Orifices | Classification of Orifices | Hydraulic Co-efficient

Definition of Orifices

Orifice is defined as the small opening on side or bottom of a tank through which any kind of fluid is flowing. The opening can be of circular, triangular or rectangular in cross section and they are named on the basis of shape accordingly. Orifices are mainly used for measuring the rate of fluid flow.

Orifice ( Wikimedia Commons)
Orifice ( Wikimedia Commons)

Classification of Orifices

Orifices are classified on the basis of many criteria such as:

  1. Size
  2. Shape
  3. Nature of discharge
  4. Nature of upstream edge etc.

These are explained briefly as below:

Classification based on shape: Orifices are classified into small orifice and large orifice depending upon the size of orifice and head of fluid in that orifice. Small orifice is the one in which has the head of fluid from the centre of orifice is more than five times the depth of orifice. Also the large orifice is the one which has the head is less than five times the depth of orifice.

Classification based on shape: Based on shape of orifice they are classified as following:

  1. Rectangular orifice
  2. Circular orifice
  3. Triangular orifice
  4. Square orifice

Classification based on nature of discharge: Based on nature of discharge they are classified as following:

  1. Free discharging orifice
  2. Submerged orifice: They are also further classified as fully submerged and partially sub merged orifices.

Classification based on nature of upstream edge of orifice: Based on nature of upstream edge of orifice they are classified as following:

  1. Sharp-edged orifice
  2. Bell-mouthed orifice

 Fluid flow in Orifice

Flow through an orifice (engineeringcivil.com)
Flow through an orifice (engineeringcivil.com)

Consider a tank with a orifice on its side as shown in figure: Let h be the head of water from the centre of orifice. The water flowing through the orifice forms a jet of liquid whose area of cross section is less than that of the orifice fitted on the side of tank.

The area of jet will gradually decreasing and at a section C, area is minimum. At this Section the streamlines are straight and parallel to each other and perpendicular to the plain of orifice. So this section is called Vena-contracta. After this section, the jet diverges and it then attracted by gravity to the downward direction.

Vena-contracta (Wikimedia Commons)
Vena-contracta (Wikimedia Commons)

The theoretical velocity V2 of fluid flowing through orifice will be:

V2= sqrt (2gh)

Where g= acceleration due to gravity

h= head of fluid

Also the actual velocity will be less than this value due to various losses.

Hydraulic Co-efficients

There are mainly 3 hydraulic coefficients, they are as follows:

  1. Co-efficient of velocity, CV
  2. Co efficient of contraction, Cc
  3. Coefficient of discharge, Cd

Co efficient of velocity, Cv

It is the ratio between the actual velocity of the jet of fluid at the vena-contracta to the theoretical velocity of fluid. It is denoted by Cv

Cv = v/ sqrt (2gh)

Generally the value of Cv varies from 0.95 to .99 for different orifices depending upon their size, shape, head etc. for calculation purposes we assume the value of Cv as 0.98 generally.

Co efficient of contraction, Cc

It is the ratio of area of the jet at vena-contracta to the area of orifice. It is generally denotes by Cc.

Cc =  ac/ a

Where ac is the area at the vena-contracta and a is the area of orifice. The value of CC varies from 0.61 to 0.69 depending upon the size, shape and head of orifice in which flow takes place. In general the value is taken as 0.64.

Co efficient of discharge Cd

It is defined as the ratio of the actual discharge from an orifice to the theoretical discharge from the orifice. It is denoted by Cd. It can be calculates as following:

Cd = Cc*Cv

The value of Cd varies from 0.61 to 0.65. In general the value of Cd is taken as 0.62 for all calculation purpose.