Efficient sealing of cooling water in mixing, milling and calendering operations. (Process Machinery).
[FIGURE 1 OMITTED]
Pressure rotary joints
In the early 1900s, a breakthrough called the rotary joint enabled the industry to use closed pressurized systems and thereby regulate temperature precisely. The rotary joint also permitted the use of steam as a heat source. Steam was injected into the roll chamber to raise the temperature of the rubber prior to mixing, milling or calendering.
Rotary joints: From rope packing to carbon graphite seals The first rotary joints were simple packing glands consisting of a rope packing compressed into a chamber and held there by a spring or gland nut. A major drawback to these rudimentary joints was the need for constant adjustment for wear. If the seal was too loose, the joint leaked. If overtightened, the packing material would be quickly destroyed due to overheating caused by friction.
The introduction of a new material - carbon graphite - led the way in the 1940s to the development of the next generation of rotary joints. The new technology became known as the pressure joint, since it used the pressure of the fluid instead of mechanical means to produce a tight seal and prevent leakage.
Unlike softer graphite materials, the new grades of carbon graphite were capable of withstanding high fluid pressures and provided excellent wear characteristics. Carbon graphite also was chemically compatible with many fluids, so its applications in the rubber industry were many and varied.
The basics of pressure rotary joints A pressure rotary joint consists of one or two carbon bearings, a carbon graphite seal with a spherical sealing face and a spring that keeps the seals together at zero pressure (figure 2).
[FIGURE 2 OMITTED]
The spherical seal allows the stationary seal member, or seat, to misalign. Carbon bearings require a running clearance between the bearing and the rotary joint sleeve. Because of this clearance, the seal seat, contained in the casing, is not perpendicular to the center line of rotation and a precise alignment of the sealing faces cannot be maintained. The spherical surfaces of the seal, similar to a balland-socket arrangement, slide against each other to maintain full contact with the seat.
Most pressure rotary joints have a balance ratio of 3.0 or higher. The seal balance ratio is determined by taking the closing, or clamping, area and dividing it by the seal face, or opening area (figure 3). As fluid pressure increases, seal forces increase dramatically, resulting in a torque curve highly dependent on pressure (figure 4). To prevent hose failures, torque rods must be installed between rotary joints to transfer forces away from the hose.
[FIGURE 3 AND 4 OMITTED]
Since fluid pressure is used to energize the seal, rotary pressure joints are self-adjusting for wear. However, seal face closings are inconsistent. At low fluid pressures, the majority of the closing force is provided by the spring and a slight dripping is not unusual.
Conversely, high fluid pressures produce high closing forces which increase friction and wear. This wear is compounded by high rotating speeds.
Spherical seal faces are made from two types of materials. Common materials such as ductile iron, bronze and steel can all be machined economically and are suitable for steam, heat transfer and clean water applications. Harder materials, which are recommended in contaminated water applications where rust and scale can cause abrasion in softer materials, must be spherically ground, an expensive operation.
Pressure rotary joints are designed for applications requiring positive sealing between fixed plumbing and continually rotating machinery. Specific examples include heat transfer applications on steam and thermal fluid systems. Rotary joints are available in sizes from 1/2" to 4", pressures to 250 psi, speeds to 400 rpm and temperatures to 600 [degrees] F.
The basics of rotary unions
A mechanical rotary union consists of two seals, one rotating and one stationary. The stationary seal is keyed in place by pins or flats to prevent it from rotating, but at the same time, allowing the seal to move axially down the center line (figure 6). A spring keeps the seal faces in contact and an o-ring seal prevents leakage as the stationary seal slides axially.
[FIGURE 6 OMITTED]
Rotary unions differ from rotary joints in three ways. First, the rotary union uses ball bearings to maintain accurate alignment of the seals. The ball bearings provide rigidity and support the thrust load generated by the fluid pressure. In a rotary joint, the spherical seal is the sealing element and thrust bearing.
