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). 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

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.
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 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.
slide9

Consequence

    • Seal is no longer failing.
    • But bearings are, every 6-9 months.
    • Axial Thrust !
slide10
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
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.
slide25

Pressure Drop across Hub = 1.2 psi

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