SPLIT MECHANICAL SEALS S076
A split mechanical seal is defined as having all components split at the time of installation.
It is important to note that no dynamic elastomers should be glued together because the hard spot that develops will not allow the elastomer to be functional.
The following illustration describes the rotating version of a split seal. It is called rotating because the spring loaded face rotates with the shaft.
Here is the stationary version of a split seal. Please note that the spring loaded face does not rotate. Finger springs are used in this design instead of the more familiar coiled type. Finger springs are often used when axial space is at a premium.
The split seal is the one seal that every consumer wanted. The main reason that people continue to use conventional jam packing in pump stuffing boxes is that no one wanted to take the pump apart just to fix a leak. Taking a pump apart involves several problems:
- The mechanic must have enough skill to put it back together again, insuring that all tolerances, balance and fits are correct. In a world of multi- craft workers that skill is rapidly disappearing.
- You must go through a complete realignment between the pump and the driver. That can take hours.
- In some instances insulation has to be removed to move the pump. This can involve serious hazardous materials disposal problems.
- In many facilities seal replacement involves many crafts. An electrician to blank out the motor, a pipe fitter to remove the piping, a rigger to bring the pump back to the shop, a mechanic to fix it and several work orders to reverse the process when the pump goes back.
- When the pump is disassembled to replace the seal, the bearings are often replaced at the same time. More often than not seal replacement often means a complete pump overhaul.
- In some cases the system has to be sterilized if the pump is disassembled. This can involve many hours of heating, flushing, etc.
Note that the split seal is an extension of the stuffing box and is not installed in a typical “outside seal” configuration. In other words, as the seal faces wear they move away from the solids in the product and not into them. You will also note that the elastomer always moves to a clean surface, as the faces wear. This is a very important feature if there are solids in the sealing fluid.
The first successful split seals were used on the US. atomic submarine Nautilus main propeller shafts back in 1954. They were of the stationary configuration and proved to be as reliable as solid seals but they were very expensive because of a lack of good technology for cutting the lapped faces or joining the O-ring seals. There are three accepted methods of joining the split elastomer components:
- Vulcanize the components together around the shaft. This is the method that was used on the atomic submarine Nautilus. Its only limitation is that you are not able to manufacture small diameter rings because the stock must go around the shaft and then through the vulcanizing tool. Present technology limits this technique to shaft diameters larger than six inches (150 mm.).
- Install extra elastomers over the shaft and into the seal assembly. You can then move them out and use them as needed. This is a good technique, but presents major difficulties in seal design.
- Use the “ball and socket” design supplied by some manufacturers or any other sensible design that makes sense to you.
Gluing O-rings or any other type of elastomer together is never acceptable for a dynamic or moving elastomer. The glue creates a “hard spot” that will prevent proper sealing.
Early split designs were cut in half by “cut off saws” and re-machined to concentricity. This involved a lot of handling and hand finishing operations that added to the original high cost. Today we use lasers, high-pressure water tools, fracturing techniques and other types of sophisticated machinery to accomplish the same thing at much lower costs. The fact is that reliable split seals are just about the same price as any balanced, O-ring, cartridge seal made out of the same materials. Spare part kits are considerably cheaper than comparable solid seal spare parts.
Depending upon the brand and size of split seal that you select, the temperature, speed and pressure limits are just about the same as any other balanced O-ring mechanical seal. The major difference comes in the sealing of vacuum. Some older design splits seals need a positive pressure to hold the faces together, so if you intend to use these seals in vacuum service they must be turned around so that atmospheric pressure is on the sealing side. There are split seals available now that can seal either pressure or vacuum. This is very important in many mixer applications.
See: Split seals, where we use them S077