Understanding Split Mechanical Seals 3-8
The split seal is the one seal that every consumer wanted. The main reason that people continue to use packing is that no one wants 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 multiple 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,t he bearings are often replaced at the same time. More often than not seal replacement means a complete pump overhaul.
- Often, the system has to be sterilized if the pump is disassembled. This can involve many hours of heating, flushing, etc..
The following illustration shows a modern split seal mounted between the face of the stuffing box and the bearing case. Note that the 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’ll 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 atomic submarine main propeller shafts back in 1954. They 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’re 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 logical design that makes sense to you.
Gluing O-rings, or any other type of elastomer together is never acceptable for a dynamic elastomer. The glue creates a “hard spot” that’ll 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 today 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 splits seals need a positive pressure to hold the faces together, so if you intend to use the seals in vacuum service, this type must be turned around sputting atmospheric pressure on the sealing side. Stationary split seal do not experience this problem.
- The pump is leaking. You can break off, or cut off the present gland and install a split seal while the leaking seal remains in the stuffing box. The pump can be back on stream in about an hour.
- You can install the split seal in a fire pump and leave the packing in place. This way you will probably not violate any fire codes.
- In most designs you’re installing the seal closer to the bearings. If you install a carbon bushing in the stuffing box of the pump you stop most of the bad affects of shaft bending and deflection. Even if you don’t use the carbon bushing you’re still better off being located closer to the pump anti friction bearing.
- Mixers and awkward locations. The savings are huge! In some instances you have to take the roof off of the building to remove the motor before you can pull the pump.
- Vertical and horizontal split pumps. You don’t have to rig a special lifting device and you only have to change one seal instead of two.
- Shallow stuffing boxes. The seal installs outside the conventional stuffing box but unlike other seals it does not seal backwards. The seal gland is an extension of the present stuffing box. CAUTION: some split seal designs are actually “outside” type seals that move the seal faces into the entrained solids as the seal face wears.
- Any time “down time” is expensive, split seals must be considered as the only sensible solution outside of installing two seals in all of your pumps.
- The pump is located in a dangerous area (radiation is a good example) and it’s important that the personnel spend as little time in the area as possible.
- If you want to measure the savings in electricity between packing and a mechanical seal, the split seal is your only choice. Take an amperage reading with the packing in the pump and when you’re satisfied you know the power being consumed by the pump, pull the packing and install the split seal. The difference in electricity consumption should pay for the seal in less than eighteen months.
- You can install a split seal in a pump that has had the shaft/ sleeve damaged by packing or a mechanical seal, and save the shaft/ sleeve replacement cost. As you can see in the first illustration the seal static elastomer sits on a non-damaged portion of the pump shaft/ sleeve.
- Large diameter shafts are a natural for split seals. Shaft damage is expensive and pump disassembly is a big problem. Many times the packing is left in the pump and the leakage tolerated because of the problems of installing solid seals on shafts larger than four inches (100 mm.). Split seals changed all of this and shaft damage can be eliminated entirely.
- If you mount a split seal on a split sleeve you can change side entering mixer seals with out emptying the mixer. See the following illustration:
- Leaking product can be costly.
- Leaking product can present a personnel hazard and a housekeeping problem..
- Leaking product can be a pollution problem.
- Seals consume less power than packing.
- There’s no adjustment needed to compensate for wear.
- In many cases the flushing water can be eliminated.
- Shaft damage or wear can be eliminated. This’ll allow you to use a solid shaft that’ll resist shaft deflection and bending.
- Leaking packing, and the water hose that is needed to clean up the area, are the major causes of premature bearing failure.
- Leakage causes a “house keeping” problem.