Instrumentation

Turbine flow meters

Turbine flow meter is a widely used flow measuring device, used in a wide range of industries for both liquids and gases.

Turbine flow meters have the ability to measure wide range of flows. Turndown ratio for turbine flow meters (i.e. the ratio maximum flow / minimum flow that can be measured) can reach values up to 30:1 or even bigger, with good accuracy. Typical error levels can be as low as +- 0.5% of measured flow rate. Accuracy flow measurement is maximized when operating under a turbulent flow profile (high Reynolds number, typically bigger than 10,000).

One of the limitations of turbine flow meters is the fact that they have to be used only with clean, low-viscosity, non-corrosive fluids. Generally, it is common practice to install a strainer upstream to a turbine flow meter for protection against particulates.

Structure of turbine flow meters

The basic structure of turbine flow meters includes a bladed rotor axially suspended in the pipe. As the fluid flows through the pipe, the rotor spins at a speed which is proportional to the fluid's velocity. The rotating blades generate a frequency signal proportional to the liquid flow rate, which is sensed by the magnetic pick-up installed outside of the pipe and transferred to the applicable transducer. Electrical pulses can also be added and totalized so that we get the aggregate flow rate over a specific time period.

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Figure 1 - Basic structure of turbine flow meters

Advantages of turbine flow meter

  • Simple, durable structure
  • Easy to install and maintain
  • Turbine meters are able to operate under a wide range of temperatures and pressures
  • Low pressure drop across the flow meter
  • Most effective in applications with steady, high-speed flows

Disadvantages of turbine flow meters

  • Require constant backpressure in order to avoid cavitation
  • Accuracy adversely affected by bubbles in liquids
  • Sensitive to changes in fluid viscosity
  • A straight run of pipe upstream and downstream the turbine meter needs to be installed to allow homogenisation of the flow pattern
  • Significant bearing wear is another drawback of turbine flow meters. Some manufacturers have developed a ''bearingless design'' in order to resolve this problem.

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