Industrial Machinery Repair Part Episode 2 Part 6 potx

Mixers and Agitators 359 is acceptable for each application. The recommended range should include adjustments for temperature, flow rates, mixing speeds, and other factors that directly or indirectly affect viscosity. Troubleshooting Table 18.1 identifies common failure modes and their causes for mixers and agitators. Most of the problems that affect performance and reliability are caused by improper installation or variations in the product’s physical properties. Table 18.1 Common failure modes of mixers and agitators THE PROBLEM THE CAUSES Surface vortex visible Incomplete mixing of product Excessive vibration Excessive wear Motor overheats Excessive power demand Excessive bearing failures Abrasives in product • Mixer/agitator setting too close to side or corner • • • • • Mixer/agitator setting too high • • Mixer/agitator setting too low • • Mixer/agitator shaft too long • Product temperature too low • • • Rotating element imbalanced or damaged • • • • • Speed too high • • • Speed too low • Viscosity/specific gravity too high • • • Wrong direction of rotation • • • 360 Mixers and Agitators Proper installation of mixers and agitators is critical. The physical location of the vanes or propellers within the vessel is the dominant factor to consider. If the vanes are set too close to the side, corner, or bottom of the vessel, a stagnant zone will develop that causes both loss of mixing quality and premature damage to the equipment. If the vanes are set too close to the liquid level, vortexing can develop. This will also cause a loss of efficiency and accelerated component wear. Variations in the product’s physical properties, such as viscosity, also will cause loss of mixing efficiency and premature wear of mixer components. Although the initial selection of the mixer or agitator may have addressed the full range of physical properties that will be encountered, applications some- times change. Such a change may result in the use of improper equipment for a particular application. 19 Packing and Seals All machines such as pumps and compressors that handle liquids or gases must include a reliable means of sealing around their shafts so that the fluid being pumped or compressed does not leak. To accomplish this, the machine design must include a seal located at various points to prevent leakage between the shaft and housing. In order to provide a full under- standing of seal and packing use and performance, this manual discusses fundamentals, seal design, and installation practices. Fundamentals Shaft seal requirements and two common types of seals, packed stuffing boxes and simple mechanical seals, are described and discussed in this sec- tion. A packed box typically is used on slow- to moderate-speed machinery where a slight amount of leakage is permissible. A mechanical seal is used on centrifugal pumps or other type of fluid handling equipment where shaft sealing is critical. Shaft Seal Requirements Figure 19.1 shows the cross-section of a typical end-suction centrifugal pump where the fluid to be pumped enters the suction inlet at the eye of the impeller. Due to the relatively high speed of rotation, the fluid collected within the impeller vanes is held captive because of the close tolerance between the front face of the impeller and the pump housing. With no other available escape route, the fluid is passed to the outside of the impeller by centrifugal force and into the volute, where its kinetic energy is converted into pressure. At the point of discharge (i.e., discharge nozzle), the fluid is highly pressurized compared to its pressure at the inlet nozzle of the pump. This pressure drives the fluid from the pump and allows a centrifugal pump to move fluids to considerable heights above the centerline of the pump. This highly pressurized fluid also flows around the impeller to a lower pres- sure zone where, without an adequate seal, the fluid will leak along the drive 362 Packing and Seals Discharge Housing Suction Back-head Impeller Shaft Balancing holes Pumping vanes Figure 19.1 Cross-section of a typical end-suction centrifugal pump shaft to the outside of the pump housing. The lower pressure results from a pump design intended to minimize the pressure behind the impeller. Note that this design element is specifically aimed at making drive shaft sealing easier. Reducing the pressure acting on the fluid behind the impeller can be accomplished by two different methods, or a combination of both, on an open-impeller unit. One method is where small pumping vanes are cast on the backside of the impeller. The other method is for balance holes to be drilled through the impeller to the suction eye. In addition to reducing the driving force behind