Pressure in Liquid

Under a static condition where there is no influence of external force, the pressure acting at any point in a fluid system is proportional to the height of the fluid column above the point.  See the figure below


An oil column of 10 feet high with a base area of 1 square inch is containing 62.4 pounds per cubic feet of oil.  If the base area is divided by 144 square inches, then the the pressure acting on every square inch will be 0.133 pound per cubic feet. If represented with the height of 10 feet, then the pressure will be 4.33 pounds per square inch or 4.33 psi.



Flow is defined as the movement of a fluid from one place to another per unit of time in GPM (Gallon Per Minute) or l//m (litre per minute).  In discussing a hydraulic system, we should not ignore the naature and characteristic of liquid fluid flow because the flow of a confined fluid in a hydraulic system will have a large influence on the whole system.

Our discussion on the flow will be focused on a closed hydraulic system having one or two cylinders which are connected to each other where fluid can flow to and from the cylinders.  The circulation in a hydraulic system takes place in a way that the fluid flows from a reservoir to the pump.  Then the fluid flows to the service sylinder and returns to the reservoir.  The flow is measured as the velocity and flow rate.


Velocity is defined as the average flow of a fluid on a certain path.  The velocity is mesured in feet per second (fpsa) or feet per minutes (fpm).  The velocity of a flow must be taken into consideration by a mechanic when determining or selecting a hydraulic line.

Flow Rate

Flow rate is defined as the volume of a fluid flowing in a path in a certain time.  Flow rate is measured in gallon per minute (gpm).

Based on the above explanation, it can be concluded that the base part of the fluid tank is the centre of pressure.  As shown in Figure 2-17, it can be concluded that for every feet of the fluid column’s height, there is an increase of fluid pressure in that column.  It is clear that the pressure depends on the height and the types of the fluid.


The relation between the discharge, surface area, and the velocity of fluid flow on a certain surface is called the moving fluid mechanics.  If the fluid flows through two pipes of the same size in the same periode of time, then the velocity of the fluid flows will be the same.

Q= Vol/t , and Vol =  A x S


Q         =          volume of flow or discharge [l/second]

Vol      =          Volume [liter ]

t           =          Time [detik]

A         =          surface area []

S          =          distance passed per second [m]


The force is defined as a measurement of power that changes or may produce change of movement in a body on which it acts or presses.

Based on Newton’s Law,

F = m . a

where        F    =    Force [N]

m   =    weight [kg]

a    =    acceleration [m/dtk²].

As Pascal’s Law says that the pressure will be distributed evenly to all directions, then the pressure can be determined using the formula below:

P = F/A

where        P    =    Pressure [N/m²]

F    =    Force [N]

A         =          Surface area []

Daniel Bernoulli, a Swiss scientist shows that in a system where there is a constant fluid flow, the energy is converted from one form into another if the diameter of the pipe changes.

He also stated that the quantities of the pressure energy and the kinetic energy at various points in a system must be constant as long as the discharge of the flow is constant.  If the diameter of  the pipe changes, then the velocity of the fluid flow will change.  Thus, the kinetic energy must be balanced by the increase or decrease in pressure.

One of the applications of Bernoulli’s Law can be found in a carburetor.

The pressure of the air flowing through the carburetor barrel decreases when passing through the throttle.  The decrease in pressure allows the materials to flow, so that the fuel will mix with the air and form fuel fog.

Based on Bernoulli’s Law, it can be concluded that energy will never change.  There are two types of energy in a flow, namely:

  1. Potential energy; the energy that is related to the level of a liquid.
  2. Kinetic energy; the energy that is related to a movement or the speed of movement.

Based on Bernoulli’s formula,

Based on the continuity and energy formulas, it can be concluded that:

  • If the diameter decreases, the velocity and the transfer of energy will increase.
  • The amount of energy is constant, the potential energy will change if the diameter decreases.
  • However, the decrease in diameter does not significantly result in the changes in the potential energy.
  • The static pressure changes with regard to normal pressure, due to flow rate.

In a hydraulic system, the pressure energy (static pressure) is very critical, since the fluid level and the velocity is too low.

Loss of Friction Factor

When a fluid is in stationary condition (immobile), the pressure acting at the rear side is the same as that acting at the front side.  However, the flow of a fluid through a system produces heat due to friction.  Thus, there will be a loss of some of energy due to the transfer of energy into thermal energy, meaning that there is a loss of pressure.


The hydraulic power can never be changed into any other forms of power without a loss of energy due to friction.

Flow in a pipe

Velocity is closely related with flow rate.  Therefore, the movement of the fluid must always be through a specific line in accordance with the specification of velocity and flow rate.  For illustration, see Figure 2-20.  A fluid pump with a capacity of 1 gpm is used to pump a fluid to two pipes of the same capacity (1 gallon) and different diameter.  Each of the pipes is emptied and supplied with the fluid every one minute to reach a flow rate of 1 gpm.  The fluid in pipe B must flow at the velocity of 2 feet/minute.  To reach the same flow rate in the other pipe with a smaller diameter, the velocity of the fluid flowing in that pipe must be increased.  When a fluid moves in a pipe, a friction will occur due to a very high velocity of the fluid.  The diameter of pipe determines the velocity of the fluid flowing in it.  This is important as a proper pipe diameter will not provide over-velocity that overheats the pipe.  For that reason, it is recommended that replacement of a pipe should be done using the procedure stated in the manufacturer’s manual and design.

In Figure 2-21, each cylinder has 1 gpm pumped through it.  The piston of the smaller pipe must travel twice farther as it moves at the velocity of twice higher than the piston of the bigger pipe.  The piston of the smaller pipe can travel in that way as the velocity of the fluid in the smaller cylinder is twice higher than that in the bigger one.  The above example shows the ratio of fluid rates in the two cylinders.  It shows that the piston of the smaller cylinder travels twice farther than that of the bigger one.

However, not all advantages are found in the smaller cylinder.  The bigger cylinder provides a larger force.  The piston of the bigger cylinder will not move fast and far, but with a great force.

There is a balance here.  When you get a higher velocity, you will get a lower force.  An assembling department of a manufacturing company should take the balance into consideration when selecting a hydraulic cyilinder.  It needs to balance the load on the moving piston to produce the required velocity and distance.


Hydraulic cylinders on a backhoe such as the boom cylinder, bucket cylinder, crowd cylinder, each of which has a different length.  Any hose has a cylinder designed in accordance with the force applied, the velocity, and the distance required to move a load.


  1. If the pump delivers a constant flow of 10 gallons per minute
  2. The valve, however, will control or limit the flow
  3. The actuator receives only 5 gallons of oil and the oil will flow only a half way in one minute.
  4. The excess flow will be diverted in excess of the discharge of 5 GPM.

Laminar and Turbulent Flows

There are two sorts of flow in both metal and non-metal (flexible) pipes:

Laminar flow.

Laminar flow is a fluid flow at a certain velocity forming a uniform layer to obtain a smooth, linear and parallel flow

Turbulent flow.

Turbulent flow is a flow in which there is an increase in the velocity of the fluid movement throughout a pipe of uniform diameter.  The nature of the flow will change after reaching a certain velocity (the critical velocity).  The flow forms a circle and turbulence that influence and obstruct each other.

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