Page 245 - Root Cause Failure Analysis

Seals and Packing 233

tion. The following recommendations for seal startup apply to most types of seal
installations and will improve their life if followed:
Caution the electrician not to run the equipment dry while checking motor
rotation. A slight turnover will not hurt the seal, but operating at full speed
for several minutes under dry conditions will destroy or severely damage
the rubbing faces.
The stuffing box always should be vented before startup, especially with
centrifugal pumps. Even if the pump has a flooded suction, it is still possible
that air may be trapped in the top of the stuffing box after the pump’s initial
liquid purge.
Where cooling or bypass recirculation taps are incorporated in the seal
gland, piping must be connected to or from these taps before startup. These
specific environmental-control features must be used to protect the organic
materials in the seal and to ensure proper performance.
Cooling lines should be left open at all times. This is especially true when
hot product passes through off-line standby equipment, commonly done so
that additional product volume or equipment change can be achieved easily.
often by simply pushing a button.
At the end of each day when hot operational equipment is shut down, it is
best to leave the cooling water on long enough for the seal area to cool
below the temperature limits of the organic materials in the seal.
Before startup, face-lubricated seals must be connected from the source of
lubrication to the tap openings in the seal gland. For double seals, it is nec-
essary for the lubrication feed lines to be connected to the proper ports
before startup for both circulatory and dead-end systems. This is very
important because all types of double seals depend on the controlled pres-
sure and flow of the sealing fluid to function properly. Even before the shaft
is rotated, the sealing liquid pressure must exceed the product pressure
opposing the seal. Be sure a vapor trap does not prevent the lubricant from
promptly reaching the seal face.
Thorough warm-up procedures include a check of all steam piping arrange-
ments to be sure that all are connected and functioning. Products that solid-
ify when cool must be fully melted before startup. It is advisable to leave all
heat sources on while the system is shut down to ensure that the product
remains in the liquid state. This facilitates quick startups and equipment
switchovers that may be required during a production cycle.
Thorough chilling procedures are necessary for some applications; for
example, applications involving liquefied petroleum gas (LPG). LPG
always must be kept in a liquid state in the seal area, and startup usually is
the most critical time. Even during operation, the recirculation line piped to
the stuffing box might need to be run through a cooler to overcome fric-
tional heat generated at the seal faces. LEG requires a stuffing-box pressure
greater than the vapor pressure of the product at pumping temperature. A 25
to 50 psi differential is generally desired.