Page 242 - Root Cause Failure Analysis

230 Root Cause Failure Analysis

The contact area of the mating faces will be increased, resulting in
increased wear and the elimination or reduction of the lubricating film
between the faces, further shortening seal life.

Shaft Runout Shaft runour refers to a bent shaft, which can lead to vibration and
poor sealing. Bearing life is greatly reduced, and the operating conditions of both
radial and thrust bearings can be affected.

Measure the shaft runout at two or more points on the O.D. of the impeller end of the
shaft by clamping the dial indicator to the pump housing. Also measure the shaft
runout at the coupling end of the shaft. If the measurement exceeds 0.002 in.,
straighten or replace the shaft.

Face Squareness With the stuffing-box cover bolted down, clamp the dial indicator
to the shaft with the stem against the face of the box. The total indicator runout should
not exceed 0.003 in. When the face of the stuffing box is “out of square,” not perpen-
dicular to the shaft axis, the result can be serious malfunction of a mechanical seal for
the following reasons:

The stationary gland plate that holds the stationary insert or seat in position
is bolted to the face of the stuffing box. Misalignment will cause the gland
to cock, causing the stationary element to cock. This results in seal wobble
or operation in an elliptical pattern. This condition is a major factor in fret-
ting, pitting, and wearing of the mechanical seal shaft packing at the point
of contact with the shaft or sleeve.
A seal that wobbles on the shaft can cause wear on the drive pins. Erratic
wear on the face contact results in poor seal performance.

Bore Concentricity With the dial indicator set up as described previously, place the
indicator stem well into the bore of the stuffing box, which should be concentric to the
shaft axis to within a 0.005 in. total indicator reading. Eccentricity alters the hydraulic
loading of the seal faces, reducing seal life and performance. If the shaft is eccentric to
the box bore, check the slop, or looseness, in the pump-bracket fits. Rust, atmospheric
corrosion, or corrosion from leaking gaskets can cause damage to these fits, making it
impossible to ensure that a stuffing box is concentric with the shaft. A possible remedy
for this condition is welding the corroded area and remachining it to proper dimensions.

Driver Alignment and Pipe Strain Driver alignment is extremely important, and
periodic checks should be performed. Pipe strain also can damage pumps, bearings,
and seals. In most plants, it is customary to blind the suction and discharge flanges of
inactive pumps. These blinds should be removed before pump-driver alignment is per-
formed or the job is incomplete.

After the blinds have been removed and as the flanges on the suction and discharge
are being connected to the piping, check the dial-indicator reading on the O.D. of the
coupling half. Observe movement of the indicator dial as the flanges are being