Page 234 - Root Cause Failure Analysis

222 Root Cause Failure Analysis

rotates with it. The spring must be made from a material compatible with the fluid
being pumped so that it will withstand corrosion. Likewise, the same care must be
taken with material selection for the 0 ring and seal materials. A carbon graphite
insertion ring provides a good bearing surface for the seal ring to rotate against. It also
is resistant to attack by corrosive chemicals over a wide range of temperatures.

Figure 18-3 shows a simple seal that has been installed in the pump’s stuffing box.
Note how the coil spring sits against the back of the pump’s impeller, pushing the
packing 0 ring against the seal ring. By doing so, it remains in constant contact with
the stationary insert ring.

As the shaft rotates, the packing rotates with it, due to friction. There also is friction
between the spring, the impeller, and the compressed 0 ring. Thus, the whole assem-
bly rotates together when the pump’s shaft rotates. The stationary insert ring is located
within the gland bore. The gland itself is bolted to the face of the stuffing box. This
part is held stationary by the friction between the 0 ring insert mounting and the
inside diameter (I.D.) of the gland bore as the shaft rotates within the bore of the
insert. In more complex mechanical seals, the shaft packing element can be secured to
the rotating shaft by Allen screws.

Having discussed how a simple mechanical seal is assembled in the stuffing box, we
must now consider how the pumped fluid is prevented from leaking out to the atmo-
sphere. In Figure 18-3, the path of the fluid along the drive shaft is blocked by the 0
ring shaft packing at Point A. Any fluid attempting to pass through the seal ring is
stopped by the 0 ring shaft packing at Point B. Any further attempt by the fluid to
pass through the seal ring to the atmospheric side of the pump is prevented by the
gland gasket at Point C and the O-ring insert at Point D. The only other place where
fluid can escape is the joint surface around Point E, which is between the rotating car-

Figure 18-3 Pump stumg box containing a simple mechanical seal (Bearings Inc. catalogue}.