Pump Design & Selection

Dick Hawrelak

Presented to UWO CBE 497

16 Oct 01

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

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.

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

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.

Typical Pump Sketch

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.

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.

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.

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 )

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

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

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.

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.

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.

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.

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.

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

Durco PUMSEL Program Input

From Quick Pump Design V1.3 enter:Design USGPMSystem head, in ft.Specific GravityPumping temp, Deg FViscosity in CentipoiseNPSH available in ft.3 points from system curve.

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.

Typical Pump Head Curves

Selected Pump

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.

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.

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.

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.

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.