Working principle of Venturi Flow Meter

Venturi Flow Meter

• When a venturi flow meter is fixed in a fluid pipeline carrying process fluid for measuring flow rate, a pressure drop exists between the venturi meters’ convergence and throat. This pressure drop is determined using a DP transmitter.This pressure drop after calibration embellishes an estimate of the fluid flow rate.
• The venturi meter works on the principle of Bernoulli’s equation and the equation of continuity.
• The Bernoulli Theorem: It says that the total amount of energy at any point in a steady, ideal flow of an incompressible fluid is always the same. The three types of energy—pressure, kinetic, and potential or datum energy—combine to make up the total energy.

Construction of Venturi Flow meter

• The main parts and areas of the venture flow meter are shown below. The Venturi flow meter opening is cylindrical in shape to meet the size of the pipe for the flow of process fluid through it this permits the venture to be fixed to the pipeline.
• After the fluid inlet, a converging conical section exists at an angle of 19 Degrees to 23 Degrees.
• Next to the converging section, there is a cylindrical section called the throat with the least cross-sectional area.
• The diverging conical section exists at an angle of 5 Degrees to 15 Degrees next to the throat area.
• At the inlet side and in the throat area of the venturi flow meter, suitable openings are provided to fix DP Transducer or U tube manometer, Differential Pressure Gauge.
• The inlet side called Convergent and in throat area are shown in Sections 1 & section 2 in the schematic diagram.

Operation of Venturi Flow Meter:

• The process fluid enters through the inlet or convergence side of the venturi flow meter with pressure P1.
• The pressure of process fluid gets reduced as the fluid enters the convergence side venturi flow meter.
• The pressure is reduced and reaches some valves after reaching the throat indicated by P2.
• Note that the pressure at the throat part of the venturi flow meter is minimum compared to the convergence side. P1>P2
• The DP transmitter is linked between the inlet side and throat section of the venturi flow meter to record the pressure difference given by δP = P1-P2.
• This pressure difference becomes an indication of the fluid flow rate through that pipeline.
• The divergent side enables the fluid to recover its Kinetic Energy and pressure back to P1.
• The angle of divergence is inversely proportional to recovery.
• If the angle of the diverging section is low then recovery is high. & Vice versa.

Overview of the Working of the Venturi Flow Meter

• Let’s study how the venturi flow meter works in the convergence, throat, & Divergence sections.
• A venturi flow meter works by determining pressure differences at two different regions.
• The pressure difference is developed by minimizing the pipe diameter to a fluid to flow with greater velocity (Fast).
• Pressure is minimum for fast-flowing process fluid compared to fluid flow with the minimum velocity at the larger section of the venturi flow meter.
• But we know that fluid velocity in the throat is high, & fluid velocity is low both at the inlet and outlet section of the venturi flow meter.

What are Piping Conditions that affect the performance of the venturi flow meter?

• Generally, in all process stations, these venturi flow meters whose flow rate needs to be determined need to be fixed at a specific distance before and after the piping components.
• This allows the flow to stabilize before the venturi.
• Since venturi flow meters usually don’t have downstream requirements.
• In fact, some flow rate controllers consist of butterfly valves integrated right into the flow meter.

Location of Valves to Venturi Flow Meters

• The valves must be located or fixed after this venturi flow meter.
• Generally, an open valve develops an uneven flow profile that causes uncertainty in flow measurement through the venturi flow meter.

Elbows location for Venturi Flow Meters increases, or Decreases.

• These elbow components are directly bolted at the outlet or divergent side (Divergence) of the venturi flow meter.
• This doesn’t affect the measurement of flow.
• For the upstream side (Convergence) an elbow fitting depends upon the number of diameters before the venturi.
• For example: 5 diameters on a 12-inch pipeline are calculated as 5 x 12 inches = 60 inches.

Below table shows the straight pipeline diameter prior to Venturi Flow Meter

The exact elbow requirement may vary based on venturi type and its beta ratio of orifice defined by the ratio of the throat diameter to inlet diameter.

Installation Concerns

1. Flow Direction
2. Orientation of pressure taps
3. Alignment of venturi to pipe

Flow direction

• In most cases, the performance of this venturi flow meter becomes very poor due to backward installation.
• All flow meters develop a differential pressure in the reverse flow direction but the pressure difference value doesn’t remain the same even in normal flow conditions.
• Every industrial flow meter is provided with a flow direction arrow mark on it.
• But some have directional arrows formed into the body.

Full Body Venturi Meters

Venturi Meters consists of

• Two sets of pressure taps are located 180 degrees apart from each other.
• Vent & Drain port located 90 degrees from these pressure taps
• In the case of horizontal pipelines
• These two sets of pressure taps must be positioned at the horizontal center line.
• Vent and drain ports must be positioned on the top and bottom.
• In the case of vertical pipelines
• A pressure connection doesn’t affect the process performance.

Insert Style Venturi Meters

• This venturi meter consists of one high and one low-pressure connection.
• Both these pressure connections within the venturi meter are fixed between the mounting flange bolt holes.
• The angle between these depends on the flange size and flange rating.

Orientation of Pressure Taps

• The orientation of pressure taps depends upon the venturi type and measuring process fluid.
• Pressure connections must be aligned to dodge air pockets in liquid applications.
• Pressure connections must be aligned to dodge condensation in air applications.

Alignment of venturi to pipe

• Venturi must be installed at the centre the meter in a pipe line for process fluid flow measurement.
• If venturi installed off centre, a disruption in flow occurs at a pipe and venturi joint that causes accuracy loss.

Proper pressure connection orientation is shown in table below.

Advantages of Venturi Flow meter

• Discharge Co-efficient is high.
• Behaviour can be perfectly concluded
• Can be installed in vertical, horizontal, or in inclined.
• Pressure drop is low about 10% of DP.
• Sensitivity is less.
• Less susceptible to damage.
• Suitable for gas flow with entrained liquid.
• Solid particles can pass through it because to its smooth structure and low cone angle. Hence, it may be applied to contaminated fluids.
• More accurate than the orifice and flow nozzle methods.

Disadvantages of Venturi Flow meter

• Costly
• High installation and maintenance cost.
• Low experimental data
• Low turndown ratio
• Large in size, hence they cant be used in limited      space.
• The venturi has to be followed by long, straight pipes for effective operation.
• It cannot be used in small-diameter pipes (70mm).

Applications of venturi flow meters

• Venturi flow meters are used
• In requirement of high pressure recovery.
• To measure high flow rates with minimum diameter pipes
• To measuring flow rates of various fluids such as water, gases, suspended solids, slurries and dirty liquids.

Frequently asked questions

What is the basic principle of Venturi meter and orifice meter?

• Venturi meter works on the principle by reduction in the cross-sectional area of the flow passage creates a pressure difference.
• Pressure difference measurement permits discharge of flow through pipe.

Why is Venturi flow meter used?

Venturi flow meter is a device used to determine the flow rate of process fluid flowing in a pipe line.