PRINCIPLES OF NAVAL ENGINEERING 



5.180 

 Figure 15-1.— Results of pressure applied to a 

 rigid bar (left) and to a column of liquid (right). 



The principle that pressure is transmitted 

 equally and undiminished in all directions 

 through a contained liquid is known as Pascal's 

 principle. This principle may be regarded as the 

 basic law or foundation of the science of hydrau- 

 lics. 



An important corollary of Pascal's principle 

 is that the transmission of pressure through a 

 liquid is not altered by the shape of the container. 

 This idea is illustrated in figure 15-2. If the 

 pressure due to the weight of the liquid is 8 psi 

 at any one point on the horizontal line H^ it is 8 

 psi at every point along line^. The pressure due 

 to the weight of the liquid at any level thus 

 depends upon the vertical distance from the 

 chosen level tothesurfaceof the liquid. The ver- 

 tical distance between two horizontal levels in a 

 liquid is known as the head of the liquid. (Since 

 various kinds of head enter into pump calcula- 

 tions, the term head is more fully discussed 

 later.) 



Pressure is defined as force per unit area. 

 Alternatively, we may say that force is equal to 

 pressure times area. Figure 15-3 shows how a 

 force of 20 pounds acting on a piston with an area 

 of 2 square inches can produce a force of ?00 

 pounds on a piston with an area of 20 square 

 inches. The system would, of course, work the 

 same in reverse. If we consider piston 2 as the 

 the input piston and piston 1 as the output piston, 

 then the output force would be 1/10 the input 

 force. 



We are now in a position to state a general 

 rule: If two pistons are used in a hydraulic sys- 

 tem, the force acting on each will be directly 

 proportional to its area, and the magnitude of 

 each force will be the product of the pressure 

 and the area. 



The second basic rule for two pistons in a 

 hydraulic system such as the one shown in fig- 

 ure 15-3 may be stated as follows: The distance 

 moved by each piston is inversely proportional 

 to the area of the piston. Thus if piston 1 in fig- 

 ure 15-3 is pushed down 1 inch, piston 2 will be 

 raised 1/10 inch. 



Consideration of the two basic rules just 

 stated leads us to another basic rule: The input 

 force multiplied by the distance through which it 

 moves is exactly equal to the output force 

 multiplied by the distance through which it moves 

 (disregarding energy losses due to friction). In 

 essence, this rule is merely another statement of 

 the general energy equation— that is, energy in= 

 energy out . 



PUMP CAPACITY.— The capacity of a pump 

 is the amount of liquid the pump can handle in a 

 given period of time. For marine applications, 

 the capacity of a pump is usually stated in gal- 

 lons per minute (gpm). 



PRESSURE HEAD. -The power required to 

 drive a pump is a function of pump capacity and 

 of the total head against which the pump oper- 

 ates. Previously we defined head quite simply 

 as the vertical distance between two horizontal 

 levels in a liquid. Since a pump may be installed 

 above, at, or below the surface of the source of 

 supply, it is obvious that other factors must enter 

 into the discussion of pressure head as applied 

 to pumps. 



When the pump is installed at the same level 

 as the free surface of the source of supply, no 

 new considerations need apply since the pump 

 merely acts on the liquid like any other applied 

 force. 



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