shaft leakage, decreasing the pressure differential between the front and rear of the impeller using one or both of the methods described above greatly decreases the axial thrust on the drive shaft. This decreased pressure prolongs the thrust bearing life significantly. Sealing Devices Two sealing devices are described and discussed in this section: packed stuffing boxes and simple mechanical seals. Packing and Seals 363 Packed Stuffing Boxes Before the development of mechanical seals, a soft pliable material or pack- ing placed in a box and compressed into rings encircling the drive shaft was used to prevent leakage. Compressed packing rings between the pump housing and the drive shaft, accomplished by tightening the gland-stuffing follower, formed an effective seal. Figure 19.2 shows a typical packed box that seals with rings of compressed packing. Note that if this packing is allowed to operate against the shaft with- out adequate lubrication and cooling, frictional heat eventually builds up to A. Packing chamber or box B. Packing rings C. Gland follower or stuffin g g land Figure 19.2 Typical packed stuffing box 364 Packing and Seals the point of total destruction of the packing and damage to the drive shaft. Therefore, all packed boxes must have a means of lubrication and cooling. Lubrication and cooling can be accomplished by allowing a small amount of leakage of fluid from the machine or by providing an external source of fluid. When leakage from the machine is used, leaking fluid is captured in collection basins that are built into the machine housing or baseplate. Note that periodic maintenance to recompress the packing must be carried out when leakage becomes excessive. Packed boxes must be protected against ingress of dirt and air, which can result in loss of resilience and lubricity. When this occurs, packing will act like a grinding stone, effectively destroying the shaft’s sacrificial sleeve, and cause the gland to leak excessively. When the sacrificial sleeve on the drive shaft becomes ridged and worn, it should be replaced as soon as possi- ble. In effect, this is a continuing maintenance program that can readily be measured in terms of dollars and time. Uneven pressures can be exerted on the drive shaft due to irregularities in the packing rings, resulting in irregular contact with the shaft. This causes uneven distribution of lubrication flow at certain locations, producing acute wear and packed-box leakages. The only effective solution to this problem is to replace the shaft sleeve or drive shaft at the earliest opportunity. Simple Mechanical Seal Mechanical seals, which are typically installed in applications where no leakage can be tolerated, are described and discussed in this section. Toxic chemicals and other hazardous materials are primary examples of applications where mechanical seals are used. Components and Assembly Figure 19.3 shows the components of a simple mechanical seal, which is made up of the following: ● Coil spring ● O-ring shaft packing ● Seal ring The seal ring fits over the shaft and rotates with it. The spring must be made from a material that is compatible with the fluid being pumped so that it will withstand corrosion. Likewise, the same care must be taken in Packing and Seals 365 Stationary ring Rotating ring Pressure chamber Static seal point stationary unit to housing Static seal point rotating unit to shaft Rotary seal point mating ring faces in contact Rotating shaft Force Force Figure 19.3 Simple mechanical seal material selection of the O-ring and seal materials. The insert and insert O-ring mounting are installed in the bore cavity provided in the gland ring. This assembly is installed in a pump-stuffing box, which remains stationary when the pump shaft rotates. A carbon graphite insertion ring provides a good bearing surface for the seal ring to rotate against. It is also resistive to attack by corrosive chemicals over a wide range of temperatures. Figure 19.4 depicts a simple seal that has been installed in the pump’s stuffing box. Note how the coil spring sits against the back of the pump’s impeller, pushing the packing O-ring against the seal ring. By doing so, it remains in constant contact with the stationary insert ring. As the pump shaft rotates, the shaft packing rotates with it due to friction. (In more complex mechanical seals, the shaft-packing element is secured to the rotating shaft by Allen screws.) There is also friction between the spring, the impeller, and the compressed O-ring. Thus, the whole assembly rotates together when the pump shaft rotates. The stationary insert ring is located within the gland bore. The gland itself is bolted to the face of the stuffing box. This part is held stationary due to the friction between the O-ring insert mounting and the inside diameter (ID) of the gland bore as the shaft rotates within the bore of the insert. 