What is an Orifice Plate?
An orifice plate is one of the most widely used primary flow measurement devices in industrial process systems. It is a thin metal plate with a precisely machined hole installed inside a pipeline to create a differential pressure when fluid flows through it.
When a fluid passes through the orifice opening, its velocity increases while the static pressure decreases. This pressure difference is proportional to the flow rate, allowing engineers to determine the fluid flow using differential pressure transmitters.
Orifice plates are widely used in industries such as:
- Oil and gas
- Chemical processing
- Power plants
- Water treatment
- Petrochemical industries
In fluid mechanics, orifices serve as crucial devices for regulating flow by mechanically restricting it. When fluid passes through an orifice plate, it encounters a pressure decrease, making it essential to accurately size the orifice plate to control the flow effectively.
It’s worth noting that the pressure drop across an orifice can also serve as a method for determining the flow rate of the fluid passing through it.
The type of orifice referred to here is typically a thin, perforated plate known as an orifice plate or diaphragm.
They are preferred because they are simple, inexpensive, reliable, and standardized under ISO 5167.
Orifice plates are a simple, low-cost flow restriction used to measure and control flow by creating a measurable pressure drop. Accurate sizing and pressure-drop calculation using ISO 5167 is essential for reliable measurement and to avoid problems like cavitation or excessive loss.
This page explains the ISO 5167 orifice equations, shows worked liquid and gas examples, and provides a free downloadable Excel calculator that computes orifice diameter, flow rate and pressure drop automatically.
How to calculate flow through an orifice plate (step-by-step)
pressure drop coefficient for an orifice=
Where
K = Pressure drop coefficient
D = Pipe diameter (m)
d = orifice diameter (m)
um = fluid velocity, before diaphragm, (m/s)
ΔHs = Pressure drop in m
The following graph also makes it possible to determine the coefficient K:
This more direct relationship is provided by ISO 5167, which is utilized in the Calculator that can be found below.
Where
qm = flow rate of the fluid through the orifice (kg/s)
C = coefficient of discharge, which for an orifice plate is typically 0.65.
β = d/D
ε = Expansibility factor (only for gas; epsilon = 1 for liquid)
D = pipe diameter (m)
d = orifice diameter (m)
ρ = fluid density (kg/m3) upstream of the orifice plate
ΔP = The pressure drop that occurs via the orifice plate is measured in Pa.
With the help of the following formula, one is able to determine the expansibility factor for gasses.
Where:
? = Expansibility factor (only for gas; epsilon = 1 for liquid)
? = d/D
p1 = upstream pressure (Pa)
p2 = downstream pressure (Pa)
Download: Free Orifice Plate Excel Calculator (ISO 5167)
How to calculate the flow through an orifice plate ?
To simplify the calculation process, we provide a free Excel calculator that performs all ISO 5167 calculations automatically.
The Excel tool calculates:
- Flow rate through the orifice plate
- Pressure drop across the orifice
- Beta ratio
- Discharge coefficient effects
- Gas expansibility factor
Inputs Required
To use the calculator, enter the following parameters:
- Pipe internal diameter (D)
- Orifice bore diameter (d)
- Fluid density
- Upstream pressure
- Downstream pressure
- Temperature
- Desired flow rate or differential pressure
The spreadsheet will automatically compute the flow rate and pressure drop across the orifice plate.
Download the tool below.
FAQs on Orifice Plate Flow and Pressure Drop
What is an orifice plate and why is it used?
An orifice plate is a flat, thin plate with a hole in the middle that is put in a pipeline to monitor fluid flow by making the pressure drop. It is frequently used since it is easy to use, cheap, and follows ISO 5167 standards.
How do you calculate orifice plate flow?
The ISO 5167 equation is used to figure out the flow rate through an orifice plate:
C is the discharge coefficient, ε is the expansibility factor, ΔP is the pressure drop, ρ is the fluid density, and β is the d/D.
What is the typical discharge coefficient (C) of an orifice plate?
C is the discharge coefficient, ε is the expansibility factor, ΔP is the pressure drop, ρ is the fluid density, and β is the d/D.
How to calculate the pressure drop across an orifice plate?
The relationship between flow rate, density, and orifice shape is used to find the pressure drop (ΔP). You can get it straight from the ISO 5167 equation or by utilizing empirical correlations.
Which standards govern orifice plate calculations?
ISO 5167 is the most used standard. It has equations for orifice plates, nozzles, and Venturi tubes.
Can I use Excel for orifice plate sizing?
Yes. The Excel Orifice Plate Calculator that you may download does flow and pressure drop calculations automatically using ISO 5167 algorithms. You just need to type in the pipe diameter, orifice size, fluid characteristics, and pressure conditions.
What inputs are required for orifice sizing?
- Pipe internal diameter (D)
- Orifice bore diameter (d)
- Fluid density or specific gravity
- Operating pressure and temperature
- Desired flow rate or allowable pressure drop
Is orifice plate suitable for both liquids and gases?
Yes, you can use orifice plates to measure the flow of both liquids and gases. When dealing with gases, the expansibility factor (ε) needs to be taken into account.
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