White paper
On
Pressure Basics
What Is Pressure??
Pressure is the amount of force applied over a defined area. The relationship
between pressure, force, and area is represented in the following formula:
P=F/A
Where:
• P = Pressure
• F = Force
• A = Area
FIG. – Demonstration of Pressure
Unit
The primary unit of pressure in the International System of Units (SI) is Pascal,
abbreviated Pa. It is defined as:
Despite the theoretically universal character of the International System of
Units, many other units of pressure are still commonly used in both science
and industry. Most popular ones are:
• Pound-force per square inch (psi or lbf/in²) – used mainly in the USA, 1
psi = 6,894.8 Pa
• Kilogram-force per square centimeter (kgf/cm², in Poland abbreviated as
kG/cm²), 1 kgf/cm² = 98,066.5
• Millimetre of mercury (mmHg), 1 mm Hg = 133.3 Pa
• Torr (Tr) – equal to 1/760th of standard atmosphere and thus practically
equal to 1 mm Hg
• Standard atmosphere (atm) – so-called standard pressure, 1 atm =
101,325.0 Pa
• Technical atmosphere (at) – defined as one kilogram-force per square
centimetre, thus 1 at = 98,066.5 Pa
• Bar (bar) – defined as 100 kilopascals, 1 bar = 100,000.0 Pa
Please note that bar, technical atmosphere, standard atmosphere and kilogramforce per square centimetre are roughly similar and for estimation purposes can
be assumed practically equal.
Why to measure pressure??
The most common reasons that process industries measure pressure are as
follow:
 Safety –
Pressure measurement and control help minimize equipment damage,
reduce the risk of personal injury, and prevent leaks of potentially harmful
process materials into the environment. Pressure measurement used to control
the level and flow of process materials helps to prevent backups, spills, and
overflows.
 Process efficiency –
Maximum process efficiency is achieved when accurate measurement
has been made. When pressure is maintained at a particular value or narrow
range of values highest process efficiency is achieved.
 Cost savings –
Precise measurement of pressure reduces the overall cost by saving the
energy consumed by pressure instruments (Pumps and compressor).
TYPES OF PRESSURE
Pressure-measurement devices can be categorized according to the measured
reference pressure. Reference pressure is the pressure measurement that is
compared to the actual measured pressure of the process. The three reference
pressures are:
 Absolute
 Gauge
 Differential
•
Absolute pressure is measured relative to the absolute zero pressure - the
pressure that would occur at absolute vacuum. All calculation involving the
gas laws requires pressure (and temperature) to be in absolute units.
Because no pressure reading can be less than a perfect vacuum, an absolute
pressure-measurement device will never have a negative reading.
•
Gauge pressure A gauge is often used to measure the pressure difference
between a system and the surrounding atmosphere. This pressure is often
called the gauge pressure. Therefore Gauge pressure is measured from
atmospheric and absolute is measured from 0 (as all absolute scales are
measured from). It is zero-referenced against ambient air pressure, so it is
equal to absolute pressure minus atmospheric pressure. Negative signs are
usually omitted.
•
Differential pressure is the difference in pressure between two points.
Absolute and gauge devices measure the difference between the pressure of the
process fluid and a reference pressure.
Differential devices take two pressure measurements of the process fluid at
different points and compute the difference.
DIFFERENTIAL
PRESSURE
GUAGE
PRESSURE
1 atm
14.7 psia
ABSOLUTE
PRESSURE
0 psia
(PERFECT VACCUM)
Fig. showing level of types of pressure
Pressure based measurement
Pressure-measurement readings can be used in many applications. Examples
include calculating flow rate, liquid level, and fluid density. This is because a
known relationship exists between pressure and density, pressure and level, and
pressure and fluid flow through a pipe.
Flow
A common use of a pressure measurement is to determine a fluid's flow rate
through a pipe. As a fluid flows through a pipe restriction, the fluid pressure
drops. The differential pressure measured is proportional to flow rate.
The flow equation used to determine flow rate for DP flow meters is based on
Bernoulli’s equation, which shows that flow rate (Q) is proportional (𝛼 ) to the
square root of differential pressure (∆P):
Q =√∆𝑃
Level
The level of a liquid in a tank or vessel can be determined from the pressure
measurement by this equation:
Height of Liquid = Pressure/ Specific Gravity
Interface Measurement
An interface is the boundary between two immiscible (incapable of being mixed)
fluids with different densities (e.g., oil and water). An interface measurement
finds the boundary between two liquids stored in the same tank.