Metal bellows sealing
An overview of metal bellows sealing 12-10
Metal bellows seals perform many functions. Unfortunately all the functions are not available in the same seal.
- They can be used to eliminate elastomers (rubber like parts) in the chemical process industry. Most chemicals and chemical combinations can be sealed with either a good grade of Viton® or ethylene propylene, but someone has to make the decision and many responsible people are not capable, so mistakes are often made.
- Most solvents present a real problem for elastomer selection. Expensive materials like Kalrez® and Chemraz are often the only solution. You would be better off if you could eliminate these special materials all together
- Metal bellows are also used to eliminate elastomers because of temperature limits. All elastomers have both an upper and lower temperature limit that prevents them from sealing many hot resins, polymers and cryogenic applications. Hot oils are another high temperature sealing problem, but their “coking” characteristics dictates cooling of the stuffing box area.
The individual bellows convolutions can be formed in different ways:
- Convoluted, stamped plates can be welded. This is the most popular type used in mechanical seals. End fittings are welded to the bellows to hold the seal faces, sleeve and gland attachments.
- The bellows can be formed by forcing a metal tube into a die. Here you are limited to ductile material that have to be stretched to conform to the die, leaving thin and thick cross sections. “Crushed formed” techniques have helped, but they still lack the reliability of welded bellows. You have seen many of these formed bellows used in commercial expansion joints.
- The bellows configuration can be plated onto a wax mold that can later be melted away to leave the bellows configuration. The resultant thin bellows section eliminates this style for mechanical seals, but they are frequently used in instrumentation.
In the following drawing we will learn the names of the individual parts of a typical “nested convolution” bellows seal
Please take a look at the following diagram for some more bellows terminology;
- A convolution is two stamped plates welded together. You can count the number of convolutions in the seal by counting the spaces between the end fittings.
- The weld bead fusing the plates together is about 2.5 times the thickness of an individual plate (0.004″ or 0.10 mm).
- The span is the width of the plate. A 0.250 inch (6 mm) span is the most popular but seldom the most sensible. Most bellows seals come in this cross section because the tooling is readily available. The wider the span the less convolutions you need to get the desired spring rate for the proper face loading. If you use too many convolutions you end up, with a “slinky toy”.
- The pitch is the distance between the plates. You measure the pitch from the center of a weld bead to the center of an adjacent weld bead. 0.040 inches (0.10 mm) is typical in mechanical seal applications
The driving end of the bellows seal can be attached to the shaft in several ways:
- Hastelloy “C” is a good choice for most pumps because of its chemical compatibility, but it may not be thick enough for a Hastelloy “C” pump. Most bellows convolutions are only 0.004 inches (0.10 mm) thick and the definition of corrosion resistant is that the material can corrode up to 0.002 inches (0.05 mm) per year.
- The 300 series of stainless steel should never be used because of the probability of chloride stress corrosion problems.
- AM350 is a heat treatable form of stainless steel that has been used successfully for many years in high temperature and cryogenic seal applications. You need a heat treated material because it has to retain its strength and spring rate at these elevated temperatures.
- Inconel 718 is a metal that has good corrosion resistant properties in an annealed form and retains some of the corrosion resistant properties after heat testament. It has become the favorite of oil refinery people because of corrosion problems they have experienced with AM350 after five or six years of service.
- Titanium, 17-4 PH and variety of other materials have been used as bellows seals. In every case you are looking for high strength and chemical resistance. A tough combination to put together.
There are several ways to retain the seal face in the bellows end fitting ;
- Elastomer seals have a built in vibration damper. Metal bellows seals lack this feature so a damper must be built in. The most common method is to let the seal face holder come into contact with the shaft when vibration starts. You can see this feature in the first illustration
- “Slip stick” vibration is the most common. It occurs if the product you are sealing is not a good lubricant (hot water as an example). The resultant “slipping and sticking” between the lapped faces causes the vibration.
- In the stationary version of the seal it is hard to get an even cooling or heating of the bellows and seal faces unless you have paid close attention to the location of the stuffing box recirculation lines.
- In abrasive, slurry service the bellows plates may prove to be too thin. Try to rotate the slurry with the bellows and you will reduce the plate wear.
- Thicker plates are always desirable, but their higher spring rate would cause the use of too many convolutions to get the desirable spring load of about 30 psi on the seal faces. When the carbon face is worn down there should still be a load of about 10 psi.on the faces to prevent vibration from causing them to open.
- Hard face retention in a holder is a persistent problem, and there are times you really need two hard seal faces. Shrinking a hard face in a metal holder has the same problems we discussed about carbon a few paragraphs back.
- When bellows seals are used in temperature extremes they should be provided with an API (American Petroleum Institute) gland or back up seal.
- Since the face holder has a different expansion and shrink rate than the seal face, high temperature applications require that the face holder be manufactured from low expansion metals such as Invar 36 or Carpenter 42 materials. These metals have poor corrosion resistance.
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