Hydraulic Principles -Fundamentals of Engineering Module #12

hydraulic principles

Simple braking system diagram.

We are getting into some stuff now aren’t we? We started off small with some basics but we are already moving into some heavy territory.

It is important as building engineers that we at least understand some concepts behind the principles of hydraulics. For example, when a new building is being designed, planners must take into account moving water or fluids around as needed.

Will a chiller need water from the roof? What size pump and piping will the system need to deliver the desired amount of flow or pressure? How far does the fluid have to travel and what obstacles are along the way?

The principles of hydraulics are everywhere – inside your home, the engine of your car, when you are using your hose outside in your garden. What happens when you fold your hose over? The pressure decreases while the velocity increases, it’s trying to get the same amount of water out of a now smaller opening. Hydraulics.

What Are Hydraulics?

Hydraulics at first was concerned with studying the behavior of water both at rest and in motion.

Hydraulics has evolved to encompass the behavior of all liquids, like oil or petroleum. Water is pretty much the standard by which most other substances are measured against. Things like specific gravity, viscosity, all change depending upon the fluid of the system. Most fluids I would guess behave at least a little bit like water but not quite. This is why the study of hydraulic principles has grown to include everything.

Modern Hydraulics

pascal's law

Blaise Pascal

The basis of modern hydraulics started with the discovery of yet another sweet law and principle courtesy of a Frenchman, Blaise Pascal.

Pascal’s Law

  • States that pressure exerted at any point upon an enclosed, non-compressible liquid is transmitted equally and undiminished in all directions.

Pascal’s law governs the behavior of the static factors concerning non-compressible fluids when taken by themselves.

For more info:  http://en.wikipedia.org/wiki/Pascal’s_law

Bernoulli’s Principle

This principle was discovered by Jacob Bernoulli, a Swiss philosopher and mathematician. The crazy cat Bernoulli was really into hydraulics and the relationship between pressure and temperature. Maybe he was a stationary engineer or boiler operator at heart?

Bernoulli’s principle governs the relationship of the static and dynamic factors concerning non-compressible fluids. Remember, Pascal’s law governed the behavior, Bernoulli’s the relationship.

what are hydraulicsThe diagram to the right shows the effect of Bernoulli’s principle. Chamber A is under pressure and is connected by
a tube to chamber B, also under pressure. Chamber A is under static pressure of 100 psi. The pressure at some point, X, along the connecting tube consists of a velocity pressure of 10 psi. This is exerted in a direction parallel to the line of flow,

Added is the unused static pressure of 90 psi, which obeys Pascal’s law and operates equally in all directions. As the fluid enters chamber B from the constricted space, it slows down. In so doing, its velocity head is changed back to pressure head. The force required to absorb the fluid’s inertia equals the force required to start the fluid moving originally.

Therefore, the static pressure in chamber B is again equal to that in chamber A. It was lower at intermediate point X.

The diagram disregards friction, and it is not encountered in actual practice. Force or head is also required to overcome friction. But, unlike inertia effect, this force cannot be recovered again although the energy represented still exists somewhere as heat.

Therefore, in an actual system the pressure in chamber B would be less than in chamber A. This is a result of the pressure used in overcoming friction along the way.

At all points in a system, the static pressure is always the original static pressure minus any velocity head at the point in question. It is also less the friction head it used in reaching that point. Both velocity head and friction represent energy that came from the original static head.

You do remember the law of conservation of energy right? Energy cannot be destroyed.

This means that the sum of the static head, velocity head, and friction, anywhere in the system, has to add up to the original static head.

Bernoulli’s Principle:

  • If a non-compressible fluid flowing through a tube reaches a constriction, or narrowing of the tube, the velocity of fluid flowing through the constriction increases, and the pressure decreases.

For more info:  http://en.wikipedia.org/wiki/Bernoulli’s_principle

Pretend we apply a force to the end of a column of confined liquid; the force is transmitted not only straight through to the other end but also equally in every direction throughout the column.

hydraulic principlesLet us now consider the effect of Pascal’s law in the systems shown in the drawing, views A and B.

If the total force at the input piston is 100 pounds and the area of the piston is 10 square inches, then each square inch of the piston surface is exerting 10 pounds of force. This liquid pressure of 10 psi is transmitted to the output piston, which will be pushed upward with a force of 10 psi.

In this example, we are merely considering a liquid column of equal cross section so the areas of these pistons are equal. All we have done is to carry a 100-pound force around a bend. However, the principle shown is the basis for almost all mechanical hydraulics.

The same principle may be applied where the area of the input piston is much smaller than the area of the output piston or vice versa. In view B, the area of the input piston is 2 square inches and the area of the output piston is 20 square inches. If you apply a pressure of 20 pounds to the 2 square-inch piston, the pressure created in the liquid will again be 10 psi. The upward force on the larger piston will be 200 pounds—10 pounds for each of its 20 square inches. Thus, you can see that if two pistons are used in a hydraulic system, the force acting on each piston will be directly proportional to its area.


That’s a wrap on hydraulics people. Not really though. This was just an intro to the principles of hydraulics, not even close to everything you need to know.

This fundamentals of engineering post covered what hydraulics are all about, it also descibed both Pascal’s law and Bernoulli’s principle. Confined system, pressure exerted in all directions, yadda yadda yadda. Later, as we get more into pumps, we will get knee deep into hydraulics. And static head, friction.

Ready for the quiz? Click here to test your brainpower with the principles of hydraulics quiz.

If you are like me, and constantly on the search for more knowledge, check out this book on the basics of hydraulics.  It will go into way more detail on the subject than I ever could.

Introduction to Hydraulics & Hydrology – $35.49

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from: RedShelf

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