Seals vs. packing
More about packing conversion 9-4
Sales people constantly preach the virtues of converting that nasty, greasy packing to a brand new, shiny, expensive mechanical seal. Their presentation makes a lot of sense and besides that the government is passing new legislation that is making conventional packing more obsolete by the minute.
If you are about to make one of these conversion in your plant there are some things that you should know to make the transition easier. Packing conversion does have its down side. Let’s look at a few of the problems. We will then discuss some of the solutions, and end this discussion with the advantages of converting to a better mechanical seal .
- Unlike packing, mechanical seals are very sensitive to installation errors.
- You have to measure carefully. If you use the improper face load the seal will either overheat or lose its spring tension before the carbon face is worn.
- The shaft or sleeve tolerance and finish are critical for many seal designs. A typical shaft tolerance for a mechanical seal should be +0.00 inches – 0.002 inches. A finish of at least 32 rms. is required to prevent a sliding elastomer or Teflon piece from hanging up.
- You cannot make the initial impeller setting for most ANSI pumps without disturbing the seal setting.
- Many seal are damaged by poor packaging during shipment or improper handling at the installation site. Seal faces are lapped to a tolerance of less than one micron (0.000039 inches). There are not very many pieces of equipment in your shop with that tight a tolerance.
- Without the radial support being supplied by the packing, the shaft can deflect severely when the pump operates outside of its very narrow operating window. This is a major problem at start up when the pump is frequently running against a throttled discharge valve.
- Packing is not sensitive to axial movement of the shaft. Seals have a very small axial movement capability.
- Mechanical seals are very sensitive to pump/ driver misalignment.
- Cavitation and other types of vibration are deadly for mechanical seals. Although never desirable this type of shaft deflection did not often cause catastrophic failure with packing as it often does with mechanical seals.
- You could change packing without having to dismantle the pump. Most seal installations require pump disassembly and all the problems associated with it.
- Many pump and mixer applications alternate between a positive stuffing box pressure and a vacuum (condensate pumps are typical). OEM seals can be blown open during the vacuum cycle.
- Most mechanical seals require some sort of an environmental control to function properly:
- Cooling to prevent a product from flashing or coking.
- Heating to prevent a liquid from solidifying or crystallizing
- Flushing to wash away solid particles from the seal components.
- Quenching to prevent crystals and ice from forming outboard of the seal interfering with its movement.
- Mechanical seals are very sensitive to fluctuating flushing pressures. Lose the pressure and you often lose the seal. Needless to say this is a very common failure.
- Seal failure is typically catastrophic and occurs at the worst possible time.
- Mechanical seals have to be centered in the stuffing box. You never worried about that with packing. If you fail to center the rotating seal’s stationary face the rotating face can run off the edge. With stationary seals the failure to center can cause a wiping action across the seal faces.
- Impeller adjustment was easy with packing. With a non-cartridge seal it is almost impossible. Remember that with ANSI pumps the impeller has to be adjusted after the power end of the pump has been installed into the wet end that was left hanging on the piping. At this point the seal has already been set screwed to the shaft or sleeve and impossible to move.
- Seal materials have to be selected for chemical compatibility with the product you are sealing and any cleaners or solvents that will be flushed through the lines. You need an in depth knowledge of mechanical seals and chemicals to do this properly because of the wide variety of materials currently being used by seal manufacturers. This was seldom a problem with choosing packing material.
- Intermittent service pumps have more problems with mechanical seals because the fluid can solidify, crystallize, become viscous, etc. when the pump is not running.
- This change in the state of the product will almost guarantee a failure the next time the pump is started. Packed pumps had the same problem with the product changing state, but the catastrophic failure at start up seldom happened. Standby pumps experience the same difficulty.
- Spare parts are always a problem with mechanical seals. The subject never came up with packing.
- The stuffing boxes of vertical pumps have to be vented after a mechanical seal is installed. There is also a problem with venting between dual seals when they have been installed in a vertical application. Packed pump vent all the time.
- Horizontally split case pumps are a nightmare for mechanical seals. You get massive misalignment between the rotating and stationary seal faces because the top and bottom halves of the casing are never lined up properly. This subject never came up when packing was in the pump.
- Most original equipment seals will damage a shaft or sleeve so you have the same problem with sleeve removal that you had with packing. When the seal damages the shaft it is called “fretting”, but no matter what name you give it, it is still shaft damage requiring a disassemble of the pump to replace either the shaft or the sleeve.
- Pumps of the same shaft size seldom take the same seal gland.
- Hydraulically balanced seals do not generate a lot of heat so they seldom require any cooling. A suction recirculation line is almost always satisfactory.
- Cartridge seals solve most of the installation problems.
- The closer you move the seal to the bearing, the less affect of vibration and shaft displacement. Newer cartridge designs are located closer to the pump bearings.
- Stationary seals and self aligning designs solve most of the problems you experience with horizontally split case pumps.
- There are plenty of non-fretting seal design on the market today so shaft damage can be eliminated.
- Most modern cartridge seals have built in connections that allow you to vent the stuffing box in vertical applications.
- Newer face materials and the latest elastomers are chemically compatible with a wide cross section of chemicals and cleaners. With few exceptions you should be able to put the same seal in every pump of the same shaft size. The exceptions are non metallic and exotic metal pumps that require a different seal metallurgy.
- Cartridge designs have solved the impeller adjustment and centering problems that were common with the ANSI standard pumps.
- Split seal designs have solved the problem of having to dismantle the pump to change a seal.
- Dual seals will solve the catastrophic seal failure problem.
There is no question that mechanical seals are better than packing if you know how to deal with the problems mentioned above. The advantages of the cartridge mounted balanced seal over conventional packings are numerous, here are a few:
- Balanced seals consume one sixth the power of conventional packing.
- Pollution and fugitive emission legislation prohibits the leakage of even small amounts of many chemicals and packing does leak.
- Most products are costly. Even a small steady leak represents substantial money.
- Leakage always presents a safety and housekeeping problem.
- The waste treatment of leakage can exceed the cost of the leakage its self.
- Packing requires cooling that is often accomplished by the use of a flush that will dilute your product. The cost of removing the flush water at some later part in the process is another costly item.
- Packing leakage is a major cause of premature bearing failure. Not only from the liquid leaking out of the pump getting into the bearings, but because a water hose is often used to wash leakage away from base plates, and that is the moisture that is our major concern
- Sleeve damage is costly not only because of the sleeve cost, but also the costs involved in the sleeve removal. Seal removal almost always means changing the bearings and wear rings also. Also many parts get broken or lost when pumps are disassembled.
- Balanced O-ring seals can seal either vacuum or pressure.
- Unlike packing, mechanical seals do not have to be adjusted to compensate for wear.
- There are many seals available today that can be installed on shafts that have been damaged by packing or fretted by other mechanical seals. Many of these new designs install outside of the stuffing box where the shaft is still in good shape.
- Split seal designs removed the last advantage of packing.
- On February 18, 2018