Pump Design & Selection

1. Pump Design & Selection • Dick Hawrelak • Presented to UWO CBE 497 • 16 Oct 01

2. Good Design References • Flow of Fluid Through Valves and Fittings by Crane. Technical Paper No. 410-C. • Centrifugal Pumps and System Hydraulics, Chem Eng’g, 4 Oct 82, p-84, by Karassik. • Chem Eng Handbook, Perry 6, Sections 5 and 6 • Primer software by Durco http://www.durcopump.com • PumpSel 6.2™ (rev .07) by Durco http://www.durcopump.com

3. Pump Design Stages • Phase 2 – Process alternatives & optimization. • Phase 3 – P&IDs, preliminary layout, approx design, specification, pre-selection, cost estimate. • Phase 4 – detailed design. • Final layout – check piping & elevations. • Final detailed hydraulic design & selection. • Suitability (NPSH, SSS, Re-circulation). • Final cost estimate.

4. Excel Pump Design Program • Plant Design Programs on CD-ROM • Quick Pump Design V1.3 • Contains Following Features • Line Sizing • Control Valve Sizing • Check Valve Sizing • Orifice Plate Sizing • NPSH, NS, SSS, HP calculations • System Head Curve • Limited Pump Selection • Used along with Durco’s PUMPSEL and PRIMER

5. Draw Sketch of Pump System • Collect physical property data (density, viscosity, vapor pressure). • Show line details, size, schedule or wall thk, check valves, control valves, block valves, reducers/expanders (may have to take a WAG). • Show origin (min.) and destination pressures (max.). • Show origin and destination elevations for static head. • Combine services where practical & economical.

6. Typical Pump Sketch

7. Size Suction and Discharge Lines • Break lines into sizable sections. There may be different sizes in any one branch. Eg 4”, 6” and 8” sections. • Estimate the number of elbows, block valves, fittings, etc. (WAG in Ph 2). “Fitting” pgm in Phase III. • Expand line sizing routine for record keeping. This will simplify phase 4 checking.

8. Line Size Equations • Re No. = 6.31 ( W ) / ( d ) / ( cP ) • Darcy friction factor f = 4 ( f Fanning ) f Darcy by “all-in-one” Chen Equation • DP100 = 0.000336 ( f )( W )^2 / ( DL ) / ( d )^5 • Max Dp100 usually limited to 1.0 psi per 100 ft. • K = ( f ) ( L / D ) for pipe • K = ( f Turb ) ( L / D ) for valves and fittings • K total = K pipe + K valves & fittings • DP = 2.8E-07 ( K total )( W )^2 / ( DL ) / ( d )^4 • Abs roughness e = 0.00015 for clean pipe, ft. • Abs roughness e = 0.0004 for dirty pipe, ft.

9. Size Check Valves in Each Discharge Line Branch • Line sizing program built-into Pump pgm. • Check pipe spec for type of check valve. • Check minimum line velocity to keep ChV in open position. Prolonged operation at reduced rates may cause ChV chatter and damage to ChV. • Operation with damaged ChV is extremely hazardous.

10. Typical Check Valve Equation • For a Swing Check Valve (see Crane, page A-27) • K = 100 ( f Turb ) for pressure drop • Minimum Velocity, fps = 35 / ( DL )

11. Size Orifice Plates in Each Branch • See Line sizing routine. • Orifice Plate pressure drop usually in three ranges. • Typically 0.5, 1.0 or 2.0 psi

12. Typical Orifice Equation • Beta = d1 / d2 should be in range 0.2 to 0.7 • d1 = orifice dia., d2 = pipe dia., inches • Re No. based on d2, the pipe diameter • W = 1891 ( d1 )^2 ( C ) (( DP )( DL))^0.5 • C = Flow Coefficient for square edged orifices (see Crane, page A-20) • C = Function of Re No. and Beta ratio • C should be in range 0.6 to 0.8

13. Control Valves • Select each branch with control valves and use line size routine to size control valves assuming Fisher Equal Percentage type valves. • Poor CV selection – no control, pump running on by-pass…may need two control valves. • If too large DP taken across control valve, it may be better to trim impeller, save CV wear & energy. • Pump program should use CV in controlling line. • DP CV / (DP CV + DP fric) = approx 0.1 to 0.3. Default DP control valve = 10 psi.

