Figure 9-6.—Stuffing box on a centrifugal pump.
snug against the stationary seal face. The rotating
seal and all of the assembly below it are affixed
to the pump shaft. The stationary seal face is held
stationary by the seal gland and packing ring. A
static seal is formed between the two seal faces
and the sleeve. System pressure within the pump
assists the spring in keeping the rotating seal face
tight against the stationary seal face. The type of
material used for the seal face depends on the
service of the pump. When a seal wears out, it
is simply replaced.
You should observe the following precautions
when performing maintenance on mechanical
seals:
. Do not touch new seals on the sealing face
because body acid and grease can cause the seal
face to prematurely pit and fail.
. Replace mechanical seals when the seal is
removed for any reason or when the leakage rate
cannot be tolerated.
. Position mechanical shaft seals on the
shaft by stub or step sleeves. Shaft sleeves are
chamfered (beveled) on outboard ends to provide
ease of mechanical seal mounting.
. Do not position mechanical shaft seals by
using setscrews.
Fire pumps and all seawater pumps installed in
surface ships are being provided with mechanical
shaft seals with cyclone separators. The glands are
designed to incorporate two or more rings of
packing if the mechanical shaft seal fails.
A water flinger is fitted on the shaft outboard
of the stuffing box glands to prevent leakage from
the stuffing box following along the shaft and
entering the bearing housings. They must fit
tightly on the shaft. If the flingers are fitted on
the shaft sleeves instead of on the shaft, ensure
that no water leaks under the sleeves.
Stuffing Box Packing.— Although most
centrifugal pumps on gas turbine ships have
mechanical seals, you should be familiar with
stuffing box packing.
The packing in centrifugal pump stuffing
boxes (fig. 9-6) is renewed following the PMS.
When replacing packing, be sure to use packing
of the specified material and the correct size.
Stagger the joints in the packing rings so they fall
at different points around the shaft. Pack the
stuffing box loosely and set up lightly on the
gland, allowing a liberal leakage. With the pump
in operation, tighten the glands and gradually
compress the packing. It is important to do this
gradually and evenly to avoid excessive friction.
Uneven tightening could cause overheating and
possible scoring of the shaft or the shaft sleeve.
On some centrifugal pumps, a lantern ring is
inserted between the rings of the packing. When
repacking stuffing boxes on such pumps, be sure
to replace the packing beyond the lantern ring.
9-5
