1. Seal and Bearing Failure on a Two-Stage Overhung Pump (3x6x13
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.
x-sec of pump showing...

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
x-sec of pump showing...

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 off-BEP operation.
If pump is back on the curve, Flow is less, Head is higher: The 0.75Pd can go to ~0.8Pd (shutoff ~ 0.9Pd)
If pump is out on the curve, Flow is more, Head is less: The 0.75Pd can go to ~0.7Pd (end of curve ~ 0.6Pd)

14. 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

15. 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

16. Calculate Axial Thrust for this Pump
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.
We are assuming the simplified 0.75*Pd on Shrouds.
Initially show all pressures on rotor, and then show the typical simplification.

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

18. Axial Thrust - As designed. (Simplified)

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

20. Axial Thrust – Plan 11 with No Balance Line

21. Axial Thrust – Plan 11 with No Balance Line

22. 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?

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

24. 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_

25. 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.

26. Pressure Drop across Hub = 1.2 psi
Therefore Pressure Behind 2nd Stg Impeller is = psi

27. Axial Thrust – Plan 13 with No Balance Line

28. 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, for roller.
Operating hours at constant speed before onset of fatigue.
L10h = ( / (60*n))*L10 n = RPM

29. Bearing L10h.

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

31. Problem Resolution Lessons Learned
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.

32. Questions ?
Thrust