The rubber bellows seal
The rubber bellows seal 11-11
Available from a number of seal companies (the Crane #1 is typical), and manufactured in a variety of materials, it looks like the following illustration:
You will find this seal used in water pumps and oil pumps, but it is seldom used with chemicals in the process industry. In recent years one manufacturer is mounting the seal on a cartridge sleeve and promoting it as a slurry seal.
To the casual observer the seal appears to have several real advantages:
- Low cost. It is probably the largest mass produced seal in the world.
- No sliding elastomer or O-ring. Nothing to “hang up” on the shaft.
- Easy to install. There are seldom any measurements to make or set screws to tighten to the shaft. Most designs are positioned against a shaft shoulder, or attached to a shoulder cast into the back of the impeller. Push it on the shoulder, and that is all there is to it !
- Availability. You can purchase these seals from a variety of manufacturers, distributors and bearing houses. Most distributors have replacement charts that will reference your present seal and recommend an equivalent design.
- No shaft or sleeve fretting. When installed properly the seal cannot frett or damage a shaft or sleeve because there is no relative movement between the rubber boot and shaft or sleeve.
Let’s look at each of these features in detail and see if they really are an advantage:
- The seal is low cost only if you purchase it manufactured from brass metal with a low grade carbon face and a Buna N rubber boot.
- The Crane company advertises they use 76 different grades of carbon in their seals and this is the seal that uses most of them. The problem surfaces with replacement seals. No reputable distributor can afford to stock 76 carbons so he often inventories the seal with a better grade of carbon, 316 stainless steel parts and possibly a fluroelastomer bellows. This makes the low cost original equipment seal (O.E.M.) an expensive replacement seal.
- Once the seal is cartridge mounted, and balanced versions of this seal require it, the seal is no longer low cost.
- No sliding elastomer.
- The rubber boot must transmit the turning torque from the shaft to the seal face and provide enough flexibility to compensate for axial motion and carbon face wear. This means that the bellows must stick to the shaft, so the lubricant you choose to install the seal becomes critical.
- Buna N is one of the few rubber compounds that has a shelf life because it is sensitive to ozone attack. The seal is packaged with the rubber boot wrapped in waxed craft paper to retard ozone attack. Once the package is opened you have about one year shelf life.
- O-ring seals offer a wide choice of elastomer materials that are readily available at low cost. The rubber boot choice of materials is very limited.
Easy to install
- Pushing the seal against a shoulder sounds very attractive, but it a real installation problem. Looking again at the illustration you can see that the rubber bellows location is critical to the operation of the seal. The mechanic can position the seal spring, but he has no control over the rubber bellows which can be either extended or compressed with respect to the seal face.
- The lubricant used to install the seal must make the rubber slippery enough to slide easily on the shaft and yet stick to the shaft to provide the turning torque to the rotating seal face. A lubricant that will attack the rubber bellows is selected for this. It will make the rubber “slimy” for about fifteen minutes and then cause the rubber to “swell up” and lock to the shaft. This means you have about fifteen minutes to assemble the seal into the stuffing box and tighten up the seal gland, which is just about impossible on a double ended pump. That is the reason you find so many of these seals sleeve mounted. Silicone grease is the first choice for lubricating rubber parts in other seals, but it must never be used in this design, because it will not attack the rubber boot.
- If the old seal was installed correctly it vulcanized to the shaft. This means that the seal has to be physically scrapped off the shaft or sleeve before a new seal can be installed. It is very common for mechanics to polish the cleaned up shaft with crocus cloth or fine emery paper, but this must be avoided because if the shaft or sleeve is too smooth the new rubber boot will not stick. You want a finish of no better than 40 rms, which is very different than the requirement of at least 32 rms (0,8 microns) used in other seal designs.
- Because of the great variety of materials used in this design, and because many of the replacement seals are sold by non professionals, it is easy to mix up the seal materials. The print that came with the seal shows part numbers and not grades of material. Call up a local distributor of these seals and he will often ask you to bring one of the seals over so they can pick out a seal that looks like it. Most distributors cannot identify the seal materials because there are many different grades of stainless steel, carbon/graphite/, ceramic, Buna N, neoprene, etc.
- If you experience damage or fretting on the shaft or sleeve under the rubber boot it means that the boot did not attach its self to the shaft. The faces had stuck together and you were sealing between the stationary rubber boot and the rotating shaft. You had used the wrong lubricant when the boot was installed on the shaft or sleeve and the rubber boot is acting like a “grease or lip” seal.
Here are some additional things you should be aware of:
- The carbon is often a loose fit in the metal holder. In many designs the carbon can be installed backwards and often is. The problem is caused by the seal packaging method. Many manufacturers will install the carbon backwards in the holder to protect the lapped face and then “bubble packed” the assembly to save packaging costs. The never tell you to turn the carbon face around at assembly. Mechanics usually install hardware the way it came out of the box. There is nothing in their experience to tell them to do it differently.
- The most common failure with this seal is to rupture the rubber bellows and experience a sudden and massive bellows failure. Well designed, balanced, O-ring seals tend to drip excessively when they begin to fail. This bellows design leaks massively at failure. Very scary!
- Buna N has a high temperature limit of about 210°F (100°C) meaning that the seal can be injured if hot water or steam is used to clean the lines. There are much better elastomers on the market for a wide range of chemical compatibility and operating temperature.
What is my personal opinion of this type of product ? It is probably the best original equipment seal (O.E.M.) made and the worst replacement seal ever designed. I do not like it for the following reasons:
- The normal failure mode is dangerous. A rubber bellows rupture is a very severe seal failure.
- The carbon seal face is thermally insulated by the rubber boot. This is never a good idea when you are trying to remove heat between the seal faces.
- The special lubricant required to install the seal makes installation different than other seals. Any time something is different, errors occur.
- The rougher shaft finish requirement makes installation different also.
- The Buna N boot is sensitive to ozone attack once the package is opened, and people like to open packages. Shops often have high levels of ozone caused by the sparking of electric motors.
- The carbon face can be put in backwards. If it can be, it will be.
- The seal is not usually hydraulically balanced, limiting its pressure and speed capability.
- Unless the seal is on a cartridge you cannot make the initial impeller setting and other impeller adjustments when using open impeller pumps.
- The seal is limited to a replacement part, and with the great variety of materials specified you will have too many replacement seals in your inventory. With just a few exceptions, you should be able to use he same seal in every pump of the same shaft size. Imagine what a difference that would make in your inventory costs and spare parts availability.
- Your spare seals should be acceptable for packing replacement as well as a replacement part for an existing application. This seal is limited to replacement only.
- The stainless steel spring is a major component of this seal. Stainless steel springs are not recommended in mechanical seals because of the possibility of chloride stress corrosion problems that can break the spring.
If you are willing to invest in a high priced, balanced design, cartridge mounted version of this seal; then there is something to be said for its performance in slurry applications where you are trying to cut down on water flushing.
Given the choice you will probably be better off with the stationary version, but you will still have to contend with all of the points mentioned above.
- On February 18, 2018