Measuring principle

A variable flow section is a relatively simple and effective flow measurement of gases and fluids. The device consists of a graduated vertical pipe up inside which there is a glass or metal float suspended in the fluid, which enters from below. The force exerted by the fluid on the float depends on the density, viscosity and fluid velocity.

Designs of variable area flowmeters

The higher the flow rate and, therefore, the force that develops, the higher the float will float inside the graduated cylinder. The space between the float and the tube wall expands as the float moves upward until the forces acting on the float and float balance remains suspended at a constant height (Fig. below).

Figure: Principle of a flowmeter measurement variable section.

The volume flow (Qv) is calculated from the following expression:

By introducing the flow coefficient

The expression for the flow rate Qv is finally:

Flowmeters tapered obey the same physical phenomenon of differential pressure flowmeters. However, the measurement principles that underlie both UPOS accountants are opposed. In the differential pressure meter area is constant and a variation in the differential pressure indicating a change in flow rate. In the variable section counters, however, the differential pressure experienced in float is constant and a variation in the cross section (the position of the float inside the tube) indicates a change in fluid velocity.

If the tube is glass itself fulfills the function of sight and lets you read the volume directly. Provided, of course, the fluid is clear, because otherwise the float is not visible.

If the tube is metallic, the position of the float can be transmitted on magnetic media on a scale outside of the tube or by magnetic induction detected and converted into electrical signal.

Some variable-section flow meters also incorporate integrated level switches or analog output for connection to a flow indicator. In addition to the counters usual graduated glass tube in many applications, all types of units can also be found in highly resistant materials to processing conditions harsh. The conventional range of materials for the measuring tube (and the liner) include steel, stainless steel, plastic (PP, PTFE), glass and hard rubber. The variable section counters are equipped with high-level microprocessors and can be operated from indicators / special keyboards.

Float Designs

The float must be maintained properly focused on the measuring tube, otherwise the flow will not give an accurate reading. For this purpose, inside the tube there are some guidelines, or the head of the float is grooved so that the fluid flow forces you to rotate to maintain its inherent stability. For this reason, this type of flow meter is also known by the name of Rotameter. The float multiple models and forms of the measuring tubes to tailor the field measured values ​​of various process conditions (Fig. below):

- You can choose a specific model and a float that takes into account the effects of fluid viscosity reading. It has a wide variety of materials for the float, because the density of this component to determine the value field of measurement stainless steel, tantalum, glass, synthetic sapphire, plastic (PE, PTFE, hard rubber, PVC) , monel and nickel, as well as special alloys.

- The sampling probe pressure in the measuring tube linearized motion of the float. The scaling depends on the type of fluid and process data, so that this information is displayed on the scale. If flow conditions change work within limits can be reset or calculate the scale to suit the new situation.

Ill.: Float Models

a = float switch (direct reading and as an indicator in glass tubes). The grooves cause the rotation angle, b = float insensitive to the viscosity, c = rotator designed to broaden the field of measurement values ​​in a +30% (but sensitive to

effects of viscosity).

Advantages and disadvantages

Advantages:

For applications with liquids, gases or steam.

- Method of flow measurement low cost by simple design.

- No power supply required.

- The devices allow monitoring peephole installation walk, easy and reliable process and flow.

- Low pressure drops.

Disadvantages:

- The accuracy of the measurement depends on the process conditions and fluid properties.

- Requires specific calibration for each fluid.

- Field limited reduced (up to 10:1).

- Sensitive to the intrusion of materials not suitable for carrying liquids solids. Only suitable for fluids at low density.

- No function has totalization.

- The meter must be installed in a vertical pipe (circulation of bottom-up), otherwise it might not work properly.

Applications

Because of its low cost, tapered devices are among the most common types of flowmeters used in industry, such as simple flow controllers. The most popular are simple designs with tapered glass tube and scale directly readable (peephole). These counters have a set price, are a simple way to indicate flow rate, and have the added advantage of not. need power. The principle is quite adequate for minimum volume flow measurement. There are no such models available for pipe sizes above 250 mm (10 ").

Flow Control:

When you are equipped with level limit switches, counters, variable section are considered "flow controllers" easy to install, suitable for almost any field. Often this type of meter proves one of the few solutions for low flow applications.

Manual flow control:

In many cases, variable-section flow meters have another key advantage. The manual valve built into its base can manually control the flow (Fig. below). This is the most rendible of control processes in simple non-automated applications.

Flow rate:

Counters with local indicator (sight glasses) are well equipped to observe and adjust paia flow in a system at a glance. The variable area meters often do not need power and are often used in chemical plants, for example with other meters and electronic equipment more sophisticated instrumentation, such as emergency control in case of power failures. And in many water plants, chemicals are added from the readings taken in simple visual flow meters variable section.

Ill.: Section flowmeters for process control variable (flow control).