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SEP-OCT 2018

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S ome material in this section may seem self- evident to recent engineering graduates. However, it is worth establishing some basic concepts as a springboard to what follows. It does not hurt to review basic material from time to time. It is surprising what one forgets—or never learned. Compressibility of all fluids Many terms used in the field of hydraulics are simplified descriptions of reality. Sometimes these didactic simplifications are useful; sometimes they are not. They help younger people over early hur- dles and hinder them in their later comprehension. The name of the term, itself, can lead to concep- tual errors. For instance, the distinction between an incompressible fluid and a compressible one is just a matter of degree. There may be orders of magnitude between the compressibility of a gas and that of a liquid; nevertheless, it is wise to bear in mind that all fluids are compressible to some degree—even the ones we call "incompressible." Later, we will show how compressibility helps ex- plain another logical inconsistency—fluid friction. Mechanism of flow It is obvious to anyone who has ever used a gar- den hose that water flows when there is a higher pressure upstream than downstream. When we open a tap, or faucet, the higher pressure in the pipe causes water to flow. This common observa- tion leads to the assumption that a higher pres- sure upstream than downstream is always associ- ated with flow. The assumption is not correct. It is true a higher pressure is required upstream to start a fluid flowing across a restriction. However, once a fluid is flowing, it has momentum. As will be developed in greater detail when we discuss the Bernoulli equation, pressure is associated with one form of energy, static energy, and energy can take on different forms. One of these forms is kinetic energy. When a high velocity fluid flows from a small diameter pipe through a swage to a larger diameter pipe, the fluid slows down because of the continuity principle. In slowing down, some of its kinetic energy is changed to static energy. This change can cause the pressure downstream to be greater than the pressure up- stream of the swage. This phenomenon is called pressure recovery. It is not necessary to have a higher pressure upstream in order to initiate flow. If a pipe is connected to an open reservoir—for instance, a water tower in a city water distribution net - work—flow will begin once a valve is opened. The pressure on the surface of the water in the reservoir is approximately equal to the pressure in a kitchen sink. In this case, there is no differ - ence in pressure between the ends of the piping system, but flow occurs. There is more than just pressure difference involved in the flow of wa - ter. An unbalanced force in the direction of the flow is necessary. 50 INTECH SEPTEMBER/OCTOBER 2018 WWW.ISA.ORG By Raymond Mulley, PE Editor's note: The Basics department this month i s a n e x c e r p t f r o m the ISA book Flow of Industrial Fluids – The- ory and Equations by Raymond Mulley.

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