Page 238 - Root Cause Failure Analysis

226 Root Cause Failure Analysis

Figure 18-6 Selecting correct packing size (Bearings Znc. catalogue).

by adding the diameter of the shaft to the packing cross-section that was
calculated in the preceding formula. For example, a stuffing box with a 4-in.
I.D. and a shaft with a 2-in. diameter will require a packing cross-section of
1 in. The centerline of the packing then would be 3 in. Therefore, the
approximate length of each piece of packing would be

PL = Centerline diameter x 3.1416 = 3.0 x 3.1416 = 9.43 in.

The packing should be cut approximately v4 in. longer than the calculated
length so the end can be bevel cut.
16. Controlled leakage rates easily can be achieved with the correct size of
packing.
17. Cut the packing rings to size on a wooden mandrel that is the same diameter
as the pump shaft. Rings can be cut either square (butt cut) or diagonally at
approximately 30". NOTE Leave at least a g6-in. gap between the butts
regardless of the type of cut used. This permits the packing rings to move
under compression or temperature without binding on the shaft surface.
18. Ensure that the gland area is perfectly clean and not scratched in any way
before installing the packing rings.
19. Lubricate each ring lightly before installing in the stuffing box. NOTE:
When putting packing rings around the shaft, use an S twist. Do not bend
open. See Figure 18-7.
20. Use a split bushing to install each ring, ensuring that the ring bottoms out
inside the stuffing box. An offset tamping stick may be used if a split bush-
ing is not available. Do not use a screwdriver.