SPLIT MECHANICAL SEALS, WHERE TO USE THEM S077
Split seals can be used to convert packed pumps to mechanical seals and to replace solid type mechanical seals. In another section of this CD I discussed the reasons you would want to convert a packed pump to a mechanical seal, now we will look at a few areas where the split seal has a real advantage over solid seals. :
- Many fire codes will not allow you to remove the packing from a fire pump. The split seal can easily be installed outside of the stuffing box with the packing left inside. Fire pumps leak a great deal and it is not unusual to have a “jockey pump” run twenty four hours a day trying to keep the fire main pressurized as this leakage occurs. Many fire pumps have been converted to a mechanical seal and in just about every case the “jockey” pump has stopped running.
- Sometimes it is just too difficult to pull a pump to change the packing sleeve or mechanical seal. Most split seal applications can be done in less than an hour, with the pump left in place.
Any time down time is expensive.
- Whenever a solid seal wears out or fails, it takes a considerable amount of time to pull the pump and change the seal. Split seals do not have this problem. In some cases this “down time” can cost thousands of dollars.
- The worker can fix the leak and then get out of the area in a hurry. Radioactive environments are an example of this problem. In many cases the old packing and sleeve, or failed mechanical seal does not have to be removed to install a Split seal.
No realignment between the pump and driver is needed because the equipment was not disassembled
- The coupling and motor do not have to be pulled and therefore realignment is no longer necessary. Even if you have the latest Laser equipment, it still takes a long time to align a driver and pump properly.
No need to overhaul the pump most of the time.
- Good split seals will not wear a shaft, so there is no need to pull the bearings unless they have been damaged. You change only the seal, not the shaft and bearings. To prevent all damage to the shaft you should replace the bearing lip or grease seals with positive face seals or the labyrinth type that work better and do not damage expensive shafts.
No damage to the pump.
- Since the pump is not being disassembled there is less chance of damaging something during the seal or sleeve change. Often there are no spare parts available for some of the older pumps. Usually there is nothing wrong with the pump&emdash; only the leakage is the problem.
The pump cannot be repacked. The stuffing box or sleeve is too far worn.
- In many cases the inside of the packing chamber has corroded or the metal has fatigued causing the packing to rotate with the shaft. The face of the stuffing box can easily be repaired with available commercial products and a split seal installed outside of the packing chamber. There is seldom any need to remove the damaged sleeve during these installation.
Mixers and agitators.
- Disassembly of this equipment is always a problem. Many times it pays to install a split bushing in the bottom of the packing chamber to help stabilize the shaft. For those mixer designs that do not require emptying to change the seal, a split sleeve can be installed under the split seal.
Systems that have to be sterilized
- In some instances a split seal can be sterilized and then installed without having to sterilize the entire system. This often happens when the pump has to be removed from the piping.
The pump insulation does not have to be removed.
- Many pumps have been insulated with asbestos packing and its removal is an involved and costly process. Split seals can usually be installed with no need to remove this installation.
Seal repair and disposal problems.
- Split seal components are easily replaceable and do not take up any volume. There is no need to send seals back to the manufacture or any other facility for rebuilding. Repair can be done by the mechanic at a cost that is usually lower than comparable size solid seals. Recent “Right to know laws” has created an urgent need for these designs.
No multiple trades needed
- Split seals can be installed by one man in less than an hour. No need for multiple trades and the multiple work orders involved.
Split seals can back up existing seals.
- If a double seal is necessary, you can usually install a split mechanical seal behind the present seal to protect the product and area in the event the present seal fails. A convection tank using anti freeze or any compatible liquid can be installed between the seals as a barrier fluid. This system works well with exotic metal pumps also. A 316 stainless steel split seal can often back up an exotic metal seal because it will only be activated when the first seal fails.
- You can install a split seal on a leaking pump and stop the leakage until you have time to replace the seal that was specified for the pump. If there is not enough room between the seal gland and the first obstruction you can cut off the existing gland and leave the leaking rotary unit attached to the shaft inside the stuffing box.
- Using split seals you can convert packed pumps at a much faster rate. In fact there is no longer any need to wait until turn around time to fix or convert leaking pieces of rotating equipment.
There are some limitations to split seals:
- At the time I wrote this article split seals were not available in all of the metallurgies and shaft sizes we find with non-split designs.
- There is some question as to whether single split seals will ever be able to pass a fugitive emission test.
There is no longer any need to speculate if a seal will work in a given application. You can always install a split seal and learn the answer during the present work shift. A typical stationary split seal application is accomplished in less than one hour with twenty to thirty minutes being more common as the mechanic gains experience.