8. Operation limits of pump-pipeline system

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8. Operation limits of pump-pipeline system

  1. 1. oe4625 Dredge Pumps and Slurry Transport Vaclav Matousek October 13, 2004 1 Dredge Pumps and Slurry Transport Vermelding onderdeel organisatie
  2. 2. 8. OPERATION LIMITS OF PUMP-PIPELINE SYSTEM REQUIRED MANOMETRIC PRESSURE MAXIMUM VELOCITY – INITIAL CAVITATION MINIMUM VELOCITY – STATIONARY BED October 13, 2004 2 Dredge Pumps and Slurry Transport
  3. 3. REQUIRED MANOMETRIC PRESSURE DETERMINATION OF SUCTION PRESSURE DETERMINATION OF DISCHARGE PRESSUREOctober 13, 2004 3Dredge Pumps and Slurry Transport
  4. 4. Hman-Q CURVE OF A CENTRIFUGAL PUMP•A rotating impeller of a centrifugal pump addsmechanical energy to the medium flowing througha pump.•As a result of an energy addition a pressuredifferential occurs in the pumped medium betweenthe inlet and the outlet of a pump.•The manometric pressure, Pman, that isdelivered by a pump to the medium, is given as ρm (Vp2 − Vs2 ) Pman = Pp − Ps + ρm ( hp + hs ) + 2 The manometric head, Hman, that is delivered by Pman a pump to the medium, is H man = ρf g October 13, 2004 4 Dredge Pumps and Slurry Transport
  5. 5. REQUIRED MANOMETRIC PRESSURE Pman = Pp − Ps + ρm (V 2 p −Vs2 ) Patm hd,pipe 2 hs,pump Ps hd,pump Pp = ? Ps = ? hs,pipe Pump-pipeline system: mean velocity V mixture density rmOctober 13, 2004 5Dredge Pumps and Slurry Transport
  6. 6. REQUIRED MANOMETRIC PRESSURE Ps = absolute suction pressure Patm hd,pipe hs,pump Ps hd,pump Vs2 Ps = Patm + ρ f ghs, pipe − ρm g ( hs, pipe − hs, pump ) − ρ f gHtotloss,s,m − ρ f hs,pipe 2 Pump-pipeline system: mean velocity V mixture density rmOctober 13, 2004 6Dredge Pumps and Slurry Transport
  7. 7. REQUIRED MANOMETRIC PRESSURE Pp = absolute discharge pressure Patm hd,pipe hs,pump Ps hd,pump Vp2 Pp = Patm + ρm g ( hd , pipe + hd , pump ) + ρ f gHtotloss,d ,m − ρ f hs,pipe 2 Pump-pipeline system: mean velocity V mixture density rmOctober 13, 2004 7Dredge Pumps and Slurry Transport
  8. 8. MAXIMUM VELOCITY IN THE SYSTEM THE UPPER LIMIT FOR A SYSTEM OPERATION:VELOCITY AT THE INITIAL CAVITATION OF A PUMPOctober 13, 2004 8Dredge Pumps and Slurry Transport
  9. 9. Static Pressure Variation Along SystemThe static-pressure (P)variation along suction anddischarge pipes connectedwith a pump (schematic).The static pressure varies due tochanges in- the geodetic height (suction pipe)- the velocity [head] (change in pipe diameter in front of the pump)and…due to the losses (both in suction anddischarge pipes). October 13, 2004 9 Dredge Pumps and Slurry Transport
  10. 10. MAXIMUM VELOCITY IN THE SYSTEMCriterion for non--cavitational operation of a systemThe no cavitation condition for a certain pump-suction pipe combination is: (NPSH)REQUIRED < (NPSH)AVAILABLEThe available Net Positive Suction Head is a total available energy head over the vapour pressure at the suction inlet to the pump during an operation at velocity Vm in a suction pipe of a certain geometry and configuration. Ps − Pvapour Vm2 ( NPSH) AVAILABLE = + ρf g 2g October 13, 2004 10 Dredge Pumps and Slurry Transport
  11. 11. MAXIMUM VELOCITY IN THE SYSTEMCriterion for non--cavitational operation of a systemThe no cavitation condition for a certain pump-suction pipe combination is: (NPSH)REQUIRED < (NPSH)AVAILABLEThe required Net Positive Suction Head is a minimum energy head a certain pump requires to prevent cavitation at its inlet. This is a head value at the incipient cavitation. Ps,min − Pvapour Vm2 ( NPSH) REQUIRED = + ρf g 2g October 13, 2004 11 Dredge Pumps and Slurry Transport
  12. 12. MAXIMUM VELOCITY IN THE SYSTEMCriterion for non--cavitational operation of a systemThe no cavitation condition for a certain pump-suction pipe combination is: (NPSH)REQUIRED < (NPSH)AVAILABLEThe (NPSH)REQUIRED-Q curve is a characteristic specific for each pump and it must be determined by a pump cavitation test. A design (dimensions, shape) and an operation (specific speed) of a pump decide the absolute suction pressure at the initial cavitation. Ps,min − Pvapour Vm 2 ( NPSH) REQUIRED = + ρf g 2g October 13, 2004 12 Dredge Pumps and Slurry Transport
  13. 13. MAXIMUM VELOCITY IN THE SYSTEMCriterion for non--cavitational operation of a systemThe no cavitation condition for a certain pump-suction pipe combination is: (NPSH)REQUIRED < (NPSH)AVAILABLEAt the incipient cavitation, the absolute suction pressure Ps,min at the pump inlet is equal to the difference between the atmospheric pressure Patm and the so-called “decisive vacuum” (Dutch: maatgevend vacuum) (Vac)d, i.e. (Vac)d = Patm – Ps,min. October 13, 2004 13 Dredge Pumps and Slurry Transport
