An orifice is defined as a small opening of any cross-section provided on the side or bottom of a tank through which a fluid can flow easily. Orifices are used to measure the rate of flow of a fluid.

The continuous stream of fluid that flows out of an orifice is called a jet of water. An orifice can have a rectangular, circular, or triangular shape.

This article explains the classification and determination of the rate of flow through an orifice.

Contents:

## Flow Through an Orifice

Consider a tank with a rectangular orifice fitted at the surface as shown in Figure-2. Let the head of the liquid above the center of the orifice be ‘H’. The liquid that comes out of the orifice forms a jet of liquid that possesses an area of cross-section less than that of the orifice. The area of the cross-section of this liquid jet goes on decreasing and attains a minimum value in section 1-1.

At section 1-1, the area of cross-section (A_{c}) is minimum, the streamlines are straight and they are parallel to each other. The streamlines are hence perpendicular to the plane of the orifice. This section is called as vena-contracta. Usually, vena-contracta is located at a distance equal to the radius of the orifice.

If the velocity of liquid at vena-contracta is given by “v”, then the theoretical velocity of flow at vena-contracta is given by:

**Classification of Orifice**

Orifices are classified based on the following categories:

### 1. Based on Size

Orifice can be classified as small or large orifice based on the size and the head of fluid in the orifice.

#### 1.1. Small Orifice

If the head of fluid is greater than 5 times the depth of the orifice, then it is categorized as small orifice. In the case of small orifice, the jet of fluid possesses a constant velocity throughout the cross-section. Hence, the discharge is calculated as:

Where, a= area of orifice, C_{d} is a hydraulic coefficient named as coefficient of discharge and g= acceleration due to gravity. The value of C_{d} varies from 0.61 to 0.65.

#### 1.2. Large Orifice

If the head of fluid is less than 5 times the depth of the orifice, then it is categorized as a large orifice. Here, the velocity over the entire cross-section of the jet of fluid is not constant, hence discharge cannot be determined by Equation-2. Hence,

The total discharge of fluid for a large orifice is given by:

**Q = ****C _{d}.b.**

**[ H**

_{2}^{3/2}– H_{1}^{3/2}]Equation-3

**2. Based on Shape**

Based on shape or cross-section of an orifice, they are classified as:

- Rectangular orifice
- Circular orifice
- Triangular orifice
- Square orifice

### 3. Based on Nature of Discharge Through Orifice

Based on the nature of discharge of the fluid, an orifice can be classified as:

#### 3.1 Fully-Submerged Orifice

A full-submerged orifice is one whose outlet is fully submerged under liquid.

The actual discharge through fully-submerged orifice is:

Where,

H1 = Height of water above the top of the orifice on the upstream side

H2 = Height of water above the bottom of the orifice

H = Difference in water level

b = width of orifice

C_{d} = Coefficient of discharge

#### 3.2. Partially Submerged Orifice

Partially submerged orifice is one whose outlet side is partially submerged in the fluid as shown in Figure-4 below.

The actual discharge through partially submerged orifice is:

Where,

H1 = Height of water above the top of the orifice on the upstream side

H2 = Height of water above the bottom of the orifice

H = Difference in water level

b = width of orifice

C_{d} = Coefficient of discharge

### 4. Based on Upstream Edge of Orifice

Based on the upstream edge of the orifice, it is classified as:

- Sharp-edged orifice
- Bell-mounted orifice

## FAQs

**What is an orifice?**

An orifice is defined as a small opening of any cross-section provided on the side or bottom of a tank through which a fluid can flow easily. Orifices are used to measure the rate of flow of a fluid.

**What is vena-contracta in an orifice?**

The liquid that comes out of the orifice forms a jet of liquid that possesses an area of cross-section less than that of the orifice. The area of the cross-section of this liquid jet goes on decreasing and attains a minimum value at a certain point.

At this point the area of cross-section (A_{c}) is minimum, the streamlines are straight, parallel to each other, and perpendicular to the plane of the orifice. This section is called as vena-contracta. Usually, vena-contracta is located at a distance equal to the radius of the orifice.

**How to calculate discharge of fluid flowing through a large orifice?**

The total discharge of fluid for a large orifice is given by:**Q = ****C _{d}.b.**

**[ H**

_{2}^{3/2}– H_{1}^{3/2}]**Read More**

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