PRINCIPLES OF FLOW & PRESSURE IN HYDRAULICS

In the previous post i had explained you about the difference between hydrodynamics and hydrostatics.Hydrostatics is nothing but hydraulics in common language.Pressure and flow are two big terms in hydraulics.In this post i will explain pressure & flow in details.

Pressure explained :

Basically pressure is created in a hydraulic system when there is a resistance to the flow in the system.If there is a load or force which is opposing the free flow of fluid in the system(friction if neglected) then only pressure is created i.e equal to the force divided by the area perpendicular to the opposing force.


pressure = force / perpendicular area in contact with force.


if there is no opposition to the flow then pressure is equal to zero.

pressure head comes from the weight of the fluid.
e.g. A column of air of one square inch as high as atmosphere gives a pressure of one atmosphere at see level i.e 76 mm of mercury or 14.7 psi. 


a pressure gauge is used to measure the pressure but it shows the gauge pressure which is the difference between absolute pressure minus the atmospheric pressure.

gauge pressure =absolute pressure - atmospheric pressure 

psig  = psia - 14.7

* Liquid always seeks a level depending on pressure.

*according to Bernoulli's theorem the sum of pressure energy kinetic energy and the potential energy is constant for a system.

*If the fluid levels are same then the potential energy will be cancelled out hence we have kinetic energy inversely proportional to the pressure energy.

principles of flow :-

Flow is an action in the hydraulic system that gives the actuator its motion.Pressure gives the actuator it's force but flow is essential for the movement.The flow is created by pump.

There are two ways to measure flow rate one is velocity of flow ( feet per second ) and flow rate (liter per minute ).

velocity is the average speed of fluid particles past a particular point in unit time.Flow rate can be defined as the volume of fluid passing a particular point in unit time.

large volumes are measured in GPM (gallons per minute) or LPM (liters per minute) and small flow rates are measured in CC per min.

1 gpm = 231 inch^3 per minute

1 lpm = 1000 cc^3 per minute.

whenever a liquid is flowing then there must be an unbalanced force to cause motion.When a fluid flows in a streamlined path in a straight path then also there is a pressure difference between the start point and downstream.That pressure drop is due to friction and friction causes the pressure drop causing the flow of fluid in the pipe.