Page 296 - Root Cause Failure Analysis

284 Root Cause Failure Analysis

Misapplication
Little attention generally is given to the selection of mechanical seals. Most plants
rely on the vendor to provide a seal that is compatible with the application. Too often
there is a serious breakdown in communications between the end user and the vendor
on this subject. Either the procurement specification does not provide the vendor with
appropriate information or the vendor does not offer the option of custom ordering the
seals. Regardless of the reason, mechanical seals often are improperly selected and
used in inappropriate applications.

Seal Flushing
When installed in corrosive chemical applications, mechanical seals must have a
clear-water flush system to prevent chemical attack. The flushing system must provide
a positive flow of clean liquid to the seal and also an enclosed drain line that removes
the flushing liquid. The flow rate and pressure of the flushing liquid will vary, depend-
ing on the specific type of seal, but must be enough to assure complete, continuous
flushing.

PACKED BOXES
Packing is used to seal shafts in a variety of applications. In equipment where the
shaft is not continuously rotating (e.g., valves), packed boxes can be used successfully
with no leakage around the shaft. In rotating applications, such as pump shafts, the
application must be able to tolerate some leakage around the shaft.

Nonrotating Applications
In nonrotating applications, packing can be installed tight enough to prevent leakage
around the shaft. As long as the packing is properly installed and the stuffing-box
gland is properly tightened, there is very little probability of seal failure. This type of
application requires periodic maintenance to ensure that the stuffing-box gland is
properly tightened or that the packing is replaced when required.

Rotating Applications
In applications where a shaft continuously rotates, packing cannot be tight enough to
prevent leakage. In fact, some leakage is required to provide both flushing and cooling
of the packing. Properly installed and maintained packed boxes should not fail or con-
tribute to equipment reliability problems. Proper installation is relatively easy and
routine maintenance is limited to periodic tightening of the stuffing-box gland.