Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

410 Centrifugal Pumps: Design and Application
the seal faces. At higher pressure, the seal faces may deflect in a positive
direction to the shaft centerline. As the seal is operated at higher pressure,
no leakage occurs. At lower pressure, however, the seal will begin
to leak because the wear pattern has changed and the seal faces are now
opening to the pressure at the outside diameter. This type of leakage from
coning or positive seal face rotation can be eliminated by upgrading seal
materials or the design of the seal rings.
Leakage due to thermal distortion is just the opposite to what is observed
when the seal faces deflect from pressure. Here, wear occurs at
the inside diameter of the seal face. The seal face continues to expand,
opening up to the liquid in the stuffing box. As the seal opens up at higher
temperatures, leakage increases and little or no contact is observed at the
outside diameter of the seal faces. This condition is corrected by increasing
the cooling to the seal faces. An illustration of damage from thermal
distortion is given in Figure 17-47. Damage to this cast iron mating ring
was eliminated by providing adequate flow to the seal faces.
Mechanical distortion may result from bolt loading from the gland
plate on the pump. Illustrations of mechanical distortion are shown in
Figures 17-48, 17-49, and 17-50. In Figure 17-48 a tungsten carbide
mating ring had been installed into a unit where the bolts had been overtightened.
This changed the complete waviness pattern at the seal faces
and resulted in two high spots 180° apart. When this condition occurs,
the separation between the mating and primary rings can result in erosion
of the softer carbon primary ring. This erosion can take two forms. If the
unit is idle with pressure in the stuffing box and the shaft is not rotated,
wire drawing can occur across the seal face. This condition is shown in
Figure 17-49. In addition to wire drawing shown on the seal faces, erosion
of the outside carbon shoulder can be seen. Erosion of the carbon
can be eliminated by redirecting the seal flush to the side of the faces, and
where possible, an abrasive separator should be added to the seal flush
line.
Wire brushing of a seal face can occur when abrasives are present in
the stuffing box and the seal is operated with a mechanically distorted
mating ring, Figure 17-50. In both cases, erosion to the seal faces can be
eliminated by not overtightening the gland plate bolts or by supporting
the gland plate outside the bolt circle. Distortion can also occur when the
top and bottom halves of a split case pump are not properly lined up. In
this case, a review of the application should be made to determine if an
abrasive separator should be included in the seal flush line.
Heat checking of a seal face can occur in a pattern through 360° as
shown in Figure 17-51. When this condition occurs a few minutes after
startup, it would indicate that the seal was operated below its working
height or that the seal was operated without any flush to the seal cavity,