  14. 14. MAXIMUM VELOCITY IN THE SYSTEMCriterion for non--cavitational operation of a systemThe upper limit for the working range of a pump-pipeline system is given by points of intersection of a pump decisive vacuum curve and a set of vacuum curves of a suction pipe for various mixture densities.The vacuum curve of a suction pipe : Vac Patm − PS = ρf g ρf g October 13, 2004 14 Dredge Pumps and Slurry Transport
  15. 15. MAXIMUM VELOCITY IN THE SYSTEMCriterion for non--cavitational operation of a systemThe upper limit for the working range of a pump-pipeline system is given by points of intersection of a pump decisive vacuum curve and a set of vacuum curves of a suction pipe for various mixture densities.The decisive vacuum curve of a pump : (Vac)d Patm − Pvapour Vm 2 = + − NPSHREQUIRED ρf g ρf g 2g October 13, 2004 15 Dredge Pumps and Slurry Transport
  16. 16. MAXIMUM VELOCITY IN THE SYSTEM Vac Patm − PS = ρf g ρf g (Vac)d Patm − Pvapour Vm 2 = + − NPSHR ρf g ρf g 2gOctober 13, 2004 16Dredge Pumps and Slurry Transport
  17. 17. MAXIMUM VELOCITY IN THE SYSTEM HOW TO AVOID CAVITATIONIn the design of a pump-pipeline system:• to reduce the static head that the pump must overcome, i.e. to put the pump as low as possible• to reduce the head lost due to flow friction, i.e. to minimize local losses and a suction pipe length• to increase pressure by using a larger pipe at the suction inlet of a pump. October 13, 2004 17 Dredge Pumps and Slurry Transport
  18. 18. MAXIMUM VELOCITY IN THE SYSTEM HOW TO AVOID CAVITATIONDuring the operation of a system (the position of a pump and a geometry of a suction pipeline can not be changed):• to reduce the head lost due to flow friction either by • diminishing the mean mixture velocity or by • reducing the mixture density in a suction pipeline. October 13, 2004 18 Dredge Pumps and Slurry Transport
  19. 19. MINIMUM VELOCITY IN THE SYSTEM THE LOWER LIMIT FOR A SYSTEM OPERATION:VELOCITY AT THE INITIAL STATIONARY BED IN A PIPE October 13, 2004 19 Dredge Pumps and Slurry Transport
  20. 20. MINIMUM VELOCITY IN THE SYSTEMCriterion for the deposit-free operation of a systemMean mixture velocity must be higher than deposition-limit velocity: Vm > Vdl. October 13, 2004 20 Dredge Pumps and Slurry Transport
  21. 21. Empirical Model for Critical Velocity (MTI)C. Diagram for Vcr: October 13, 2004 21 Dredge Pumps and Slurry Transport
  22. 22. Empirical Model for Critical Velocity (MTI)B. Correlation for Vcr: 1  1   Cvd  6 Ss −1 Vcrit = 1.7  5 −  D   dmf   Cvd + 0.1 1.65   In the equation dmf [mm], D [m] and Vcr [m/s]. October 13, 2004 22 Dredge Pumps and Slurry Transport
  23. 23. MINIMUM VELOCITY IN THE SYSTEM HOW TO AVOID STATIONARY BED1. If the pipeline is composed of sections of different pipe sizes: the mixture flow rate must be maintained at the level assuring a super-critical regime (Vm > Vdl) in the pipe section of the largest pipe diameter.2. If the solids concentration fluctuates along a pipeline: the mixture flow rate must be maintained at the level assuring a super-critical regime in the section of an extreme concentration. For a prediction, use the highest value of the deposition-limit velocity from the entire range of expected solids concentrations. October 13, 2004 23 Dredge Pumps and Slurry Transport
  24. 24. MINIMUM VELOCITY IN THE SYSTEM HOW TO AVOID STATIONARY BED3. If during a job a pipeline is prolonged or coarser solids are pumped: the flow rate supplied by a dredge pump might become insufficient to assure a super-critical regime in a pipeline. Then two solutions must be considered:• to pump mixture at much lower concentration; this will lead to lower frictional losses and thus higher flow rate that might be high enough to avoid a thick stationary bed in a pipeline• to install a booster station; this increases the manometric head provided by pumps and increase a flow rate. October 13, 2004 24 Dredge Pumps and Slurry Transport
  25. 25. EFFECT OF PUMP POSITION ON OPERATIONAL LIMITSIf a pump (e.g. a submerged pump) is placed to a lower position within a pump-pipeline system: • the suction pipe becomes shorter • the geodetic height over which a mixture has to be lifted becomes smaller. October 13, 2004 25 Dredge Pumps and Slurry Transport
  26. 26. EFFECT OF PUMP POSITION ON OPERATIONAL LIMITSOctober 13, 2004 26Dredge Pumps and Slurry Transport
  27. 27. EFFECT OF PUMP POSITION ON OPERATIONAL LIMITSOctober 13, 2004 27Dredge Pumps and Slurry Transport
  28. 28. OPERATION LIMITS AND PIPE LENGTH langste persleiding constant toerental kortste persleiding 850The maximum length of a manometrische druk (kPa) 750 discharge pipeline is limited by the 650 deposition-limit velocity. 550 werkgebied 450The minimum length of a onderkritisch bovenkritisch discharge pipeline is 100 maatgevend vacuüm limited by the decisive 75 vacuüm (kPa) vacuum of a pump. 50 zuigleiding- 25 karakteristiek 0 1 Qkritisch 2 3 Qmaatg. vacuüm debiet (m 3/s) October 13, 2004 28 Dredge Pumps and Slurry Transport

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