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!
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
 Oversized pumps have greater NPSHR
(Net Positive Suction Head REQUIRED)
 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
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
Upgrade Bearing Protection
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
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
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
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