Industrial Machinery Repair Part Episode 2 Part 7 pdf

384 Packing and Seals Table 19.1 Common failure modes of packing and mechanical seals THE PROBLEM THE CAUSES Excessive leakage Continuous stream of liquid No leakage Shaft hard to turn Shaft damage under packing Frequent replacement required Bellows spring failure Seal face failure Packed box Nonrotating Cut ends of packing not staggered • • • Line pressure too high • Not packed properly • • • Packed box too loose • • Packing gland too loose • • Packing gland too tight • • • • • Rotating Cut end of packing not staggered • Line pressure too high • Mechanical damage (seals, seat) • • • • Noncompatible packing • • • Packing gland too loose • Packing gland too tight • • • Mechanical seal Internal flush Flush flow/pressure too low • • Flush pressure too high • • • • Improperly installed • • • Induced Misalignment • Internal flush line plugged • • Line pressure too high • • Physical shaft misalignment • Seal not compatible with application • External flush Contamination in flush liquid • • External flush line plugged • • Flush flow/pressure too low • • flush pressure too high • • • • Improperly installed • • Induced misalignment • • • Line pressure too high • • Physical shaft misalignment • • • Seal not compatible with application • • Source: Integrated Systems Inc. Packing and Seals 385 Seal Flushing When installed in corrosive chemical applications, mechanical seals must have a clear water flush system to prevent chemical attack. The flushing system must provide a positive flow of clean liquid to the seal and also provide an enclosed drain line that removes the flushing liquid. The flow rate and pressure of the flushing liquid will vary depending on the specific type of seal but must be enough to assure complete, continuous flushing. Packed Boxes Packing is used to seal shafts in a variety of applications. In equipment where the shaft is not continuously rotating (e.g., valves), packed boxes can be used successfully without any leakage around the shaft. In rotating applications, such as pump shafts, the application must be able to tolerate some leakage around the shaft. Nonrotating Applications In nonrotating applications, packing can be installed tightly enough to pre- vent leakage around the shaft. As long as the packing is properly installed and the stuffing-box gland is properly tightened, there is very little probabil- ity that seal failure will occur. This type of application does require periodic maintenance to ensure that the stuffing-box gland is properly tightened or that the packing is replaced when required. Rotating Applications In applications where a shaft continuously rotates, packing cannot be tight enough to prevent leakage. In fact, some leakage is required to provide both flushing and cooling of the packing. Properly installed and main- tained packed boxes should not fail or contribute to equipment reliability problems. Proper installation is relatively easy, and routine maintenance is limited to periodic tightening of the stuffing-box gland. 20 Precision Measurement Introduction Precision measurement is an important part of any maintenance procedure. Without micrometers, telescopic gauges, dial calipers, edge finders, and other precision measuring tools, the job cannot be done correctly. The areas covered in this chapter are: 1 The proper use of an outside micrometer 2 The proper use of an inside micrometer 3 The proper use of telescopic gauges 4 The proper use of dial calipers Micrometers Precision measurement is an important part of the correct installation of equipment. One of the most important precision measurement tools available to the technician is the micrometer. A difference of 0.001" may not seem important for most purposes, but some parts of equipment or tools must fit even more closely than that, even as close as .0001". The most common type of micrometer is operated by a screw that has 40 threads to the inch. Each revolution of the screw moves the measur- ing spindle 0.025". A scale revolving with the screw is divided into 25 parts and indicates, therefore, the fractions of a turn in units of 0.001". Outside Micrometer A Vernier scale micrometer can measure objects to .001" or .0001". Measure- ments for the outside micrometer are taken on the outside of an object like a shaft (see Figure 20.1). Precision Measurement 387 54321 3 1 0 2 22 23 24 Figure 20.1 Outside micrometer Frame Thimble SpindleAnvil Lock 54321 1 2 3 22 23 24 0 Sleeve Ratche t stop Figure 20.2 