White paper On Pressure Basics

Transcription

1 White paper On Pressure Basics

2 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: Where: P = Pressure F = Force A = Area P=F/A 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

3 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 = 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.

4 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.

5 GUAGE PRESSURE DIFFERENTIAL PRESSURE 1 atm 14.7 psia ABSOLUTE PRESSURE 0 psia (PERFECT VACCUM) Fig. showing level of types of pressure

6 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: Interface Measurement Height of Liquid = Pressure/ Specific Gravity 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.