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SEP-OCT 2018

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nal elevation of the surface of the reservoir. The incremental internal energy flows as heat through the pipe walls and is lost to practical use. It warms up the atmosphere. In a well-insulated pipe, it remains with the fluid for a longer period of time and is seen as a higher fluid temperature. Logical inconsistency of fluid friction The term "fluid friction" is commonly used to explain losses in mechanical forms of energy associated with fluid flow. A little knowledge of physics shows it is a poorly chosen term. Friction is a macro - scopic phenomenon—such as when two blocks of wood slide over one another, or when the rubber hits the road. There is a definite resisting force to be overcome— the force of friction. Fluid molecules do not touch Fluids are made up of molecules, and molecules do not touch in the fluid state. Collisions of gas molecules are actually re- pulsions that prevent the molecules from touching. Liquid molecules are less ener- getic than gas molecules. They slide past one another or remain at an equilibrium distance. The equilibrium distance is estab- lished by a balance between an attractive force and a repulsive force. If molecules do not touch, how can there be "friction"? The answer is found in the compressive forces that occur when local flow is slowed down by obstructions of any kind. These obstructions can be large objects, such as valves or fittings; they can be eddies within the flowing stream; they can be other mol- ecules that have diffused between two lay- ers of fluid moving at different velocities. n ABOUT THE AUTHOR Raymond Mulley, PE, was the author of the book, where he shared his more than 30 years of professional experience, which included Flour Daniel, Dravo Chemplants, International Nickel, and private practice in the field of process hazards and control. Mulley passed away in 2008. AUTOMATION BASICS 52 INTECH SEPTEMBER/OCTOBER 2018 WWW.ISA.ORG If the valve is opened, the water will flow through it by virtue of the pressure difference across it. The pressure upstream of the valve will decrease for two reasons. First, some of the pressure energy has been converted to kinetic energy, the energy a body has by virtue of its motion. Second, some of the energy of the water has been converted to internal energy because of what is commonly called "fluid friction" in the upstream piping. The increase in inter- nal energy is accompanied by an increase in temperature—hence, the loose term, "thermal energy." This loose term shortly will be explained more fully. The kinetic energy is recoverable. It can be transformed back to static (pressure) energy by closing the valve or to poten - tial energy by connecting an open, vertical pipe to an elevation equal to that of the reservoir. The incremental internal energy is not completely recoverable to one of the mechanical forms of energy. This is the rea - son the water in the previously mentioned vertical pipe cannot quite reach the origi - ...once a fluid is flowing, it has momentum Build it, See it, Price it, Order it. Point and click or tap to specify valve type, end connections, actuator, and accessories. Model number, price, description, and photo all update instantly! Try one today at: We Make Valve Automation Easy! Online Valve Configurators Quickly & Easily Specify a Complete Automated Valve Assembly on Your PC, Tablet, or Phone Wherever you see this image

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