connection.
4. Pipe Sizing.
5. Process Temperature (min & max)
6. Flow Range (min & max)
7. Pressure Range (min & max)
8. Accuracy Range.

What happens to accuracy when you are above or below the range of the meter?

• The bottom section of the flow range is the critical point. Below this point, the K factor changes with a change in a flow rate change.
• Since, each meter has a different performance it cannot be easily predicted.

What is the life expectancy of the meter?

• In oil and gas applications, the flow meter spans for more than 25 years with minimum maintenance.
• But, in other applications, a simple disposable device with a one to two-year lifespan is perfectly acceptable.

Is working and maintenance of turbine meter easy?

• Yes, Turbine Flow Meters are easy to install, maintain, durable and versatile.
• Suitable for gas and liquid measurement. 
• Offers a wide flow and application range ability. 

What is the life span of rotor and vane?

• The life span of the rotor and vane kit depends on application type and meter size. 
• Clean lubricating fluid spans longer than dirty, sandy, non-lubricating products.

Can we use the meter with propane, butane, CO2, and gas?

• Yes – the product must be in the liquid phase.
• It is used above the 60% range of the meter.
• Precautions must be taken to avoid gas flashing that causes catastrophic failures.