Calibration of Venturi and Orifice Meters HIRIZZA JUNKO M. YAMAMOTO Department of Chemical Engineering, College of Engineering and Architecture, Cebu Institute of Technology – University N. Bacalso Ave. Cebu City, 6000 Philippines This experiment aims to be calibrate both the venture apparatus and the orifice apparatus. The coefficient of discharge of a sharp orifice is obtained and Reynolds number is calculated. It is then plotted in a graph. The coefficient of discharge of a venturi is also obtained and plotted against the corresponding calculated Reynolds number. The pressure drop is also plotted against the water flow rate. In order to calibrate flow meters specifically the venturi and orifice flow meters, a known volume of fluid is used to pass to measure the rate of flow of the fluid through the pipe. Venturi meters consist of a vena contracta-shaped, short length pipe which fits into a normal pipe line. Orifice meters, on the other hand, consists of a thin plate with a hole and is placed at the middle of the pipe and behaves similarly to a venturi meter.
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Calibration of Venturi and Orifice Meters
HIRIZZA JUNKO M. YAMAMOTO
Department of Chemical Engineering, College of Engineering and Architecture, Cebu Institute of Technology – University
N. Bacalso Ave. Cebu City, 6000 Philippines
This experiment aims to be calibrate both the venture apparatus and the orifice apparatus. The
coefficient of discharge of a sharp orifice is obtained and Reynolds number is calculated. It is
then plotted in a graph. The coefficient of discharge of a venturi is also obtained and plotted
against the corresponding calculated Reynolds number. The pressure drop is also plotted
against the water flow rate. In order to calibrate flow meters specifically the venturi and
orifice flow meters, a known volume of fluid is used to pass to measure the rate of flow of the
fluid through the pipe. Venturi meters consist of a vena contracta-shaped, short length pipe
which fits into a normal pipe line. Orifice meters, on the other hand, consists of a thin plate
with a hole and is placed at the middle of the pipe and behaves similarly to a venturi meter.
1. Introduction
An orifice meter is a thin plate with a hole in the middle that is placed in a
pipe through which the fluid flows. It increases the velocity of the fluid as it flows
through it, which decreases the pressure. It is a conduit and a restriction to create a
pressure drop. An hour glass is a form of orifice. [1] For orifice meter, as NRe
increases, C should decrease since friction increase and a greater head loss results.
(a) (b)
Figure 1. (a)Orifice Meter Device, (b)Orifice Meter Diagram
A nozzle, venturi or thin sharp edged orifice can be used as the flow
restriction. In order to use any of these devices for measurement it is necessary to
empirically calibrate them. That is, pass a known volume through the meter and note
the reading in order to provide a standard for measuring other quantities. Due to the
ease of duplicating and the simple construction, the thin sharp edged orifice has been
adopted as a standard and extensive calibration work has been done so that it is
widely accepted as a standard means of measuring fluids. Provided the standard
mechanics of construction are followed no further calibration is required. The
minimum cross sectional area of the jet is known as the “vena contracta.”
A venturi meter uses a narrowing throat in the pipe that expands back to the
original pipe diameter. It creates an increase in the velocity of the fluid, which also
results in a pressure drop across that section of the pipe. It is more efficient and
accurate than the orifice meter. The long expansion section (diffuser) enables an
enhanced pressure recovery compared with that of an orifice plate, which is useful in
some metering applications. As NRe increases in fluid flow, C should increase since
friction effects decrease and flow rate approaches the theoretical. [2]
Figure 2. Venturi Meter Diagram
The hydrostatic equation is applicable to all types of flowmeters (venturi and orifice)
(Equation 1). By Bernoulli’s equation, the cause of the pressure drop is determined to be the
increase of velocity of the pipe flow (Equation 2). By aggregating the hydrostatic, Bernoulli’s
and continuity equations, the theoretical flow rate passing through the venturi meter can be
calculated. Bernoulli’s equation is an energy balance equation and is given as:
WhereP1 is the pressure of the fluid flow as it enters the meter,ρ is the density of the flowing fluid,V1 is the upstream velocity of the flow,G is gravitational acceleration,z1 is the height of the fluid as it enters the meter,P2 is the pressure of the fluid at the throat of the meter,V2 is the velocity of the flow at the throat andz2 is the height of the fluid at the throat of the meter.
Considering a horizontal application, gravitational potential energy is neglected
because there is no change in height of the fluid and Bernoulli’s equation can be rewritten as:
P1/ρ + V1 2/2 = P2/ρ + V2 2/2 (Equation 2)
Bernoulli’s equation can then be rearranged to solve the energy balance in terms of the
velocities of the flow at state 1 and state 2.
ΔP/ρ = V2 2/2– V1 2/2 (Equation 3)
Where:
ΔP is the pressure difference P1– P2
Because the pressure drop, ΔP, and the velocities V1 and V2 cannot be measured directly, the
hydrostatic equation (Equation 4) and the continuity equation (Equation 5) are employed. The
Δh variable of the hydrostatic equation is the difference in height of the air over water
manometer due to pressure and is measured directly.
ΔP = ρgΔh (Equation 4)
Qth= V1A1= V2A2 (Equation 5)
Equation 5 is rearranged to solve for V1 and is written as follows:
V1= V2A2/A1= V2(D22/D12) (Equation 6)
Where:
D2 is the diameter of the throat of the venturi meter and D1 is the diameter of the upstream
region of the meter. Still, the velocity at state 2 is unknown and can be solved for by
rearranging the continuity equation to be substituted into Equation 3.
V22= (Qth/A2)2 (Equation 7)
where
Qth is the theoretical flow rate
Substituting Equation 4 in for ΔP and Equation 6 in for V1, the Bernoulli equation (Equation
3) becomes:
gΔh = [V22/2 - V2(D22/D12)] (Equation 8)
Tidying up Equation 8 yields the following:
2gΔh = V22 [1 - (D22/D12)] (Equation 9)
Substituting Equation 7 into Equation 9 yields the following:
2gΔh = Qth2/A22 [1 - (D22/D12)](Equation 10)
Rearranging to solve for the theoretical flow rate