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
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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.

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Pump Cross Section
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The Problem What Happened ?

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

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The Problem What Happened ?

• Pump was simplified to only use the Plan 11.
• The "Plan 13" was removed. Which actually was a
  Balance Line.

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The Problem What Happened ?

• Original Pump

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The Problem What Happened ?

• Plan 11, with
• Balance Line Removed
• No flow through the seal seal failure

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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.

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

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

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Axial Thrust Pressure Distribution on Impeller
Shrouds

• Rule of thumb 0.75Pd (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...

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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

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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)

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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
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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) -gt

lbf
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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 0.75Pd on
  Shrouds.
• Initially show all pressures on rotor, and then
  show the typical simplification.

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Axial Thrust - As designed. (all pressures on
rotor)
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Axial Thrust - As designed. (Simplified)
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Axial Thrust - As designed. (Fully simplified)
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Axial Thrust Plan 11 with No Balance Line
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Axial Thrust Plan 11 with No Balance Line
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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?

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Plan 13 with no Balance Line

• Cross section area of each Restriction

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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_
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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.

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

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Axial Thrust Plan 13 with No Balance Line
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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
• CBasic dynamic Load Rating (from Bearing Tables)
• PEquivalent Dynamic Bearing Load
• pExponent, 3 for ball, 3.333 for roller.
• Operating hours at constant speed before onset of
  fatigue.
• L10h (1 000 000/ (60n))L10 n RPM

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Bearing L10h.
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Bearing L10.

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

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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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Questions ?