SUBJECT: Why don’t good seals wear out? 9-9
We know that a mechanical seal is supposed to run until the carbon wears down, but our experience shows us this never happens with the original equipment seal that came installed in the pump. We buy an expensive new mechanical seal and that one doesn’t wear out either. What is wrong? Was the new seal a waste of money?
Not really. You are doing something that appears logical, you are trying to solve the seal problem by purchasing a different seal, but that is like trying to get a good paint job on an automobile by buying a good brand of paint.
Here are the things you would have to do in no particular order:
- Prepare the body. This is the most costly part involving metal repair, rust removal, sanding, masking etc.
- Buy a good brand of paint. All paint is not the same, and like anything good it will cost more money than many other brands.
- Apply the paint correctly. This means exactly the right amount of air pressure and a technique that guarantees no drips or runs. It also means a super clean paint room and frequent sanding between primer and finish coats. Needless to say the paint job can be ruined in this step.
- Take care of the paint after it has been applied. This means that you have to keep the car washed and waxed and garaged in bad weather. It also means frequent touch ups and paying attention to small details.
If you did those four things correctly, how long can a paint job last on an automobile? Obviously for years. Step outside and watch the cars go by and you will see evidence of people that are not doing those four things. In fact it is so rare that when we see an older car that looks good, we stare at it.
Getting good seal life involves four steps also. They should be obvious, but let’s look at them any way.:
- Prepare the pump for the seal, that’s the body work
- Purchase a good seal, the good paint.
- Install the seal correctly, apply the paint correctly
- Apply the correct environmental control if necessary (and it probably is),washing and waxing.
We will look at each of these subjects in detail and hopefully begin to increase the life of our mechanical seals to the point where most of them wear out. We will be discussing seals for centrifugal pumps in this paper, but the information applies to just about any kind of rotating equipment including mixers and agitators.
Prepare the pump for the seal
- Do an alignment between the pump and driver. Use a laser aligner. A “C or D” frame adapter is an even better choice.
- Dynamically balance the rotating assembly. You can use most vibration analysis equipment to do this. Check with your supplier if you do not have the program.
- Make sure the shaft is not bent. Rotate it between centers.
- Avoid shaft sleeves. A solid shaft is less likely to deflect and is much better for a mechanical seal.
- Reduce pipe strain where ever possible.
- Use a “center line” design pump if the product temperature is greater than 200°F (100°C). This will reduce some pipe strain problems at the pump.
- Use pumps with a low ratio. This is extremely important with intermittent service pumps.
- Use an oversize stuffing box. Avoid tapered designs. Give the seal lots of room.
- Try to get the stuffing box face as square to the shaft as possible. There are facing tools available to do this.
- Reduce vibration by any techniques you know or can learn.
- Do not let the pump cavitate. The seal faces will bounce open and possibly become damaged.
- Water hammer can occur if power is lost to the pump while it is running. Maybe you can take some preventative action to avoid water hammer problems.
- Be sure the mass of the pump/motor pedestal is at least five times the mass of the hardware sitting on it.
- Be sure there are ten diameters of pipe between the pump suction and the first elbow.
- Be sure the base plate is level and grouted in place.
- Keep the open impeller adjusted to lessen vibration and internal recirculation problems.
- Make sure the bearings have the proper amount of lubrication and that water and solids are not penetrating into the bearing cavity. Replace the grease or lip seals with labyrinth or face seals.
- Avoid discharge recirculation lines connected to the stuffing box. In most instances suction recirculation will be better.
- If the pump has wear rings, check their clearance.
- Make sure the wetted parts of the pump are manufactured from corrosion resistant materials. Cleaners and solvents in the lines sometimes cause problems that the designer never anticipated.
- Seal off any air that might be leaking into the suction side of the pump and remove any that might be trapped in the volute.
Purchase a good seal
- Use hydraulically balanced designs that seal both pressure and vacuum.
- If you are going to use an elastomer in the seal try to use an o-ring. They are the best shape for lots of reasons, but don’t let any one spring load the o-ring or it will not flex or roll as it should.
- Use non fretting seal designs. Shaft fretting is a major cause of premature seal failure.
- Stationary seals (the springs do not rotate with the shaft) are better than rotating seals (the springs rotate) for sealing fugitive emissions and any other fluids.
- If the seal has small springs keep them out of the fluid or they will clog easily. There are plenty of seal designs that have this non-clogging feature.
- A wide hard face is excellent for the radial movement we see in mixer applications and those seals that are physically positioned a long way from the bearings.
- You will need some sort of vibration damping for high temperature metal bellows seals. They lack the elastomer that normally performs that function.
- Use designs that keep the sealing fluid at the seal outside diameter, or centrifugal force will throw solids into the lapped faces and restrict their movement when the carbon wears.
- Use unfilled carbons for the seal faces. They are the best kinds and the cost is not excessive.
- Be sure you can identify all of the seal materials. It is impossible to troubleshoot a “mystery material”. Do not let the supplier tell you that his material is proprietary. If that is his attitude find another supplier or manufacturer, otherwise you deserve all of the problems you are going to have.
