losses.html

<h1>System Losses</h1>
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The ideal energy required to move a fluid under open, atmospheric conditions is the product of the fluid weight and the elevation through which it is lifted.  For example, the ideal energy required to lift ten gallons of water (weighing 83.3 lbs) by 120 feet is 83.3 x 120 = 10000 ft-lbs, or a little over 3 kilocalories. 
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In fluid systems, useful hydraulic energy is associated with three parameters - elevation, pressure, and velocity.   It is common to express these energy parameters, both individually and in combination, in terms of energy per unit weight, or "head".   In the early 1700's, Daniel Bernoulli and Leonhard Euler developed a relationship, now commonly called the Bernoulli equation, that states that the combined energy associated with elevation, pressure, and velocity in a frictionless streamline (i.e., ideal) flow is constant, even though the individual components may change.  
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Ideal systems, by definition, have no friction.  In real fluid systems, there are many sources of friction, including the piping through which the fluid is moved, pipe components, such as valves, elbows, tees, and internal friction within the fluid.  In other words, essentially anything the fluid touches is a source of friction.  In order to overcome these sources of friction, additional energy input is necessary.  The magnitude of the energy required to overcome the friction is dependent on a variety of factors, including the fluid velocity and viscosity, pipe length, valve(s) type and position, and pipe quality (smoothness).   Because of this ubiquitous friction, some of the energy is always converted to heat during flowing processes, so that the hydraulic energy at the end of a flow path is less than that at the beginning; the difference being the frictional loss.
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The total head that must be supplied by the pump in a fluid system is that needed to overcome the combination of static (pressure plus elevation) head, frictional head, and velocity head components.  The total fluid power required to move fluid at a given mass flow rate through a system is proportional to the total system head times the flow rate.  Obviously, the greater the frictional head, the more energy/power required to overcome it.
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One of the things needed by the Pumping System Assessment Tool (PSAT) in establishing what the system "requires" is the required head.  In the case of a real world system, what head should be used?  One possibility, in the case of a system which is dominated by static head, is to simply use the static head (i.e., the elevation plus pressure head).  
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At the other extreme, where the system is a closed loop, recirculating system, the static head will be, ideally, zero, meaning that essentially all of the head to be developed is that needed to overcome friction.  What head can be used as the "required" value in such a case?  Or for that matter, what would be the required head in any system as a function of the flow rate?  For example, if a system is normally operated at 1500 gpm and 184 feet of head, but flow is being regulated by a control valve.  If the valve is fully opened, the flow rate increases to 2000 gpm, and head is measured to be 180 feet.  What is the required head at 1500 gpm in this system - in other words, what would the required head at 1500 gpm be if the valve losses were eliminated?
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Some systems are sufficiently complex that a detailed system model is needed to answer that question.  But many real-world systems can be closely approximated by simple system curves.  One of the tools included with PSAT is a system curve estimator.  Armed with any two operating points, an estimate of the full system curve can be developed.  Assuming that we know that the system noted in the paragraph above has a static head of 40 feet, the system curve estimator, which can be accessed from the primary PSAT panel, can be used to tell us that the required head at 1500 gpm should be about 138 feet - 75% of the current operating head.  The difference is being consumed by the control valve. 
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Understanding the nature and potential merits of eliminating unnecessary system losses can be extremely important to reducing the energy consumption in pumping systems.  For many systems, PSAT can help quantify both the hydraulic losses and the potential energy savings that can be derived from their elimination.
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