366 Packing and Seals Figure 19.4 Pump stuffing box seal How It Prevents Leakage Having discussed how a simple mechanical seal is assembled in the stuffing box, we must now consider how the pumped fluid is stopped from leaking out to the atmosphere. In Figure 19.4, the O-ring shaft packing blocks the path of the fluid along the drive shaft. Any fluid attempting to pass through the seal ring is stopped by the O-ring shaft packing. Any further attempt by the fluid to pass through the seal ring to the atmospheric side of the pump is prevented by the gland gasket and the O-ring insert. The only other place where fluid can poten- tially escape is the joint surface, which is between the rotating carbon ring and the stationary insert. (Note: The surface areas of both rings must be machined-lapped perfectly flat, measured in light bands with tolerances of one-millionth of an inch.) Sealing Area and Lubrication The efficiency of all mechanical seals is dependent upon the condition of the sealing area surfaces. The surfaces remain in contact with each other for the effective working life of the seal and are friction-bearing surfaces. As in the compressed packing gland, lubrication also must be provided in mechanical seals. The sealing area surfaces should be lubricated and cooled Packing and Seals 367 with pumped fluid (if it is clean enough) or an outside source of clean fluid. However, much less lubrication is required with this type of seal because the frictional surface area is smaller than that of a compressed packing gland, and the contact pressure is equally distributed throughout the interface. As a result, a smaller amount of lubrication passes between the seal faces to exit as leakage. In most packing glands there is a measurable flow of lubrication fluid between the packing rings and the shaft. With mechanical seals, the faces ride on a microscopic film of fluid that migrates between them, resulting in leakage. However, leakage is so slight that if the temperature of the fluid is above its saturation point at atmospheric pressure, it flashes off to vapor before it can be visually detected. Advantages and Disadvantages Mechanical seals offer a more reliable seal than compressed packing seals. Because the spring in a mechanical seal exerts a constant pressure on the seal ring, it automatically adjusts for wear at the faces. Thus, the need for manual adjustment is eliminated. Additionally, because the bearing surface is between the rotating and stationary components of the seal, the shaft or shaft sleeve does not become worn. Although the seal will eventually wear out and need replacing, the shaft will not experience wear. However, much more precision and attention to detail must be given to the installation of mechanical seals compared to conventional packing. Nevertheless, it is not unusual for mechanical seals to remain in service for many thousands of operational hours if they have been properly installed and maintained. Mechanical Seal Designs Mechanical seal designs are referred to as friction drives, or single-coil spring seals, and positive drives. Single-Coil Spring Seal The seal shown back in Figure 19.4 depicts a typical friction drive or single- coil spring seal unit. This design is limited in its use because the seal relies on friction to turn the rotary unit. Because of this, its use is limited to liquids such as water or other nonlubricating fluids. If this type of seal is to be used 368 Packing and Seals with liquids that have natural lubricating properties, it must be mechanically locked to the drive shaft. Although this simple seal performs its function satisfactorily, there are two drawbacks that must be considered. Both drawbacks are related to the use of a coil spring that is fitted over the drive shaft: ● One drawback of the spring is the need for relatively low shaft speeds because of a natural tendency of the components to distort at high surface speeds. This makes the spring push harder on one side of the seal than the other, resulting in an uneven liquid film between the faces. These cause excessive leakage and wear at the seal. ● The other drawback is simply one of economics. Because pumps come in a variety of shaft sizes and speeds, the use of this type of seal requires inventorying several sizes of spare springs, which ties up capital. Nevertheless, the simple and reliable coil spring seal has proven itself in the pumping industry and is often selected for use despite its drawbacks. In regulated industries, this type of seal design far exceeds the capabilities of a compressed packing ring seal. Positive Drive There are two methods of converting a simple seal to positive drive. Both methods, which use collars secured to the drive shaft by setscrews, are shown in Figure 19.5. In this figure, the end tabs of the spring are bent at Figure 19.5 Conversion of a simple seal to positive drive [...]