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Seal and Bearing Failure on a Two-Stage Overhung Pump (3x6x13.5 CJA 2 Stage)

Seal and Bearing Failure on a Two-Stage Overhung Pump (3x6x13.5 CJA 2 Stage). John Schmidt, PE CSS Field Engineering Sulzer Pumps (US), Inc. Outline. What Happened ? In short: User 'moved / simplified' piping on the pump, which affected Axial Thrust.

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Seal and Bearing Failure on a Two-Stage Overhung Pump (3x6x13.5 CJA 2 Stage)

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  1. Seal and Bearing Failure on a Two-Stage Overhung Pump (3x6x13.5 CJA 2 Stage) John Schmidt, PE CSS Field Engineering Sulzer Pumps (US), Inc

  2. Outline • What Happened ? • In short: User 'moved / simplified' piping on the pump, which affected Axial Thrust. • Mini-tutorial on calculating Axial thrust for this pump.

  3. Pump Cross Section

  4. The Problem – What Happened ? • Pump design assumed by the user to have both a seal piping Plan 13 and a Plan 11.

  5. The Problem – What Happened ? • Pump was simplified to only use the Plan 11. • The "Plan 13" was removed. [Which actually was a Balance Line.]

  6. The Problem – What Happened ? • Original Pump

  7. The Problem – What Happened ? • Plan 11, with • Balance Line Removed: • No flow through the seal = seal failure

  8. The Problem – What Happened ? • Since Seal Failure occurred: • Changed the seal piping from a Plan 11 to a Plan 13. • Plan 13: • Flush is restored through the Seal. • But Balance Line not restored.

  9. Consequence • Seal is no longer failing. • But bearings are, every 6-9 months. • Axial Thrust !

  10. Axial Thrust: • Mainly a function of: Pressure distribution on the rotor. • Also: Momentum force.

  11. Axial Thrust: Pressure Distribution on Impeller Shrouds • Rule of thumb: 0.75*Pd (differential pressure) for the shroud pressure profile • (from the wear-ring-labyrinth to Impeller OD) • Used in this Case Study. • But in reality it is more complicated...

  12. Axial Thrust: Pressure Distribution on Shroud • Dependent on fluid dynamics in Side-Rooms: • Off-BEP operation • Leakage direction and amount. • Side-room geometry. • Rotor to Case Alignment. • For Example:

  13. Example: Effect of leakage on Pressure Distribution Actual pressure profile is decreased because: greater swirl in side-room due to fluid entering side-room with high pre-rotation. Actual pressure profile is increased because: Less swirl in side-room due to fluid entering hub seal with no pre-rotation. pressure profile if rotation factor assumed to be 0.5 pressure profile if rotation factor assumed to be 0.5

  14. Axial Thrust: Momentum Force • Very low. Is typically not included. • Thrust due to momentum change. • Momentum Force =(Capacity2 x density) / [(Eye Area)x722] • Momentum force in lbf. • Capacity in GPM, • Density in SG • Eye Area in Square Inches. • 722 is unit conversion factor. • Assumes 90 deg turn of fluid. • For This Pump (at design flow) -> lbf

  15. We are assuming the simplified 0.75*Pd on Shrouds. Initially show all pressures on rotor, and then show the typical simplification. What do we Need to start ? Cross section Diameters of wear ring labyrinths. Pressures Suction Pressure = 111 psi Differential Pressure = 340 psi Flush plan General idea of leakage direction, labyrinth clearances, leakage flow, etc. Calculate Axial Thrust for this Pump

  16. Axial Thrust - As designed. (all pressures on rotor)

  17. Axial Thrust - As designed.(Simplified)

  18. Axial Thrust - As designed. (Fully simplified)

  19. Axial Thrust – Plan 11 with No Balance Line

  20. Axial Thrust – Plan 11 with No Balance Line

  21. Plan 13 – with no Balance Line • In Series Flow: • Hub Seal, • Throat Bush, • Plan 13 Orifice • What is the pressure behind the 2nd Stage Impeller?

  22. Plan 13 – with no Balance Line • Cross section area of each Restriction:

  23. Plan 13 – with no Balance Line • The Orifice is greatest restriction (by far) • Find flow rate through the orifice (assume water): General / Simple Equation for Orifice _careful with units_

  24. Plan 13 – with no Balance Line • Flow rate through this line is ~5 GPM (or less if we include other restrictions and resistances..) • Given 5 GPM flow what is the pressure drop across the hub seal? • Re-Arrange Orifice Eqn, solve for pressure across hub seal gap.

  25. Pressure Drop across Hub = 1.2 psi • Therefore Pressure Behind 2nd Stg Impeller is = 338.8 psi

  26. Axial Thrust – Plan 13 with No Balance Line

  27. Bearing L10h. • The most simple method for Bearing Life Calculation is "L10" ISO or AFBMA equation for basic rating life: L10 = (C/P)p • L10 = basic rating life, millions of revolutions • C=Basic dynamic Load Rating (from Bearing Tables) • P=Equivalent Dynamic Bearing Load • p=Exponent, 3 for ball, 3.333 for roller. • Operating hours at constant speed before onset of fatigue. • L10h = (1 000 000/ (60*n))*L10 n = RPM

  28. Bearing L10h.

  29. Bearing L10. • Alternative, Use C/P [basic load dynamic rating / dynamic load] and nomograph from the bearing supplier.

  30. Problem Resolution • User realized the issue after reading about pump axial thrust and better understood what they had affected. • Solution: Pump User returned the pump to the original configuration and reliability was improved. Lessons Learned • Continuing Education / pump training should be included in the maintenance/operation/reliability sections of any plant in order to achieve success. • Modifications can have un-intended consequences. • You should contact the OEM as necessary.

  31. Thrust Questions ?

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