Seal and bearing failure on a two stage overhung pump 3x6x13 5 cja 2 stage
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Seal and Bearing Failure on a Two-Stage Overhung Pump (3x6x13.5 CJA 2 Stage) PowerPoint PPT Presentation


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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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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

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.


Pump cross section

Pump Cross Section


The problem what happened

The Problem – What Happened ?

  • Pump design assumed by the user to have both a seal piping Plan 13 and a Plan 11.


The problem what happened1

The Problem – What Happened ?

  • Pump was simplified to only use the Plan 11.

  • The "Plan 13" was removed. [Which actually was a Balance Line.]


The problem what happened2

The Problem – What Happened ?

  • Original Pump


The problem what happened3

The Problem – What Happened ?

  • Plan 11, with

  • Balance Line Removed:

    • No flow through the seal = seal failure


The problem what happened4

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.


Seal and bearing failure on a two stage overhung pump 3x6x13 5 cja 2 stage

  • Consequence

    • Seal is no longer failing.

    • But bearings are, every 6-9 months.

    • Axial Thrust !


Seal and bearing failure on a two stage overhung pump 3x6x13 5 cja 2 stage

  • Axial Thrust:

    • Mainly a function of: Pressure distribution on the rotor.

    • Also: Momentum force.


Axial thrust pressure distribution on impeller shrouds

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...


Axial thrust pressure distribution on shroud

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:


Example effect of leakage on pressure distribution

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


Axial thrust momentum force

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


Calculate axial thrust for this pump

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


Axial thrust as designed all pressures on rotor

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


Axial thrust as designed simplified

Axial Thrust - As designed.(Simplified)


Axial thrust as designed fully simplified

Axial Thrust - As designed. (Fully simplified)


Axial thrust plan 11 with no balance line

Axial Thrust – Plan 11 with No Balance Line


Axial thrust plan 11 with no balance line1

Axial Thrust – Plan 11 with No Balance Line


Plan 13 with no balance line

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?


Plan 13 with no balance line1

Plan 13 – with no Balance Line

  • Cross section area of each Restriction:


Plan 13 with no balance line2

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_


Plan 13 with no balance line3

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.


Seal and bearing failure on a two stage overhung pump 3x6x13 5 cja 2 stage

  • Pressure Drop across Hub = 1.2 psi

  • Therefore Pressure Behind 2nd Stg Impeller is = 338.8 psi


Axial thrust plan 13 with no balance line

Axial Thrust – Plan 13 with No Balance Line


Bearing l10h

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


  • Bearing l10h1

    Bearing L10h.


    Bearing l10

    Bearing L10.

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


    Problem resolution

    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.


    Questions

    Thrust

    Questions ?


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