14. Typical Control Valve Equations • Cv = ( USGPM )( SG / DP )^0.5 • Cv = Liquid Sizing Coefficient. • SG = Specific Gravity. • DP = Pressure Drop (10 psi default) • Typical control valve is an equal percentage type valve. • Cv depends on valve size, % valve opening, and flow.

15. Blocked-in Operation. • Determine features required for blocked-in operation. • Low flow shut-down. • High temperature shut-down. • Recycle plus cooling. • Pumps can explode in a short period of time if left running while blocked-in and no high temperature shut-down is provided. • Pump explodes, pieces rocket 275m, hits truck, kills driver. • Pump leaks under high pressure, liquid catches fire and destroys plant.

16. Suction Conditions • Determine NPSH available. • NPSH = SP – VP + HL – DP friction all in ft of liq. • Boiling liquids, SP = VP. Raise height or reduce DP. • Poor NPSH causes pump cavitation, high vibration & ultimately pump failure (hazard). • Pump fails to perform as designed without NPSH available greater than NPSH required. • Typically, NPSH avail 12 ft. vs 10 ft. req’d. • Pump Vendor will tell you NPSH required based on pumps selection.

17. System Head Curve • Determine Controlling Branch – I.e the one that requires the maximum differential head. • Determine the system curve for all items except the control valve. • For Dp at reduced USGPM, assume DP is proportional to the square of the flow. • Include static head.

18. Pump Selection • Hundreds of pumps to select from. • Which selection is best? • Which RPM to use? • What HP size & type of motor to select, explosion proof, TEFC? • Download Durco PUMPSEL and PRIMER on internet (program is free). • http://www.durcopump.com

19. Durco PUMSEL Program Input • From Quick Pump Design V1.3 enter: • Design USGPM • System head, in ft. • Specific Gravity • Pumping temp, Deg F • Viscosity in Centipoise • NPSH available in ft. • 3 points from system curve.

20. PUMPSEL Output • Selects all available pumps • Gives Impeller sizes • Gives HP and NPSH Required • Gives a cost estimate (PRIMER) • Gives options for types of pumps • Gives all kinds of help on all features. • PUMPSEL is a must for any design group. • Program also available from Gould.

21. Typical Pump Head Curves

22. Selected Pump

23. Suction Specific Speed, SSS • SSS is an Index number descriptive of the suction charateristics of a pump impeller. • SSS = (rpm)(Q @ BEP)^0.5 / (NPSH @ BEP)^0.75 • Pumps operating at SSS greater than 11,000 had a high failure frequency (hazard). • Low capacity operation causes inlet recirculation, impeller erosion, shaft deflections, bearing failures and seal problems which lead to leakage. • Pump program predicts recirculation as % of SSS.

24. Dissolved Gases • Absorbed gas follows Henry’s Law xa = (pp / Pt) / H. • Dissolved gases are like entrained bubbles. Residence time in suction vessel may be too short. • Dissolved gases causes problems similar to NPSH cavitation. • Prevent vapor entrainment with vortex breakers.

25. Material Transfer • Need multiple checks on quantity of material transferred to storage. • Weigh scales, level checks, mass = (flow rate)(time) on computer. • Time control EBVs to minimize Water Hammer problems.

26. Excess Flow Protection • Pumps cannot be allowed to run out on the impeller curve, may burnout motor if motor not selected for runout. • May need excess flow protection.

27. Repeat Design in Phase IV • All of the above details are checked again in Phase IV Engineering. • Necessary to have good documentation. • Poor Phase III Design & Selection means rework during expensive Phase IV stage.