Orfice is used in orifice meter, which measures discharge of fluid flowing through the pipe. Here, orifice (i.e. orifice plate) is sharp edge circular opening having hole diameter as 0.4 to 0.8 times diameter of pipe. Preferably, it is taken 0.5 times the diameter of pipe.
Construction and Working of Orifice Meter
Orifice is made up of materials like stainless steel, phosphor bronze etc., to withstand against corrosive effect of flowing fluids. A U-tube differential mercury manometer is connected between two sections, one on upstream side and other at vena contracta on the downstream side of orifice plate. Orifice meter works on same principle as that of Venturimeter i.e. the pressure head (energy) is converted into kinetic energy in an accelerated flow. Thus, orifice meter works on the principle of converting pressure energy (head) into kinetic energy by reducing cross-sectional area of flow passage, This leads to pressure difference, which is measured by means of U-tube differential manometer.
Fig. 3.2: Orifice Meter
Flow rate of fluid can be measured using the equation/ formula given below.
\[{{\text{Q}}_{\text{act}}}=\frac{{{\text{C}}_{\text{d}}}{{\text{A}}_{1}}{{\text{A}}_{2}}}{\sqrt{\text{A}_{1}^{2}-\text{A}_{2}^{2}}}\cdot \sqrt{2\text{gh}} \]
Where,
Cd = Coefficient of discharge = 0.64
h = Pressure difference in m
A1 = Area of pipe in m2
A2 = Area of Vena-contracta in m2
Advantages of Orifice Meter
- It can be used with differential pressure devices.
- It is available in many materials.
- It has well known and predictable characteristics.
Disadvantages of Orifice Meter
- It causes relatively high permanent pressure loss.
- It tends to clog. Therefore, its use is restricted in slurry applications.
- Its accuracy depends upon the care taken during its installation.
- It is subjected to erosion, corrosion and scaling.
Applications of Orifice Meter
- Widely used in the applications, where space availability is a problem.
- Useful for wide range of pipe diameters ranging from 15 mm to 1.5 m diameter pipes.