The second difference between the two types of seals is that mechanical seals can be designed to be under-balanced, where the balance ratio is below 1.0. The fluid pressure can be balanced by changing the seal's geometry (figure 7).
[FIGURE 7 OMITTED]
Finally, balancing lowers the face contract forces and reduces torque. So, unlike pressure seals (rotary joints), mechanical seals (rotary unions) do not require torque rods.
When designing mechanical seals, the objective is to use a combination of three elements - balance ratio, contact forces and seal geometry - to develop a thin layer of fluid across the entire seal face width. This layer of fluid, acting as a hydrodynamic bearing, increases seal performance by further reducing torque and lubricating the seal faces. In fact, when compared to a rotary joint, the film developed in a rotary union can reduce torque by a factor of 12. The fluid film separating the seal faces may only be several millionths of an inch thick, making small changes in the seal geometry critical in the performance of the rotary union (figure 5).
Due to the enhanced service life provided by separating the seal faces with a sufficient fluid film, mechanical seals can be made from a wider range of materials. Rather than being limited to carbon graphite, as is the case with pressure seals, the seal designer can choose from bronze and iron, as well as harder materials such as tungsten carbide, aluminum oxide and silicone carbide.
Mechanical rotary unions are specifically designed for water-based applications, particularly those involving high rotating speeds and fluid pressures. Rotary unions are available in sizes from 3/8" to 4", pressures to 150 psi, speeds to 3,500 rpm and temperatures to 225 [degrees] F.
Design factors for rotary unions
In order to maintain the necessary layer of fluid film, rotary unions are carefully manufactured to strict tolerances. For example, seal faces are ground and lapped flat to .000033" (0.8 microns) or 3 helium light bands. This precision produces a degree of flatness that makes the rotary union leak-tight at both high and low pressures.
In addition to strict manufacturing tolerances, mechanical seal faces must be kept flat in a working environment where temperatures and pressures are constantly changing. If the fluid film is disturbed, the seal will be running dry and wear rates will increase. For materials with self-lubricating properties - such as carbon graphite - this is not a major concern. However, running dry with hard face materials can cause catastrophic results.
Hard face materials are preferred in maintaining seal face flatness. That is because they have both a high coefficient of elasticity (to prevent seal face distortion) and a low thermal expansion value (to minimize heat deflection). Even more important, hard face materials resist abrasive damage caused by contaminants in water, a major source of wear in rotary unions.
Seal face flatness can also be accomplished through design. Most rotary unions are designed for internal pressure where the fluid pressure is on the inside of the mechanical seal. This design produces pressure forces and temperature deflections in a counter-clockwise direction. If the sum of these deflections is great enough, the fluid film is eliminated and face contact will occur at the inside diameter of the face.
Conversely, mechanical seals may be designed for external pressure. In this design, the seal is installed over the shaft facing the opposite direction. Pressure forces on the seal deflect clockwise, while temperature deflections are counter-clockwise. These opposing forces minimize seal face deflections and the fluid film thickness is maintained.
Rotary union or rotary joint: Selecting the right seal
Due to modem design tools, such as finite element geometry used to develop the optimum seal face geometry, mechanical rotary unions are more advanced technically than rotary joints. For applications involving high rotating speeds and pressures, the rotary union is the obvious choice. In the rubber industry, where water systems may not be filtered, mechanical seals made from hard face materials offer superior resistance to the abrasive elements found in most water sources. However, there are engineers who still prefer rotary joints for many applications.
With a wide range of rotary unions and rotary joints on the market today, selection is often difficult. It is best to call upon the services of a seal application engineer at a local supplier to help match the proper seal with the application at hand. By analyzing the application and then matching it with the best seal design and selection of materials, maximum performance and seal life can be achieved. Most complaints of short seal life arise because the wrong seal has been selected. Often, a simple change is all it takes to correct the problem and increase the performance of the rotary union.
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|Comment:||Efficient sealing of cooling water in mixing, milling and calendering operations. (Process Machinery).|
|Date:||Dec 1, 2001|
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