In common usage liquid pressure is expressed in"pounds per square inch." The derivation of this term isbest shown by reference to an open-top vessel where thesurface of the liquid is exposed to the atmosphere. If, forexample, a tube one inch square, or having a cross-sectionarea of one square inch, is filled to a vertical height of2.31 feet (27.7 inches) with water having a specificgravity of 1.0, the water will weigh one pound and thepressure at the bottom of the tube will be one pound persquare inch. A 2.31 foot-high column of mercury, whichhas a specific gravity of 13.55 would weigh 13.55 poundsand the pressure would be 13.55 pounds per square inch.Likewise, a petroleum oil having a specific gravity of lessthan water, will exert correspondingly less pressure. Thegeneral formula for pressure is :

Head in feet X·Sp. Gr.Lb. per Sq. Inch == --------

2.31

Pressure in a liquid is proportional to the depth aloneand is not influenced by the size or shape of the contain­ing vessel. This principle is illustrated in Figure 1.

Pressure in a liquid is due to the pull of gravity. How­ever, cohesion is so weak in the liquid that the pressureis released to go in any direction it can. In a solid, co­hesion overcomes this force, and the entire mass hangstogether and presses downward only.

Since pressure in a liquid is equal in all directions atanyone point, it is evident that at any depth the upwardpressure is equal to the downward pressure. This upwardpressure of liquids is manifested when any two containerswith open tops, 'regardless of shape or size, are connectedbelow the liquid levels. The liquid in one container willflow into the other until it reaches the same height in both.When the liquid heights become equal, the "up" pressureon one is equal to the"down" pressure on the other.

A liquid seeks its own level, and any free liquid isalioays level on its surlaee. The speed with which a liquidseeks its own level depends upon the viscosity of the liquid(see explanation of viscosity, page 7). A liquid with ahigh viscosity, such as cylinder oil, will actually pile upfor a time when run through a pipe into a free tank whilea liquid such as gasoline seeks its level immediately. (SeeFigure 2).

Application of Liquid Pressure

If a liquid is put under pressure in a closed container,pressure is exerted alike in all directions-downward andupward. The hydraulic press works by the transmission of

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liquid pressure based upon the above principle. Becauseof the difference in pressure exerted by different liquids,the process of settling is made possible. Two liquids ofdifferent densities and that will not mix may be run intothe same tank and the denser liquid will settle to thebottom of the tank and may be drawn off under the lessdense liquid. Because of the difference of specific gravity(density) of the two materials, one will float on the other.Oil and water are separated in this manner.

Liquids at Rest and in Motion

There are two fundamental pressure conditions ofliquids; the static pressure or head where liquids are atrest, and the dynamic pressure or head where liquids arein motion.

Static Head-Static head is the height from a givenpoint in a column or body of still liquid to the surfaceof the liquid. This height is generally expressed in feet.For water, the pressure in pounds per square inch at thegiven point can be calculated by dividing the static head,in inches, by 27.7, or dividing the head in feet by 2.31;for example, a head of 46.2 feet of water equals 20 poundsper square inch.

Dynamic Head-Dynamic head is a measure of theenergy of motion of a flowing liquid. It is, in effect, thestatic head required to accelerate the liquid to its flowingvelocity. In Figure 3, the distance from the top of thetank to ground level is the static head. As the liquidflows down through the pipe, it loses static head but gainsvelocity, or dynamic head. At the point of discharge, thedynamic head begins converting back to static head asthe liquid rises until all dynamic head is converted tostatic and then falls back to the ground. The "loss ofhead" represents friction losses in pipe and air.

Stated another way, the liquid in the tank has potentialenergy-that is, energy of position with respect to theground level. This is converted to kinetic energy or energyof motion as the liquid drops through the pipe to groundlevel. The kinetic energy at ground level is equal to theoriginal potential energy minus friction losses.

A familiar example is the garden hose. With the faucetturned on and the nozzle turned off, the hose is filled withwater at line rressure (static head or potential energy).Open the nozzle. Line pressure is now used up in movingthe water (converted to dynamic head or kinetic energy) .There is no pressure (static head or potential energy) atthe nozzle. It is all energy of motion, and this energy isused up in carrying the water through the air.