Short Answer:

An orifice meter is a device used to measure the rate of flow of a fluid (liquid or gas) through a pipe. It works on the principle of Bernoulli’s theorem, which relates pressure and velocity in a moving fluid. The orifice meter consists of a thin plate with a sharp-edged hole (orifice) placed across the flow, causing a pressure drop that helps determine the flow rate.

In simple words, when a fluid passes through the orifice plate, its velocity increases, and pressure decreases. This difference in pressure between the upstream and downstream sides of the orifice plate is measured and used to calculate the flow rate accurately.

Detailed Explanation:

Orifice Meter

An orifice meter is one of the most common and simple devices used to measure the discharge or flow rate of fluids in a pipeline. It operates based on Bernoulli’s equation and the principle of continuity, which relate the flow velocity, pressure, and cross-sectional area of the flow.

It is an economical, compact, and easy-to-install flow measuring device, widely used in industries for both gases and liquids. The orifice meter is particularly suitable for applications where moderate accuracy is sufficient, and low installation cost is desired.

1. Principle of Orifice Meter

The orifice meter works on Bernoulli’s principle, which states that the total energy (pressure energy, kinetic energy, and potential energy) in a steady, incompressible, and non-viscous flow remains constant along a streamline.

According to Bernoulli’s equation,

When the fluid passes through a small opening (orifice) in the pipe, the velocity of the fluid increases while pressure decreases. The difference in pressure between the upstream section (before the orifice) and the downstream section (after the orifice) is used to determine the rate of flow.

The flow rate can be calculated using the following formula:

where,

•  = discharge or flow rate (m³/s)
•  = coefficient of discharge (ranges between 0.60 to 0.65)
•  = area of the orifice (m²)
•  = area of the pipe (m²)
•  = difference in pressure head (m)

2. Construction of Orifice Meter

An orifice meter consists of the following main parts:

1. Orifice Plate:
   • It is a thin, circular plate made of metal (usually stainless steel) with a sharp-edged circular hole at the center.
   • The diameter of the hole is smaller than the pipe diameter, creating a restriction to the flow.
   • It is mounted between two pipe flanges.
2. Flanges and Tappings:
   • Pressure tappings are provided at two points — one before and one after the orifice plate.
   • The upstream tapping measures the static pressure before the fluid enters the orifice, and the downstream tapping measures the pressure after it passes through the orifice.
3. Manometer:
   • A differential manometer (often containing mercury or water) is connected between the two tappings to measure the pressure difference ( ).
4. Pipe Section:
   • A straight section of pipe is used before and after the orifice plate to ensure smooth flow and accurate measurement.

The arrangement ensures that the pressure difference can be measured accurately for calculating the flow rate.

3. Working of Orifice Meter

The working of the orifice meter is explained as follows:

1. Fluid Flow through Orifice Plate:
   When fluid passes through the orifice plate, it experiences a sudden contraction, causing an increase in velocity and a decrease in pressure.
2. Formation of Vena Contracta:
   After passing through the orifice, the fluid stream converges to a minimum cross-sectional area called the vena contracta. At this point, the velocity of the fluid is maximum, and pressure is minimum.
3. Pressure Difference Measurement:
   The pressure before and after the orifice plate is measured using a differential manometer. This difference in pressure ( ) is directly related to the flow rate.
4. Calculation of Flow Rate:
   Using Bernoulli’s equation and continuity principle, the flow rate   is calculated from the measured pressure difference.
   The discharge depends on the size of the orifice, the density of the fluid, and the measured head difference.

4. Types of Orifice Plates

The orifice meter may use different types of orifice plates depending on the application:

1. Concentric Orifice Plate:
   • The hole is at the center of the plate.
   • Used for clean liquids and gases.
2. Eccentric Orifice Plate:
   • The hole is off-center.
   • Suitable for fluids with suspended solids.
3. Segmental Orifice Plate:
   • The hole is in the shape of a segment near the bottom.
   • Used for fluids containing gas or solid particles.

5. Advantages of Orifice Meter

1. Simple design and easy to install.
2. Low manufacturing and installation cost.
3. Compact and requires less space.
4. Suitable for both liquids and gases.
5. Reliable for high-pressure and high-temperature applications.

6. Disadvantages of Orifice Meter

1. Causes significant energy loss due to the sudden contraction of flow.
2. Accuracy is lower compared to Venturi meters.
3. Cannot be used for highly viscous or dirty fluids, as the orifice may clog.
4. Requires frequent calibration and maintenance.
5. Flow coefficient varies with Reynolds number.

7. Applications of Orifice Meter

1. Used in water supply systems to measure discharge in pipelines.
2. Widely used in chemical and petroleum industries for fluid flow measurement.
3. Applied in steam and gas flow measurement in boilers and turbines.
4. Used in HVAC systems to measure air flow.
5. Helpful in laboratory experiments for fluid mechanics studies.

8. Comparison with Venturi Meter

While both the orifice meter and the Venturi meter work on the same Bernoulli principle, they differ in performance:

• The orifice meter is simpler and cheaper, but it has more energy losses.
• The Venturi meter is more accurate but costlier and larger.
  Therefore, the orifice meter is preferred in situations where economy and simplicity are more important than high accuracy.

Conclusion

In conclusion, an orifice meter is a simple and economical flow measuring device that works based on Bernoulli’s theorem. It uses a thin plate with a small orifice to create a pressure drop, which is then used to calculate the flow rate. Although it has higher energy losses compared to a Venturi meter, it remains widely used due to its simplicity, low cost, and easy installation. The orifice meter is ideal for many engineering applications, including water, air, and gas flow measurement in industrial and laboratory systems.