In some cases, compression packing is used to seal light to medium duty pumps on applications that prove to be difficult for mechanical seals. The pump seal is formed by the packing being squeezed between the inboard end of the stuffing box and the gland. A common packing installation is shown in Figure 1 less the impellor. A static seal is formed at the ends of the packing rings and at the inside diameter of the stuffing box. The dynamic seal is formed between the packing and the shaft sleeve. Load applied to the packing deforms the packing against the shaft sleeve controlling leakage. Some leakage is necessary to cool and lubricate the packing. The amount of leakage will depend on the material of construction for the packing, operating conditions and the mechanical condition of the pump. The packing applied must be able to withstand the variables of the equipment such as runout, misalignments and thermal growth of the equipment without an appreciable increase in leakage. The number of rings may vary from application to application depending on the objective of the sealing system. A lantern ring is used on most applications to introduce a lubricant to the packing. Packing rings from the lantern ring to the bottom of the box seal the fluid being pumped. The packing rings between the gland and lantern ring seal the lubricant used with the system.
Repacking a piece of equipment is a basic skill that is not often used by those responsible for rotating equipment. The steps for making a successful installation are just as important as those for a mechanical seal. The packing size for an existing piece of equipment must be determined. This can be done by using the following formula:
After the proper size has been determined and the material of construction is available, removing the old packing and assembling the new packing can begin.
To begin to remove the packing, loosen and remove the gland plate from the stuffing box, Figure 2.
Continue the removal process by using a packing puller to begin removing the packing rings. See Figure 3.
The next step is to remove the lantern ring and continue removing the packing rings as shown in Figure 4.
To ensure the success of the new packing installation, the shaft sleeve must be inspected. Check for scoring and nicks. If the shaft sleeve cannot be cleaned up, it must be replaced.
The next step is to identify the proper packing and material. Take the packing and wrap it tightly around a mandrel that is the same size as the shaft sleeve. The number of wraps on the mandrel will equal the number of rings that were installed in the stuffing box. Cut the packing as shown in Figure 5. Be sure to use a sharp blade when cutting the packing.
|The packing is now ready to be installed into the pump. Assemble the cut packing rings to the pump shaft sleeve by twisting the rings as shown in Figure 6.
After the packing ring has been installed into the bore of the stuffing box, use a split ring to move the packing through the stuffing box, Figure 7.
Firmly seat the packing at the bottom of the stuffing box. Continue to firmly seat each of the packing rings. Be sure that the split ends of the packing are staggered. This will eliminate unwanted leakage through the packing ends.
Reassemble the split lantern ring onto the pump shaft sleeve. Be sure that it is positioned under the flush connection in the stuffing box and complete the assembly of the remaining packing rings. See Figure 10.
Reassemble the split gland plate onto the pump, Figure 11. Be sure the gland is loose enough so that there will be some leakage initially on start-up. Leakage should be generous upon start-up. Tighten to minimize leakage. If packing begins to overheat at start-up, stop the pump and loosen the packing until leakage occurs. Restart only if the packing is leaking.
The unit is now ready to be put into service.
Figure 12 illustrates the completed assembly with the tools necessary to change the packing. The items shown are the split lantern ring, the mandrel, and the packing.
It cannot be over-emphasized that to achieve good packing life, the pump must be in good condition. Shafts should be checked for runout and eccentricity to be sure that they are within manufacturer's recommended tolerances. Surfaces in contact with the packing must be finished to the correct smoothness and tolerance.
The following is a list of common packing problems that will affect packing life:
For more information on packing and its application, visit the John Crane website at www.johncrane.com or contact Al McGovern at AMcGovern@johncrane.com
- No liquid delivered by pump
Cause: Lack of prime. Packing is too loose allowing air to leak into suction. Solution: Tighten or replace packing.
- Pump takes too much power
Cause: Packing is too tight.
Solution: Release the gland pressure and retighten reasonably. Be sure to allow a good stream of leakage.
- Pump leaks excessively
Cause: Shaft sleeve scored.
Solution: Remachine sleeve or replace. Replace worn packing.
- Stuffing box overheats
Cause: Packing is too tight or not lubricated.
Solution: Release gland pressure and retighten. Be sure packing leakage is present. Replace packing if damage has occurred and normal leakage cannot be maintained.
- Packing wears too fast
Cause: Packing is not properly lubricated.
Solution: Loosen packing to allow some leakage. Remachine or replace shaft sleeve.