2. Orifice meter
The venturi meter described earlier is a reliable flow
measuring device.
Furthermore, it causes little pressure loss.
For these reasons it is widely used, particularly for large-
volume liquid and gas flows.
However this meter is relatively complex to construct and
hence expensive.
Especially for small pipelines, its cost seems
prohibitive, so simpler devices such as orifice meters
are used.
4. Orifice meter
It is also known as Pipe orifice & Orifice plate.
It may be installed in pipeline with a
minimum of trouble and expense.
Pipe orifice is a device used for measuring the
rate of flow of a fluid through a pipe.
It consists of a thin,circular plate with a hole
in it.
5. Orifice meter
The plate is held in the pipeline
between two flanges.
Pipe orifice is a cheaper device as
compared to venturimeter.
The diameter of orifice is generally 0.5
times the diameter of the pipe (D),
although it may vary from 0.4 to 0.8
times the pipe diameter.
7. Discharge in orifice meter
Let d1 = diameter at section 1
p1 = pressure at section 1
v1 = velocity at section 1
A1= area at section1
d2, p2, v2, A2 are the corresponding values at section 2.
Applying Bernoulli’s equations at sections 1 and 2, we get
2 2
2
1 2
g 2g g 2g
p1
v1
z
p
v2
z
2 2
2 2
2 2
2
2 1
2g
2g
v v1
p1 p
z z g 1 g 2
v1
h
v2
v 2gh v where h is the differential head.
8. Let A0 is the area of the orifice.
Coefficient of contraction, cC
A2
A0
1 2
0 cA C
v
A1
By continuity equation, we have
A1v1 A2v2
v
Hence,
2 2
2
2 c
A2
C2
v2
A1
2gh
C2
A1
v 2gh 0 c 2
v 2
1
A0
Discharge in orifice meter
9. Thus, discharge, 2
2 c
C2
A1
Q A2v2 v2 A0Cc
A0Cc 2gh
1
A0
If Cd is the co-efficient of discharge for orifice meter, which is defined as
2
2
1
A0
2
2
2
2
2
2
d c
c
c
A1
C2
A1
C2
A1
A1
C C
1
A0
1
A0
Cc Cd
1
A0
Discharge in orifice meter
10. Hence,
The coefficient of discharge of the orifice meter is much
smaller than that of a venturimeter.
Discharge in orifice meter
d
A0 A1 2gh
Q C
A2
A2
1 0