Defining parts of a micrometer Standards Standards are used to check the accuracy of the micrometers. These are precision blocks that are cut to an exact measurement. The micrometer is then used to measure the standard. The measurement on the micrometer must match that of the standard. If there is any variation then the micrometer must be adjusted. Let’s take a look at the names for the specific parts of the micrometer (see Figure 20.2). The scale on the sleeve is graduated in .025". The scale on the thimble is graduated in .001". See Figures 20.3 and 20.4. Now let’s see if we can put the two parts together and come up with a measurement. Write down the measurement for the following drawing. See Figure 20.5. 388 Precision Measurement .100" 198765432 .200" .300" .400" .500" .600" .700" .800" .900" 1.000" .025" .050" .075" Figure 20.3 Vernier scale .003" .002" .001" 1 2 0 22 23 24 3 .024" .023" .022" Figure 20.4 Micrometer scale To find the measurement we start at the sleeve and add .600" + .075" + .000" = .675" Vernier Scale When using a micrometer, there may be a need to take measurements that are closer than .001". When this is necessary, a micrometer with a Vernier scale is used. A Vernier scale will make measurements to within .0001 (one ten- thousandth) of an inch. The Vernier scale is located on top of the sleeve and is read by lining up the lines on the sleeve with those on the thimble. In the Figure 20.6, we can see that the reading is .350", but we know that in order to take the reading to within .0001" we must also line up the lines Precision Measurement 389 012 20 0 5 10 15 3456 Figure 20.5 Micrometer 0 5 20 3210 0 1 2 Vernier scale Thimble Sleeve scale Figure 20.6 Micrometer readings on the thimble with the lines on the sleeve. So, our actual reading would be .3501". Inside Micrometer In addition to outside micrometers, you must also become familiar with inside micrometers. Inside micrometers work the same as outside micro- meters, except that they measure inside dimensions. See Figure 20.7. 390 Precision Measurement 234561 12 13 14 15 16 17 18 Figure 20.7 Inside micrometer Telescopic Gauges Another tool used for precision measurement is the snap, or telescopic, gauge. The telescopic gauge measures inside dimensions by adjusting to the correct bore size, then measuring it with a micrometer. See Figure 20.8. Dial Caliper Another tool that is widely used is the dial caliper. The dial caliper can take inside, outside, and depth measurements. The only drawback is that it is not as accurate as the micrometer (measurements to within .001"). See Figure 20.9. Precision Measurement 391 54321 1 2 22 23 24 3 0 Figure 20.8 Telescoping gauges measuring inside diameter Ϫ0ϩ 50 .001" 10 20 30 4060 70 80 90 Figure 20.9 Dial caliper Performance Exercises Outside Micrometer Now let’s do some performance exercises with the outside micrometer. You will need a box of drill bits, drill rods, feeler gauges, or calibration standards that have the size written in thousandths of an inch (.001") and 392 Precision Measurement 54321 1 2 22 23 24 3 0 Figure 20.10 Exercise 1 an outside micrometer. Measure each drill bit and compare the reading that you get to the size on the drill index. Write down the answers that you get and keep them, because you will need them in exercises that follow. See Figure 20.10. Inside Micrometer Now let’s try some inside measurements with the inside micrometer. Remember, this is similar to the outside micrometer, only you are measuring inside dimensions. You will need an assortment of pillow-block bearings to perform these exercises. First note the dimension that is stamped on the outside of the bearing, then convert this from fractions to decimals. To do this simply divide the top number (numerator) by the bottom number (denominator). The problem will look like this: Let’s say that the bearing size is 3 4 ". Just divide the top number by the bottom number, which will give you the decimal equivalent. 3 4 =.75 Your measurement should be .75 on the micrometer scale. Remember to write down your answers, as you will need them in another exercise. See Figure 20.11. Telescopic Gauges Let’s see how using a set of snap gauges compares to using an inside micrometer. Using the same pillow-block bearings, turn the end of the handle counter- clockwise, squeeze together the snap gauge, and turn the handle back clockwise (this will lock the gauge). Then insert the snap gauges inside the Precision Measurement 393 7 0 654321 Figure 20.11 Exercise 2 54321 1 2 22 23 24 3 0 Figure 20.12 Exercise 3 [...]