Page 241 - Root Cause Failure Analysis

Seals and Packing 229

erly installed and maintained. Inspection of the equipment before seal installation can
prevent potential seal failure and reduce overall maintenance expenses.

Equipment Checkpoints
The preinstallation equipment inspection should include the following: stuffing-box
space, lateral or axial shaft movement (end play), radial shaft movement (whip or
deflection), shaft runout (bent shaft), stuffing box face squareness. stuffing box bore
concentricity, driver alignment, and pipe strain.

Stuffing-Box Space To properly receive the seal, the radial space and depth of the
stuffing box must be the same as the dimensions shown on the seal’s assembly drawing.

End Play A mechanical seal cannot work properly with a large amount of end play
or lateral movement. If the hydraulic condition changes (as frequently happens), the
shaft could “float,” resulting in sealing problems. Minimum end play is a desirable
condition for the following reasons:

Excessive end play can cause pitting, fretting, or wear at the point of contact
between the shaft packing in the mechanical seal and the shaft or sleeve
O.D. As the mechanical seal driving element is locked to the shaft or sleeve,
any excessive end play will result in either overloading or underloading the
springs, causing excessive wear or leaks.
Excessive end play as a result of defective thrust bearings can reduce seal
performance by disturbing both the established wear pattern and the lubri-
cating film.
A floating shaft can cause chattering, which results in seal-face chipping.
especially the carbon element. Ideal mechanical seal performance requires a
uniform wear pattern and a liquid film between the mating contact faces.

Measure end play by installing a dial indicator with the stem against the shoulder of
the shaft. Use a soft hammer or mallet to lightly tap the shaft on one end and then on
the other. Total indicated end play should be between 0.001 and 0.004 in.

Whip or Deflection Install the dial indicator as close to the radial bearing as possi-
ble. Lift the shaft or exert light pressure at the impeller end. If more than 0.002 to 0.003
in. of radial movement occurs, investigate bearings for radial bearing fit (especially the
bore). An oversized radial bearing bore caused by wear, improper machining, or corro-
sion will cause excessive radial shaft movement, resulting in shaft whip and deflection.
Minimum radial shaft movement is important for the following reasons:

Excessive radial movement can cause wear, fretting, or pitting of the shaft
packing or secondary sealing element at the point of contact between the
shaft packing and the shaft or sleeve O.D.
Extreme wear at the mating contact faces will occur when excessive shaft
whip or deflection is present due to defective radial bearings or bearing fits.