Sizing liquid flow pipelines easily


Published on Newsletter, vol 212 - Sizing liquid flow pipelines easily. Designing pipe work for liquid flow requires calculating the pressure losses along the pipe due to pipe friction. Once the losses are known the right pipe diameter can be selected. If full engineering calculations are not possible the conservative approach is to select pipe sizes that produce a velocity of less than 3 m/sec.

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Sizing liquid flow pipelines easily

  1. 1. FEED FORWARD FLYER ADVANCING THE TECHNICAL KNOWLEDGE OF MAINTENANCE TRADESMEN AND PLANT OPERATORS. SIZING LIQUID FLOW PIPELINES EASILYABSTRACTSizing liquid flow pipelines easily. Designing pipe work for liquid flow requires calculating the pressure losses along the pipe dueto pipe friction. Once the losses are known the right pipe diameter can be selected. If full engineering calculations are not possiblethe conservative approach is to select pipe sizes that produce a velocity of less than 3 m/sec. Keywords: pressure loss, viscosity,friction factor.A ‘rule-of-thumb’ to use if you need to install liquid carrying pipelines without engineered drawings is to look for the largestsupplying nozzle (usually a pump discharge or vessel outlet) and go up one standard pipe size for the main trunk line from thenozzle to the delivery point. For branches being supplied off the main trunk go up one pipe size if they are smaller than the maintrunk line. Reduce down at nozzles and trunk line tees as necessary.It is a handy rule-of-thumb but not the best way. The correct way to size liquid carrying pipelines is to calculate the pressure lossesin the pipe from beginning to end.NECESSARY REQUIREMENTS TO SIZE A PIPELINEPipe sizing depends on –- Liquid velocity – fast flows have more pressure loss.- Liquid viscosity (slipperiness) – the more viscous the liquid the more the friction. Temperature changes affect viscosity, usually decreasing with rising temperature.- Pipe length – longer pipes have more pressure loss.- Surface area – bigger diameters have lower velocities.- Pipe wall roughness (surface finish) - rough pipes have more friction until complete turbulence is reached.- Liquid vapour pressure – friction can cause such pressure loss the liquid boils and creates vapour bubbles.- Presence of bubbles – gas bubbles take up space.- Corrosive and erosive properties of the liquid – some liquids destroy piping at high velocities.HOW TO CALCULATE PIPE SIZEEngineers use the formula below (1) to calculate pressure loss in liquid carrying pipelines. Its use requires specialised knowledge.However an easier method is to calculate the pipe velocity from the flow rate using various pipe diameters in the much simplersecond formula (2). Then select the diameter that produces a liquid velocity between 1.5 m/s (5 ft/s) to a maximum of 3 m/s (10ft/s). Hl = f l V 2 (1) d2gWhere Hl is the head loss (m or ft) f is the pipe wall friction factor depending on the liquid’s viscosity (dimensionless) l is the equivalent pipe length (m or ft) V is the liquid velocity (m/s or ft/s) d is the pipe diameter (m or ft) g is the gravitational constant (9.8 m/s2 or 32 ft/s2) Q=VA (2)Where Q is the volume flow rate (m3/s or ft3/s) V is the velocity (m/s or ft/s) A is the area of the pipe bore (m2 or ft2)Once the flow rate is decided or known, the pipe diameter can be calculated from the area. Minimise the cost of dischargepipelines by using smaller bore pipe with velocities at the high end of the recommended range. But to keep pressure loss low onsuction piping use velocities from the lower end of the range.WHERE PRESSURE LOSSES COME FROM IN PIPESLiquid moving in a pipe has to push its way along pipe walls, around bends, through valves, past projections and enclosed items.Throughout its progress friction robs energy. Three factors affect pressure loss in pipes.Surface Friction – Along walls the liquid has to overcome friction. Wall friction depends on surface roughness. The higher theprojections into the liquid the more the friction. At low velocities a laminar sub layer of slow flowing liquid covers the roughness.But at high velocities the sub layer thins as eddies in the turbulent center flow extend to the projections. Figure 1 shows the flow atthe pipe wall.Direction Changes – Liquid that is forced to change direction loses energy. When a liquid goes around a 90o bend its momentum
  2. 2. has to be redirected. From going at velocity in one direction it now has to go at the same velocity in a completely differentdirection. To do this it converts pressure into the energy it needs to change direction. A pressure loss results. The more suddenthe change in direction the greater the energy it needs and the higher the corresponding pressure losses.Liquid viscosity – A liquid that easily slides requires less energy to move than one that does not want to flow. Honey is thousandsof times more viscous than water. Pushing honey through pipes, around corners and past valves requires far more energy than forwater. If the honey was warmed it would then flow easier and the pressure loss would be less. TURBULENCE EDDIES SURFACE LIQUID ROUGHNESS VELOCITY LAMINAR PROFILE SUBLAYER INSIDE PIPE PIPE WALL LAMINAR SUBLAYER Figure 1 Effects of Surface Roughness on Liquid FlowMINIMISE FRICTION IN PIPELINE CONSTRUCTIONDuring design, procurement and installation of a pipeline follow the recommendations in Table 1. Select 100 meters of 100mm (4”) plastic pipe containing smooth water moving at 1 m/s (3.25 ft/s) will lose 900mm bore pipes. (35”) of pressure. The same flow in a 100mm steel seamless pipe will lose 1000mm (39”) pressure. Minimise Use long radius elbows; use pipe reducers; minimise DESIGN flow the number of flanges; use full bore straight through disruptions. valves; use pipe size branch tees and reduce down on the branch (a must on suction lines); use instrumentation which does not project into the flow. Keep flow Select pipe sizes that are practicable while keeping velocities flow velocities reasonable. low. Cut gasket Make the gasket bore hole to the pipe bore diameter. holes to the Do not let the gasket project into the liquid flow path bore as it will cause unnecessary friction and turbulence. diameter. CONSTRUCTION Clean off Weld splatter sits on the pipe surface and disrupts the weld liquid flow pattern. The splatter can come lose and splatter. damage down stream instruments and equipment. Keep butt Like gaskets cut with the diameter less than the pipe weld peaks bore, welds with high peaks project into the liquid low. flow. Try keep welds below the height of the sub- layer thickness. Make ‘Lobster back’ 90o bends are used when large diameter multiple long radius bends are not available. Make them out of segment equal sized 15o segments instead of the usual 22.5o mitered segments bends.Mike Sondalini - Maintenance Engineer Web Address: E-mail Address: Because the authors and publisher do not know the context in which the information presented in the flyer is to be used they accept no responsibility for the consequences of using the information contained or implied in any articles.