- Try to keep elastomers away from the seal face. The elastomer is the one part of the seal that is the most sensitive to heat, and the temperature is hottest at the faces.
- Any dangerous or expensive product should be sealed with dual seals. Be sure the hydraulic balance is in both directions or you are gambling that one of the faces might open in a pressure reversal or surge.
- If the design has a carbon pressed into a metal holder, be sure the carbon was pressed and not “shrunk in”. Pressed carbon will shear to conform to irregularities in the metal holder&emdash;helping to keep the lapped faces flat.
Install the seal correctly
- Cartridge seals are the only design that makes sense if you want to make impeller adjustments and they are a lot easier to install because you do not need a print, or take any measurements to get the correct face load.
- Cartridge dual seals should have a pumping ring built in. Use buffer fluid (lower pressure) between the seals when ever possible to avoid product dilution problems. Avoid any type of oil as a buffer fluid because of oil’s low specific heat and poor conductivity.
- Keep the seal as close to the bearings as possible. There is usually room to move the seal out of the stuffing box and then use the stuffing box area for a support bushing to help stabilize the rotating shaft. Depending upon the application you will have to decide if this support bushing has to be retained axially.
- Split seals make sense in just about any application that does not require dual seals or fugitive emission sealing (leakage measured in parts per million)
- Split seals are the only design to use on double ended pumps or otherwise you will have to replace both seals when only one seal has failed. They also allow you to change seals without having to do a re-alignment with the pump driver.
- Do not lubricate seal faces at installation. Keep solids off the lapped faces. If there is a protective coating on the seal faces be sure to remove it prior to installation
- Rubber bellows seals require a special lubricant that will cause the bellows to stick to the shaft. It is normally a petroleum based fluid, but you can check with your supplier to be sure.
- Rubber bellows seals require a shaft finish of no better than 40 RMS, or the rubber will have difficulty sticking to the shaft.
- In a vertical application, be sure to vent the stuffing box at the seal faces. You may have to install this vent because the pump manufacturer never provided it. Many cartridge seals have a vent built in that you can connect to the pump suction or some other low pressure point in the system.
Take care of the seal
- The seal would prefer to be sealing a cool, clean, lubricating liquid. We seldom have one of those to seal so maybe you can apply an environmental control in the stuffing box area to change your product into a cool, clean, lubricating liquid:
- If you are using a jacketed stuffing box, be sure the jacket is clean. Condensate or steam are the best fluids to circulate through the jacket. Install a carbon bushing in the end of the stuffing box to act as a thermal barrier that will help to stabilize the stuffing box temperature.
- Flushing is the ultimate environmental control. It causes product dilution, but if you are using the correct seal you won’t need much flush. Four or five gallons per hour (notice I said hour not minute) should be enough for that type of seal.
- Keep the fluid moving in the stuffing box to prevent a build up of heat.
- Suction recirculation will remove solids that are heavier than the product you are sealing. Since that is the most common slurry condition, use suction recirculation as your standard. Learn where not to use it also.
- Discharge recirculation will allow you to raise the pressure in the stuffing box to prevent a fluid from vaporizing between the lapped faces. Try not to aim the recirculation line at the lapped faces, it could injure them. If you are using a metal bellows the recirculation line can act as a sand blaster and cut the thin bellows plates.
- If the product is too hot, cool the stuffing box area, There are lots of ways to do this. Check other sections of the Technical Series for ideas.
- It is important to remember that these environmental controls are often more important when the pump is stopped because soak temperatures and shut down cooling can change the stuffing box temperature drastically, causing the product to change state.
- Dangerous products will need an A.P.I. type gland if you elect not to use dual seals. The disaster bushing that is part of the A.P.I. configuration will protect the seal from physical damage if you should lose a bearing when the pump is running.
- Insure that the API connections are made correctly. It is easy to mix up the four ports and get the flush or recirculation line into the quench port.
- Do not put too much steam or water through the quench connection or it will get into the bearing case. Leakage out the drain connection is often perceived as a seal failure by operators. Be sure they know the difference.
Does any one ever do all of these four things? Unfortunately not. If we did, eighty five or ninety percent of our seals would be wearing out rather than the ten or fifteen percent that wear out now. The prematurely failed seal with plenty of carbon face left, continues to be the rule.
The most common excuse we hear to explain our lack of good seal life is that there is never time to do it right, followed by the cliché, “but there is always time to fix it”. Most of us do one or two of the necessary steps and experience an increase in our seal life. There is nothing wrong with an increase in seal life, but that is a long way from wearing out seals.
Think about it for a minute. If the seal is lasting a year, how big can the problem be? The temperature cannot be too high or the pressure too severe. If that were true it wouldn’t take a year to fail the seal. The product can’t be too dirty for the same reason.
We often find the problem is as simple as a seal design that is fretting the shaft, causing a leak path through the damaged sleeve or shaft. Other times we find that the flush that is used to clean the lines once a year is the culprit, and no one is changing the seal materials to reflect this threat to the seal components.