...394 Precision Measurement 80 90 Ϫ0ϩ10 20 001" 70 30 50 60 40 Figure 20 .13 Exercise 4 20 0 400 600 20 0 800 400 600 1.000 Figure 20 .14 Exercise 5 bearing bore just as you did with the inside micrometer Turn the handle counterclockwise to unlock the gauge Holding the gauge perpendicular... pump Pumps 401 Figure 21 .7 Multistage centrifugal pump Components Centrifugal pumps vary in design and construction from simple pumps with relatively few parts to extremely complicated pumps with hundreds of individual parts Some of the most common components found in centrifugal pumps are wearing rings, stuffing boxes, packing, and lantern rings These components are shown in Figure 21 .8 and are described... pump casing, where it is collected in the outer part of the pump casing called the volute Discharge The volute is a region that expands in cross-sectional areas as it wraps around the pump casing The purpose of the volute is to collect the liquid discharged Impeller eye Suction Impeller Figure 21 .1 Centrifugal pump Volute 396 Pumps Single Double Figure 21 .2 Single and double volute from the periphery... diameter of the bearing bore See Figure 20 . 12 Dial Caliper A dial caliper is similar to an inside and outside micrometer, but it can take both inside and outside measurements with just one device A dial caliper has two measurement scales: the scale on the long flat body is graduated in 100 of an inch, and the round dial is graduated in 001 of an inch See Figures 20 .13 and 20 .14 Wrap-Up Exercise Using the same... displacement pump consisting of a single reciprocating piston in a cylinder with a single suction port and a single discharge port, as shown in Figure 21 . 12 Reservoir Reservoir Suction Suction Discharge Discharge Suction stroke Discharge stroke Figure 21 . 12 Reciprocating positive displacement pump operation ... directed out along the impeller blades in a direction at right angles to Pumps 399 Volute Impeller Volute Figure 21 .4 Radial flow centrifugal pump Impeller Figure 21 .5 Typical axial flow centrifugal pump the pump shaft The impeller of a typical radial flow pump and the flow is illustrated in Figure 21 .4 Axial Flow In an axial flow pump, the impeller pushes the liquid in a direction parallel to the pump shaft... discharge connection Figure 21 .2 illustrates the two types of volutes Centrifugal pumps can also be constructed in a manner that results in two distinct volutes, each receiving the liquid that is discharged from a 180 degrees region of the impeller at any given time Pumps of this type are called double volute pumps In some applications the double volute minimizes radial forces imparted to the shaft and... Packing Lantern ring Pump casing wearing ring Inlet Volute Figure 21 .8 Components of a centrifugal pump Suction eye Suction eye Single-suction Suction eye Double-suction Casing Impeller Single-suction Double-suction Figure 21 .9 Single-suction and double-suction impellers Enclosed impellers are also referred to as shrouded impellers Figure 21 .10 illustrates examples of open, semi-open, and enclosed impellers... indications that a centrifugal pump is cavitating: 1 2 Fluctuating discharge pressure and flow 3 ● Noise Fluctuating pump motor current Steps that can be taken to stop pump cavitation include: 1 Increasing the pressure at the suction of the pump 2 Reducing the temperature of the liquid being pumped 3 Reducing head losses in the pump suction piping Pumps 4 07 4 5 ● Reducing the flow rate through the pump Reducing... the required specifications A typical centrifugal pump characteristic curve is shown in Figure 21 .3 There are several terms associated with the pump characteristic curve that must be defined Shutoff head is the maximum head that can be developed Pumps 3 97 Pump head Shutoff head Pump runout Flow rate Figure 21 .3 Centrifugal pump characteristics curve by a centrifugal pump operating at a set speed Pump . Figure 20 .1). Precision Measurement 3 87 54 321 3 1 0 2 22 23 24 Figure 20 .1 Outside micrometer Frame Thimble SpindleAnvil Lock 54 321 1 2 3 22 23 24 0 Sleeve Ratche t stop Figure 20 .2 Defining parts. Measurement 393 7 0 654 321 Figure 20 .11 Exercise 2 54 321 1 2 22 23 24 3 0 Figure 20 . 12 Exercise 3 394 Precision Measurement Ϫ0ϩ 50 .001" 10 20 30 4060 70 80 90 Figure 20 .13 Exercise 4 .20 0 .400. .600" .70 0" .800" .900" 1.000" . 025 " .050" . 075 " Figure 20 .3 Vernier scale .003" .0 02& quot; .001" 1 2 0 22 23 24 3 . 024 " . 023 " . 022 " Figure