SUBJECT: The large stuffing box 7-9
Next to stabilizing the pump shaft, without question the single most effective action you can take to increase the life of your mechanical seal, is to replace the present narrow stuffing box with one of the newer, more open designs. Look at the following diagram.
This stuffing box was designed to accommodate the 5/16' to 3/8" (8 to 10 mm.) packing that you find in most of the standard design pumps, such as ANSI, DIN and ISO. When these pumps are converted to a mechanical seal it leaves very little clearance between the out side diameter of the mechanical seal and the inside diameter of the stuffing box.
Clearances of 0.015 inches (0,4 mm) are typical. Further compounding the problem is the fact that many products stick to the inside of the stuffing box rough casting, restricting the clearance even more.
Centrifugal force is trying to throw solids away from the moveable seal components and the lapped seal faces. If the seal movement is restricted, the seal faces will open allowing the solids to penetrate between them. Seal faces are lapped to three helium light bands of flatness (0.000034" or just under one micron).
There is an axial play in the bearings of 0.002" to 0.005" (0,05 to 0,15 mm) so any restriction of the seal movement will open the seal faces enough to let plenty of solids in. It is these small solids that cause most of the face damage we see in premature seal failures.
The narrow design stuffing box has a flushing connection that is located approximately in the middle of the packing set. Clean lubricant is introduced to:
When this flushing location is used with a mechanical seal:
The problem is identifiable when you notice a rubbing mark around the rotating portion of the seal and a partial rubbing mark around the component described in the above paragraph.
In a vertical application the standard lantern ring location will not vent air away from the seal faces. This will cause the seal faces to run dry and possible be damaged by the heat that will be generated at conventional motor speeds. If the dynamic elastomer ( the rubber part) is located close to the seal faces it will almost certainly be damaged during any dry running period. Look for evidence of the elastomer changing weight, shape, or appearance. One solution to the problem of a restricted stuffing box area is to open the space around the seal. One method of doing this is to install an enlarged or bored out stuffing box. Now the solids have some place to go when centrifugal force acts upon them
You can argue the merits of an enlarged or tapered stuffing box. I like the enlarged type because I have seen many seals ruined when abrasives were drawn to the narrow end of atapered box. I have also seen what appears to be cavitation damage at the narrow end that could be caused by high velocity fluid vaporizing. Regardless of the design you choose look for these features:
In most applications you will connect the circulation fitting to the suction side of the pump rather than the higher pressure discharge side. This is called suction recirculation and with this arrangement you can take advantage of the fact that the stuffing box pressure is higher than the pump suction, causing the fluid to flow form behind the impeller (where it has been centrifuged clean), through the stuffing box, to the lower pressure suction side of the pump. Whenever you use a mechanical seal This suction recirculation should be your normal set up.
Discharge recirculation is the term we use to describe a line connected between the top of the stuffing box and the discharge, or higher pressure side of the pump. We use this arrangement when suction recirculation would not make any sense. As an example :
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