Concept of Fluid Pressure

Pressure is defined as the force exerted by a fluid at rest per unit area on which the force acts. It is expressed in pound-force per square inch or psi in the U.S. Customary system. In the SI system, it is measured in pascals. Since a pascal is very small, pressure is commonly expressed as kilopascals. Other units of pressure include inches of water, inches of mercury, millimeters of mercury, atmospheres, torr, bars, and millibar. Fluid pressure is commonly measured by pressure gages that measure the pressure of the fluid relative to the atmospheric pressure.

This is known as gage pressure, denoted by Pgage. The total fluid pressure, also called the absolute pressure, pabsolute, is the sum of the gage pressure, and the atmospheric pressure, patmospheric.

Pabsolute = Pgage + Patmospheric

In the U.S. Customary system, the gage pressure is denoted by the letter g added after the unit of pressure, such as psig. The absolute pressure is denoted by adding the letter a after the unit of pressure, such as psia. In the S I system, the words gage or absolute may be added, such as kilopascals gage or kilopascals absolute. If nothing is added after the units of pressure, it is assumed to be absolute pressure.

The standard atmospheric pressure is one atmosphere. This is equal to

  • atm
  • 14.696 psia
  • 407.1 in. of water gage
  • 29.921 in. Hg
  • 760.0 mm Hg
  • 760.0 torr
  • 101.3 kPa
  • 1.013 bar.

When the absolute pressure is below the atmospheric pressure, you have a vacuum. A vacuum pressure gage usually measures the vacuum pressure relative to the atmospheric pressure. A vacuum pressure, pvacuum, is the difference between the atmospheric pressure, patmospheric, and

the absolute low pressure, by pabsolute. Vacuum pressures are always denoted as positive numbers.

pvacuum = patmospheric - pabsolute

Pressure Drop:

In case of real fluids there is friction between the flowing fluid and the wall of the conduit in which the fluid is flowing. This friction loss depends on the roughness of the inside wall of the pipe the fluid is flowing in. The loss is greater for pipes with greater internal roughness, for constant diameter. There is also internal friction between the particles of the fluid that oppose the fluid flow. This is due to the shear stress in the fluid. This friction loss depends on the viscosity of the fluid. The greater the viscosity, the greater the friction loss will be.

Friction converts the kinetic, potential, and the pressure energies, which are called the mechanical energy of the fluid, into heat. Friction will result in a loss of pressure as the fluid flows through a pipe. The difference between the pressure upstream, P1, and pressure downstream, P2, is called the pressure drop, DP.

DP = P1 - P2

Hence the pressure downstream for flow of fluid is always less than the upstream pressure. Whenever the fluid flows through equipment, such as a heat exchanger or a distillation unit, there will always be a pressure drop for the fluid flow. The only time that there is no pressure drop in the fluid flow, is when there is a pump that increases the pressure in the fluid entering the pump. In this case the pressure of the fluid leaving the pump is greater than the pressure of the fluid entering it. Whenever the fluid comes in contact with the atmosphere, like when the fluid flows into the atmosphere, the pressure on the liquid is atmospheric.

Remember that fluid will only flow from the region at a higher pressure to a region at lower pressure, except in case of a pump. This is quite important when you are looking at flow rates and direction of flows in the designing of a process.