SEAL LIFE, INCREASING IT S021
Seals that came installed as original equipment in the pump should last until the sacrificial carbon face has worn away. This is the definition of a worn out seal. If a seal leaks before the carbon face has worn down we call it a failed seal. In practice we find that seals seldom wear out, most of them fail and as a result we spend a good deal of time and money looking for better designs. Wouldn’t it be nice if there was such a thing as the ideal mechanical seal? Some consumers have spent years looking for this magic seal and their failure rate in seals still exceeds 85%.
Getting a good seal installation is similar to getting a good paint job on an automobile. It is more involved than just the purchase of a good brand of paint. To get a good paint job on your automobile you need to solve four problems:
- The body must be prepared properly.
- You must have good quality paint.
- The paint must be applied correctly.
- You have to take care of the paint once it has been applied.
If you have addressed these four separate problems you can get a paint job that will last many years. Neglect some of these steps and the life of the paint will diminish accordingly.
Seal application follows four steps also, and if you neglect any of the steps your seal life will be shortened accordingly. Here are the four steps you must follow:
- Make sure the pump is ready to receive a mechanical seal.
- Select a good seal, manufactured from identifiable materials.
- Install the seal correctly.
- Hook up the proper environmental controls to insure long life.
In the following paragraphs we will investigate each of these steps. I will not be going into great detail because these subjects will be covered individually in this book. This brief discussion, however, should encourage you to investigate the subject of seal life in more detail.
This procedure could take many hours or even days, but it is the necessary first step because without it the paint would not look as nice as we want.
To prepare the pump for a mechanical seal we would want to do a number of things also. They would include:
- Reduce shaft vibration and displacement. The more the shaft moves the more likely the seal faces are to open and allow destructive solids to enter between the lapped faces.
- Either operate close to the pump’s best efficiency point (BEP), use a shaft with a low , or use a double volute pump design.
- Avoid sleeved shafts. The shaft diameter was reduced and weakened to install the sleeve. If you use a corrosion resistant shaft with a non-fretting, balanced O-ring mechanical seal there is no need for a shaft sleeve.
- Either align the pump/driver properly or install a “C’ or “D” frame adapter and don’t worry about misalignment.
- Check for excessive pipe strain. A centerline wet end conversion can solve the problem if temperature growth is the cause.
- Make sure the shaft is not bent. Most people have no luck in trying to straighten a bent or warped shaft.
- Check that the rotating parts have been dynamically balanced.
- Make sure the bearings are in good shape.
- Check for vibration. There are many sources.
- Different types of cavitation.
- Foundations with too small a mass.
- Piping arrangements.
- Harmonic vibration caused by nearby hardware
- Seal slip stick
- Non-laminar or turbulent flow through the pipes.
- Water hammer.
- · Either bore out the packing chamber or install a large seal chamber to give the seal room to move, and utilize centrifugal force to throw solids away from the lapped faces. A one-inch (25 mm) radial clearance around the outside diameter of the seal is both desirable and practical to obtain.
- · In most cases install a line from the bottom of the stuffing box back to the suction of the pump. One of the times you will not do this is when you are pumping a fluid close to its vapor point.
- · Be sure the shaft is of the correct tolerance and finish where the elastomer seals to the shaft.
- · Be sure to vent the stuffing box back to the pump suction any time you have a mechanical seal in a vertical pump installation. If you fail to provide this vent, the seal chamber will become full of air, causing overheating in the seal area.
A good seal will incorporate the following features when ever possible:
- Both dynamic and hydraulic balance.
- Non-clogging features.
- Springs positioned out of the sealing fluid, to prevent clogging.
- The elastomer moves to a clean surface and away from any solids as the seal face wears.
- The pumping fluid should be at the outside diameter of the rotating component to take advantage of centrifugal force.
- Two way hydraulic balance must be used in multiple seal designs. This feature is necessary to prevent the seal faces from opening with fluctuating system and barrier fluid pressures
- Vibration damping must be provided to prevent damage to the faces and stop the lapped faces from separating. O-rings are a natural vibration damper. Bellows seals must have one installed.
- Excessive motion capability must be provided when a seal is used in sleeved bearing equipment and long shaft designs such as those found on mixers and agitators.
- To decrease your inventory, slotted glands should be used to adapt the seal to the widest range of pump brands. Slotted glands also help to center the seal to the rotating shaft.
- The seal face should not be insulated by an elastomer, any type of rubber, or a gasket
- Carbon faces should be pressed into a metal retainer and never shrunk in.
- Universal seal materials should be specified for the widest range of applications and minimum customer inventory.
- Failure protection in the form of either an API (American Petroleum Institute) type gland or a back up seal should be part of every seal application.
- The thinnest, shortest design that will satisfy the sealing requirements should be part of any good seal design.
- Proper packaging and storage will insure that the seal is in good shape at the time of installation.
- The seal should be boxed in a reusable container.
- It should be insulated to survive a one-meter drop with out damage to the lapped seal faces.
- Protective material must be placed on the lapped faces in non-cartridge designs.
- Proper labeling is required to prevent unnecessary box opening.
There are several similar problems associated with good seal installation:
- You must not damage the lapped faces or cut the elastomer.
- Be sure to compress the seal to the proper face load. Check the print that came with the seal for this critical dimension.
- Be sure the shaft diameter is of the proper tolerance and finish, and the dynamic elastomer will seal properly.
- With open impeller pumps you will have to make the initial impeller adjustment and additional adjustments for impeller or volute wear and thermal growth. Remember that these adjustments will also affect the mechanical seal compression. Cartridge seals and sleeve mounted split seals are the only sensible method of getting correct face compression. These designs should be the standard in any modern plant.
We take care of a mechanical seal by providing environmental controls that will allow the pumping fluid to stay at the correct temperature and pressure to prevent it from changing to a solid, vapor or crystal that would damage the mechanical seal. These environmental controls are especially important with intermittent service pumps because the constant starting and stopping is always harmful to a mechanical seal. These controls include:
- Controlling the temperature in the stuffing box. Especially when the pump is stopped.
- Controlling the pressure in the stuffing box.
- Removing solids from the stuffing box.
- Eliminating air or oxygen outboard the mechanical seal that is sealing the fluid.
- Quenching to wash away any leakage and to control the temperature outboard the seal
How long can a good paint job last on an automobile? The answer would have to be many years if you did the four things mentioned in the above paragraphs. Obviously most people do not take the proper care with their investment, and the cars you see on the street are the result.
All companies would like to get better seal life and the method of doing it is not complicated. Take a look at most of the seals you have removed from consumer pumps and notice that in 85 % of the cases there is very little carbon wear. It is reasonable to expect that with proper attention you should be able to wear out 85% of your seals and experience only a 15% failure rate.
The seal life you get will be directly related to your interest in addressing these four subjects.