This type of meter is one of the most accurate that have been developed. For this reason, its use is widespread in measurement applications count refined oil flow. Accurate models are expensive to manufacture and calibrate.

However, other cheaper models are available for water consumption and routine applications of flow measurement in industrial plants. All models are characterized by their high level of repeatability, but are sensitive to the disruptive effects due to fluid properties and flow.

Measuring principle

The following figure shows the basic elements. All types of turbine meters consist of a set of rotating blades fixed to a central pivot. The group is mounted in the center of the body of the flowmeter. The fluid kinetic energy is transmitted to the turbine wheel, which rotates with a velocity proportional to flow. The turbine wheel is known as the "rotor" in the conventional turbine meters and "reel" in mechanical meters.

Figure: Example of a turbine meter. Clearly seen with the rotor (propeller turbine) and the induction sensor, which has the rotor turns.

The rotor speed is counted by mechanical or inductive depending on model. In conventional turbine meters, each time a propeller blade passes the sensor, it generates a pulse corresponding to a fixed volume of fluid. The number of pulses gives the amount of fluid that has circulated in a known time interval and frequency of the pulses is an indicator of the flow velocity.

The rotor shaft is generally parallel to the direction of fluid flow. On some models, however, the rotor is mounted vertically to the direction of flow (see Figure below). The blades are inclined at an angle (p) with respect to the direction of fluid flow to exert a torque on the rotor. The volume flow is calculated resulting from the rotation, according to the following expression:

This simple expression shows that the number and shape of the turbine blades are the most important factors in the speed of the rotor. Moreover, the fluid velocity is not constant for the entire diameter of the pipe. This allows us to observe that the forces acting on the turbine blades are complex. The higher speed is generated near the center and at the tips is a certain drag. The balance between driving force and the drag force (which also contributes to the friction of the pivots) rotor speed remains constant for a fixed rate.

The theory can write a general expression that relates the number of pulses generated (n) and flow (Q). The following equation expresses this relationship:

The first term of the second member (A) depends on the linear momentum is the key term for high speed flow. The second term (B / Q) accounts for the effects of viscosity and flow at the ends of the blades. Becomes important in the lower third of the curve. The last sum (C/Q2) depends on the mechanical drag forces, aerodynamic and pivots on the sensor. It is the dominant adding flow at low speeds and is a term of delay. The relative balance of these three summands corresponds to a low value of n / QPAR low flow increases to a maximum ("hump") and then tends to a constant value of n / Q to high flows. The complete theory is actually much more complex because the influence of the velocity profile and the blade geometry of complicated calculation involving integrals to find the values ​​of the constants A, B and C of the equation. The figure below shows the characteristic curve of the turbine flowmeter. The pressure drop (Ap) corresponds to the equation Bemouffi, increasing with the square of the flow Q.

Figure: A typical characteristic curve of the turbine flowmeter. A = initial flow, b - Minimum flow for repeatability, c = minimum flow linear behavior, d = position of maximum count factor (hump), e = field linear behavior measurement values, shaded area = area of linear behavior

From the above it follows that the turbine meters are sensitive to the effects of viscosity, particularly in relation to the different models of blades. The blades at right angles to provide greater angular velocity, but the spiral (oblique) are much less sensitive to the effects of viscosity. All models should be kept below a maximum value of 30 cP viscosity or, otherwise, the counter loses linear behavior. The following figure shows the change in the characteristic curve for two types of blade for various viscous fluids.

Ill.: Characteristic curves of turbine meters (solid line) for different viscosities. Bottom (B): Effect of viscosity on rotors with helical blades

In general, small counters (DN <50 / 2 ") are more affected by the viscosity and do not reach values ​​as fields as wide as the great, even on thin liquids and light oils. This is because the moments Delay and power are proportionally higher pivot.