Pipe sizing

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  • 1. Major & Minor Losses Under Supervision of:Prof. Dr. Mahmoud FouadBy students: Mahmoud Bakr 533 Mohammed Abdullah 511 Moaz Emad 619 Mohammed Nabil Abbas 525

2. Applications 3. How big does thepipe have to be to carry 3 a flow of x m /s? 4. Bernoulli's Equation The basic approach to all piping systems is to write the Bernoulli equation between two points, connected by a streamline, where the conditions are known. For example, between the surface of a reservoir and a pipe . outlet The total head at point 0 must match with the total head at point 1, adjusted for any increase in head due to pumps, losses due to pipe friction and so-called "minor losses" due to entries, exits, fittings, etc. Pump head developed is generally a function of the flow through the system 5. Bernoulli's Equation 6. Friction Losses in Pipes Friction losses are a complex function of the systemgeometry, the fluid properties and the flow rate in the system. By observation, the head loss is roughly proportional to the square of the flow rate in most engineering flows (fully developed, turbulent pipe flow). This observation leads to the Darcy-Weisbach equation for head loss due to friction 7. For laminar flow, the head loss is proportional tovelocity rather than velocity squared, thus the friction factor is inversely proportional to velocity 8. Turbulent flow For turbulent flow, Colebrook (1939) found animplicit correlation for the friction factor in round pipes. This correlation converges well in few iterations. Convergence can be optimized by slight under-relaxation. 9. The familiar Moody Diagram is a log-log plot of the Colebrook correlation on axes of friction factor and Reynolds number, combined with the f=64/Re result from laminar flow. The plot below was produced in an Excel spreadsheet 10. An explicit approximation 11. Pipe roughness pipe material glass, drawn brass, copper commercial steel or wrought iron asphalted cast iron galvanized iron cast iron concrete rivet steel corrugated metal PVCpipe roughness (mm) 0.0015 0.045 0.12 d Must be 0.15 dimensionless! 0.26 0.18-0.6 0.9-9.0 45 0.12 12. Calculating Head Loss for a Known Flow From Q and piping determine Reynolds Number,relative roughness and thus the friction factor. Substitute into the Darcy-Weisbach equation to obtain head loss for the given flow. Substitute into the Bernoulli equation to find the necessary elevation or pump head 13. Calculating Flow for a Known Head Obtain the allowable head loss from the Bernoulli equation, then start by guessing a friction factor. (0.02 is a good guess if you have nothing better.) Calculate the velocity from the Darcy-Weisbach equation. From this velocity and the piping characteristics, calculate Reynolds Number, relative roughness and thus . friction factor Repeat the calculation with the new friction factor until sufficient convergence is obtained. Q = VA 14. "Minor Losses" Although they often account for a major portion of the head loss, especially in process piping, the additional losses due to entries and exits, fittings and valves are traditionally referred to as minor losses. These losses represent additional energy dissipation in the flow, usually caused by secondary flows induced by curvature or recirculation. The minor losses are any head loss present in addition to the head loss for the same . length of straight pipe Like pipe friction, these losses are roughly proportional to the square of the flow rate. Defining K, the loss coefficient, by 15. . K is the sum of all of the loss coefficients in thelength of pipe, each contributing to the overall head lossAlthough K appears to be a constant coefficient, itvaries with different flow conditions: Factors affecting the value of K include .,the exact geometry of the component .the flow Reynolds number , etc 16. Some types of minor losses Head Loss due to Gradual Expansion (Diffuser) (V1 V2 ) 2 hE = K E2g 2 2 V A hE = K E 2 2 1 2 g A1 KE0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0020 40 60 80 diffusor angle () 17. Sudden Contraction 21 V2 hc = 1 2 C 2g c V2V1 flow separationlosses are reduced with a gradual contraction = Ac C cA2 18. Sudden ContractionCc1 0.95 0.9 0.85 0.8 0.75 0.7 0.65 0.6 = CAorifice 2 gh0.81 19. Entrance Losses Losses can be reduced by accelerating the flow gradually and eliminating thehe = K e K e 1.0 K e 0 .5vena contracta K e 0.04V2 2g 20. Head Loss in Bendspressure High Head loss is a function ofthe ratio of the bend radius to the pipe diameter (R/D) Velocity distribution returns to normal several pipe diameters downstreamPossible separation from wallRD Low pressurehb = K bKb varies from 0.6 - 0.9V2 2g 21. Head Loss in Valves Function of valve type and valveposition The complex flow path through valves can result in high head loss (of course, one of the purposes of a valve is to create head loss when it is not fully open)hv = K vV2 2g 22. To calculate losses in piping systems with both pipefriction and minor losses use 23. Solution Techniques Neglect minor losses Equivalent pipe lengths Iterative Techniques Simultaneous Equations Pipe Network Software 24. Iterative Techniques for D and Q (given total head loss) Assume all head loss is major head loss. Calculate D or Q using Swamee-Jain equations Calculate minor losses Find new major losses by subtracting minor lossesfrom total head loss 25. Solution Technique: Head Loss Can be solved directlyhminor = K Re =V2hminor = K2g4Q Df =8Q 2 g 2 D 40.25 2 5.74 + log 3.7 D Re 0.9 hl = f + minor h hhf = f8LQ 2g 2 D 5 26. Solution Technique: Discharge or Pipe Diameter Iterative technique Set up simultaneous equations in Excel Re =4Q Dhminor = Kf =0.25 2 5.74 log + 0 .9 3.7 D Re 8Q 2 g 2 D 4hl = f + minor h hhf = f8LQ 2g 2 D 5Use goal seek or Solver to find discharge that makes the calculated head loss equal the given head loss. 27. Example: Minor and Major Losses Find the maximum dependable flow between thereservoirs for a water temperature range of 4C to 20C.Water25 m elevation difference in reservoir water levels Reentrant pipes at reservoirs Standard elbows2500 m of 8 PVC pipe 1500 m of 6 PVC pipeSudden contraction Gate valve wide open 28. Directions Assume fully turbulent (rough pipe law) find f from Moody (or from von Karman)Find total head loss Solve for Q using symbols (must include minorlosses) (no iteration required) Obtain values for minor losses from notes or text 29. Example (Continued) What are the Reynolds number in the two pipes? Where are we on the Moody Diagram? What value of K would the valve have to produce toreduce the discharge by 50%? What is the effect of temperature? Why is the effect of temperature so small? 30. Example (Continued) Were the minor losses negligible? Accuracy of head loss calculations? What happens if the roughness increases by a factorof 10? If you needed to increase the flow by 30% what could you do? Suppose I changed 6 pipe, what is minimum diameter needed?