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:
- Lubricate the packing.
- Cool the packing and shaft to prevent heat from being conducted to the bearings.
- Prevent air from entering the stuffing box if it is running with a negative pressure. This happens anytime the pump is lifting liquid, pumping from an evaporator or condenser, etc..
- Try and keep solids from entering and destroying both the packing and the shaft or sleeve.
When this flushing location is used with a mechanical seal:
- The clean flush enters the product stream unrestricted, diluting the product.
- The shaft radial movement can cause the rotating parts of the seal to contact a stationary part of the narrow clearance stuffing box, causing the lapped faces to open and the solids to penetrate, or to possibly damage one of the seal components. There are many causes for shaft radial movement and it’s impossible for you to prevent all of them :
- Operating the pump off of the best efficiency point (bep.)
- Pump and motor misalignment.
- The shaft is bent.
- The rotating assembly (shaft, sleeve, mechanical seal, impeller, coupling etc.) was not dynamically balanced.
- The seal or sleeve is not concentric with the shaft.
- Water hammer.
- Pressure surges.
- The stuffing box is not centered to the shaft.
- The seal gland bolt circle is not concentric to the shaft.
- This radial displacement of the shaft and seal can cause the stationary portion of the seal to be hit by the rotating shaft or the rotating portion of the seal to contact:
- Solids built up in the stuffing box.
- A protruding gasket between the seal gland and the stuffing box face.
- A protruding gasket between the halves of a split case pump.
- A loose piece of hardware in the stuffing box.
- A protruding flush connection.
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:
- Will the entire seal (especially the lapped faces) be located in the largest diameter portion of the stuffing box?
- Is the circulation connection located at or above the seal faces in a vertical application?
- The circulation connection should be located at the bottom or close to the bottom of the stuffing box so that it can be connected to the suction side of the pump or to some other low pressure point in the system for most of your applications.
- Is there a facility for installing a restrictive bushing in the end of the stuffing box? You will need one for high temperature and slurry applications. Will this bushing be positively retained or is it loose and able to blow out with pressure fluctuations? A loose bushing can interfere with the operation of the mechanical seal.
- Is a cooling jacket available for the large stuffing box? You will need one in many applications to keep the product cool when the pump is running, or warm when the pump has stopped. Caustic and heat transfer oils are good examples of these applications.
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 :
- You are pumping a fluid at or near its vapor point. A suction recirculation line will lower the pressure in the stuffing box and possibly cause the product to vaporize between the seal faces.
- The pump has a flow through semi- open impeller that adjust to the back plate rather than the volute of the pump. This causes the stuffing box pressure to equalize with suction pressure preventing a flow in the suction recirculation line. Duriron is a good example of this design.
- If the solids in the fluid have a very low specific gravity (they float) centrifugal force will not work throw the solids out to be removed by the suction recirculation line. They will tend to stay close to the seal, restricting its movement.
- Most single stage, double suction pumps are designed with the stuffing boxes at suction pressure. As is the case with the flow through, semi- open impeller, a clean flush is often required.