Turbine flow meters measure the rate of flow in a pipe or process line via a rotor that spins as the media passes through its blades. The rotational speed is a direct function of flow rate and can be sensed by a magnetic pick-up, photoelectric cell, or tachometer.

Volumetric vs. Velocity Flow Rates

Turbine flow meters are typically used to measure volumetric flow rates, but may also provide flow velocity. 

Volumetric turbine flow meters measure flow rate in units of volumetric flow, for example, mL/min. 

Velocity turbine flow meters measure flow rate in units of velocity, for example, ft/sec.

Mechanics

Turbine flow meters work analogous to a windmill, a turbine spins on a rotor with an axis of symmetry that is parallel to the flow direction. The flow of media through the flow meter causes the turbine to rotate. As the turbine rotates, each blade of the turbine passes a sensor. The speed at which the turbine rotates is directly proportional to the volumetric flow as well as the rate at which the blades of the turbine pass the sensor.

Image Credit: efunda

Signal

The sensor used to provide a signal can either be a magnet pick up, photoelectric cell or tachometer.

A magnetic pick-up sensor is excited as each blade of the turbine passes, causing an alternating current (AC) to form. The frequency of the alternating current (AC) is proportional to the rate at which the blades of the turbine pass the sensor and ultimately the flow of media through the meter.

A photoelectric cell also senses the motion of the turbine blades. Instead of indirectly measuring an implied current, the sensor produces electrical pulses as the blades pass the sensor. The pulses are counted or totalized over a time period to sense the rotational velocity of the turbine.

A tachometer, like the photoelectric cell, also directly senses the presence of the turbine blades. A tachometer is most often an electromechanical device that produces electrical pulses as the blades pass the sensors and ultimately sense the rotational velocity of the turbine.

Media

Turbine flow meters are ideally suited to handle clean media (gases or liquids) with a low kinematic viscosity. In the ideal application the only force acting on the turbine is the laminar flow of media through the meter. Mixed phase media, viscous drag, and turbulent flow patterns can all interfere with the dynamic behavior of the turbine, which is essential in providing accurate flow measurements.

Viscous Drag

Viscous drag is a retardant force that acts on the boundary layer between fluids and solid surfaces. Viscous fluids, fluids that resist flow, are fluids that have a higher level of internal friction and subsequently exert a significant sheer stress tangent to the direction of flow. This stress causes an increased amount of friction to exist between the fluid and solid surfaces, such as a pipe wall.

Image Credit: Technische Universität Darmstadt

Reynolds Number (Re)

Reynolds number is a dimensionless value used to describe the flow characteristics of a fluid and the tendency to experience a considerable amount of viscous drag. The importance of the Reynolds number is that it determines what flow regimes will exists for fluids of known density and absolute viscosity traveling at a given velocity. A higher value is correlated with turbulent flow regimes while a lower number is correlated with laminar flow regimes. It is defined by the following equation:

Image Credit: Flow Technology

Turbulent vs. Laminar Flow

Laminar and turbulent flow patterns are two flow regimes that describe the flow of media through space.

Laminar flow patterns move through a pipe analogous to a coherent beam of light. There is no interference between fluid layers as they easily flow past one another. They possess a vector gradient where the fluid stream furthest from any retardant forces or solid surfaces moves fastest. In observation there is a parabolic shape to the flow pattern

Turbulent flow patterns have a significant amount of fluid circulation across the fluid stream. The circular flow patterns, or eddy currents, are due to viscous drag and nonlinear forces acting on the fluid stream. The disrupted flow pattern moves as one body while media is constantly circulated across the fluid stream. 

Effect of Turbulent Flow

Turbulent flow causes a retardant force to be exerted on the turbine blades, working against the fluid momentum which provides a driving torque on the turbine. In turbulent flow patterns, fluid slips past the turbine and the rate at which it rotates is no longer proportional to the flow rate. Since viscous drag and turbulent flow is conditional to the absolute viscosity, fluid density, and fluid velocity, turbine flow meters must be properly calibrated to handle a specific media. 

*Turbine flow meters, when properly calibrated and installed, are capable of providing some of the highest accuracies of all flow meters. 

Important Parameters for Turbine Flow Meters

Important parameters to consider when specifying turbine flow meters include velocity flow range, volumetric flow range, operating pressure, maximum fluid temperature, fluid viscosity and pipe diameter. 

• Velocity flow range only applies to turbine flow meters that measure fluid velocity. Fluid velocity is a measurement of distance traveled per unit time.
• Volumetric flow range only applies to turbine flow meters that measure volumetric flow. Volumetric flow is a measurement of the volume of fluid that flows per unit time.
• The operating pressure is the maximum head pressure of the process media the meter can withstand. 
• The maximum fluid temperature is the maximum sustained media temperature that the meter can withstand. It is usually dependent on construction and liner materials. 
• Fluid viscosity rating describes the range of fluid viscosities that the meter can handle.  
• Pipe diameter is important to consider, as the meter should be sized to the piping and mounting options that it is to be used with.

Resources:

Universal Viscosity Curve Theory (PDF)

High Viscosity Flowmeters: Solution to a Sticky Problem

Types of Fluid Flow Meters

Image Credit: CAMERON Valves & Measurement