Transcript Improving Mechanical Seal Reliability

Improving Mechanical
Seal Reliability
William “Doc” Burke
A.W. Chesterton-Chicago
(800) 244-7325
Seals 101
Single seals are made of the same 7
pieces: gland, stationary face, rotary face,
secondary elastomers, drive mechanism,
spring face-loader, & gasket
 Keeping the seal faces together is the
main focus
 Proper selection of materials includes
solutions used for cleaning systems!
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Pump Operation
Most seal problems come from 1 of 2
causes: Pump is not operating near its
BEP, or the pump is Cavitating
 Pumps are often oversized in anticipation
of future needs, then run against a
throttled discharge: this creates Shaft
Deflection!
 Oversized pumps have greater NPSHR
(Net Positive Suction Head REQUIRED)
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Cavitation
 Air
ingestion is common in WW
 Internal recirculation is often
mistaken as Cavitation
 Cavitation is a frequent cause of
bearing failure & seal faces chipping
 NPSHR varies by the square of any
speed change
Dynamically Balance Rotating
Assemblies
Dynamic balance of entire rotating
assembly is crucial
 A 14” rim automobile tire has a
circumference of 6.5 feet: it rotates 812
RPM @ 60 MPH, and we notice the loss of
a 1 ounce weight
 Shaft should be within 0.001” Total
Indicator Runout (TIR) at impeller end to
assure good seal life
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Upgrade Bearing Protection
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Lip seals were designed in the 1940’s for car
water pumps
Pump bearings are designed to provide 30-40
years life based on their L10 life!
Contamination, not over-loading, kills most
bearings (SKF says 36%)
Installation & Removal concerns
(SKF says 34%)
Cool the oil, NOT the bearing, in hot running
applications
Alignment
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Within 0.002” on all 4 planes: vertical angular,
vertical parallel, horizontal angular, and
horizontal parallel
Couplings transmit torque & resolve thermal
growth: they allow mis-alignment to destroy
much more expensive bearings & seals by
transmitting vibration!
Buy or Rent a laser, and get training for your
people to use it properly!
Shoot the couplings with a thermal gun and see
the wasted HP energy being used!
Re-building Procedures
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Always use new gaskets & o-rings
Create & follow a “Pump Repair Checklist” to document the
work done, the parts changed, and changes made off OEM
specification! Make sure the machinist signs his work order
as a matter of pride
Use OEM parts wherever possible to assure tolerances/fits &
materials used are correct: stacking tolerances can lead to
major headaches!
All materials are readily identifiable from the OEMs, but
often are unclear from “pirate/bandit” houses
Be wary of hardened sleeves: make sure you are using set
screws which can bite into the harder materials!
Clearances & Tolerances
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Set impeller clearances by OEM specification: this can be a
severe problem with component seal designs
Case wear rings typically have a clearance of 0.0010.002”/shaft diameter inch. Change them once this
clearance doubles.
Coating the volute or the impeller can be a very good thing,
so long as the impeller & rotating assembly are re-balanced
and the clearances are not affected!
Eliminate pipe strain whenever it is identified: no comealongs or fork lifts should be required!
Identify turbulence-causing elbows & poor piping
arrangements as you move through the plant, & consider
flow-disrupters (static mixer in-line)
Install & Understand Seal
Environmental Controls
Flush: MUST be 15-20 psi higher than
stuffing box pressure to work
 Steam Quench
 Discharge Recirculation
 Suction recirculation
 Vent vertical applications
 Be wary of trapped air in stuffing boxes on
horizontal pumps
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