Pressure, in the physical sciences, is referred to as the perpendicular force per unit area, or the stress at a point within a confined fluid. In SI units of this quantity, pressure is measured in pascals and one pascal equals one newton per square meter. Atmospheric pressure is close to 100,000 pascals.
The pressure exerted by confined gas results from the average effect of the forces produced on the container walls by the rapid and continual bombardment of the huge number of gas molecules on the wall of the container. The absolute pressure of a gas or liquid is the total pressure it exerts is defined by the total pressure as the sum of the algebraic value of the pressure, including the effect of atmospheric pressure. An absolute pressure of zero corresponds to empty space or a complete vacuum.
To calculate the pressure loss in a pipe it is necessary to compute a pressure drop, usually in the fluid head, for each of the items that cause a change in pressure. However, to calculate the friction loss in a pipe, for example, it is necessary to calculate the friction factor to use in the Darcy-Weisbach equation which determines the overall friction loss.
The friction factor itself is dependent on internal pipe diameter, the internal pipe roughness, and Reynold's number which is in turn calculated from the fluid viscosity, fluid density, fluid velocity, and the internal pipe diameter.
There are therefore a number of sub-calculations that must take place to calculate the overall friction loss. Working backward we must know the fluid density and viscosity properties, know the pipe diameter and roughness properties, calculate Reynold's number, use this to calculate the friction factor using the Colebrook-White equation, and finally plug in the friction factor to the Darcy-Weisbach equation to calculate the friction loss in the pipe.
The pressure at the end of the pipe is given by the following equation and the pressure drop is calculated (where all items are specified in m Head of Fluid):
P[end] = P[start] - Friction Loss - Fittings Loss - Component Loss + Elevation[start-end] + Pump Head
Example 1: A tank is filled with oil is of up to 1m in height. Calculate the pressure exerted on the bottom of the tank. (take the Acceleration due to Gravity = 9.8 m/s2, Density of water = 1000 kg / m3).
Acceleration due to Gravity = 9.8 m/s2
Density of water = 1000 kg / m3,
The pressure can be calculated as:
P = ρ × g × h
P = 1000 × 9.8 × 1m
P = 9800 Pascal.
Hence the pressure at the bottom of the tank is 9800 Pascal.
Example 2: A force of 1200 N acts on the surface of an area of 10 cm2 normally. What would be the thrust and pressure on the surface? (take the Acceleration due to Gravity = 9.8 m/s2, Density of water = 1000 kg / m3).
Force F = 1200 N, Area A = 10 cm2 = 10 ×10-4 m2 = 10-3 m2
Thrust = Normal pressure = F = 1200 N
Pressure P = F/A 1200N10−3m21200N10−3m2
= 1.2 ×× 106 N/m2
Hence the pressure exerted is 1.2 ×× 106 N/m2
1. What is standard pressure?
Standard pressure is the value of pressure defined by scientific and metrological organizations which are 100 kPa or 101.325 kPa.
2. What are the different types of pressure?
Osmotic pressure, Hydrostatic pressure, aerostatic pressure are the different forms of pressure.
3. What is the standard unit of pressure?
Pascal is the standard form of pressure.
4. How do you find the total pressure of a mixture?
For a mixture of ideal gases, the total pressure exerted by the mixture equals the sum of the pressures that each gas would exert on its own.
5. What are kPa units?
kilopascal (kPa), one thousand times the unit of pressure and stress in the meter-kilogram-second system (the International System of Units [SI]).