... (Figure 19. 12) 23 Tighten up the gland bolts with a wrench to seat and form the packing to the stuffing box and shaft Packing and Seals 375 Figure 19. 12 Proper lantern ring installation 24 Loosen the gland nuts one complete turn and rotate the shaft by hand to get running clearance 25 Retighten the nuts finger tight only Again, rotate the shaft by hand to make sure the packing is not too tight 26 Contact... Stagger the butt joints, placing the first ring butt at 12 o’clock; the second at 6 o’clock; the third at 3 o’clock; the fourth at 9 o’clock; etc., until the packing box is filled (Figure 19.10) NOTE: When the last ring has been installed, there should be enough room to insert the gland follower 1 to 8 3/ 16 inches into the stuffing box (Figure 19.11) 22 Install the lantern ring in its correct location within... shaft with a 2" diameter will require a packing cross-section of 1" The centerline of the packing would then be 3" Packing and Seals 373 Therefore, the approximate length of each piece of packing would be: Packing Length = Centerline Diameter × 3.14 16 = 3 0 × 3.14 16 = 9.43 inches The packing should be cut approximately 1 " longer than the calculated 4 length so that the end can be bevel cut 16 Controlled... freely Tighten the gland bolts one flat at a time until the desired leakage is obtained and the pump runs cool 3 76 Packing and Seals 27 Clean up the work area and account for all tools before returning them to the tool crib 28 Inform operations of project status and complete all paperwork 29 After the pump is in operation, periodically inspect the gland to determine its performance If it tends to leak... TIR is greater than +/−0 0 02 inches, the pump shaft should be straightened 15 Determine the correct packing size before installing using the following method (Figure 19.8): Measure the ID of the stuffing box, which is the OD at the packing (B), and the diameter of the shaft (A) With this data, the packing crosssection size is calculated by: Packing Cross Section = B−A 2 3 72 Packing and Seals Indicator... problems 6 Carefully remove the lantern ring This is a grooved, bobbin-like spool piece that is situated exactly on the centerline of the seal water inlet connection to the gland (Figure 19 .6) NOTE: It is most important to place the lantern ring under the seal water inlet connection to ensure the water is properly distributed within the gland to perform its cooling and lubricating functions Figure 19 .6 Lantern... Figure 19.9 20 Use a split bushing to install each ring, ensuring that the ring bottoms out inside the stuffing box An offset tamping stick may be used if a split bushing is not available Do not use a screwdriver “S” Twist Wrong Figure 19.9 Proper and improper installation of packing 374 Packing and Seals Figure 19.10 Stagger butt joints 1/8" to 3/ 16" Figure 19.11 Proper gland follower clearance 21 Stagger... should be lightly oiled before the seal is assembled to allow the seal parts to move freely over it This is especially desirable when assembling the seal collar because the bore of the collar usually has only a few thousandths of an inch clearance Care should be taken to avoid getting the collar cocked ● Install the rotary unit parts on the shaft or sleeve in the proper order ● Be careful when passing... the packing rings to size on a wooden mandrel that is the same diameter as the pump shaft Rings can be cut either square (butt cut) or diagonally (approximately 30 degrees) NOTE: Leave at least a 1 " 6 gap between the butts regardless of the type of cut used This permits the packing rings to move under compression or temperature without binding on the shaft surface 18 Ensure that the gland area is...Packing and Seals 369 90 degrees to the natural curve of the spring These end tabs fit into notches in both the collar and the seal ring This design transmits rotational drive from the collar to the seal ring by the spring . follower 1 8 to 3/ 16 inches into the stuffing box (Figure 19.11). 22 Install the lantern ring in its correct location within the gland. Do not force the lantern ring into position (Figure 19. 12) . 23 Tighten. butt joints 1/8" to 3/ 16& quot; Figure 19.11 Proper gland follower clearance 21 Stagger the butt joints, placing the first ring butt at 12 o’clock; the second at 6 o’clock; the third at 3 o’clock;. Centerline Diameter × 3.14 16 = 3. 0 ×3.14 16 = 9.43 inches The packing should be cut approximately 1 4 " longer than the calculated length so that the end can be bevel cut. 16 Controlled leakage