Industrial Machinery Repair Part 14 pps

Steam Traps 439 Table 22.1 Common failure modes of steam traps THE PROBLEM THE CAUSES Trap will not discharge Will not shut-off Continuously blows steam Capacity suddenly falls off Condensate will not drain Not enough steam heat Traps freeze in winter Back flow in return line Back-pressure too high • Boiler foaming or priming • • Boiler gauge reads low • Bypass open or leaking • • Condensate load greater than design • Condensate short-circuits • Defective thermostatic elements • Dirt or scale in trap • • Discharge line has long horizontal runs • Flashing in return main • • High-pressure traps discharge into low-pressure return • Incorrect fittings or connectors • • Internal parts of trap broken or damaged • • • • Internal parts of trap plugged • • Kettles or other units increasing condensate load • Leaky steam coils • No cooling leg ahead of thermostatic trap • • Open by-pass or vent in return line • Pressure regulator out of order • Process load greater than design • Plugged return lines • Plugged strainer, valve, or fitting ahead of trap • Scored or out-of-round valve seat in trap • Steam pressure too high • System is air-bound • Trap and piping not insulated • Trap below return main • • Trap blowing steam into return • Trap inlet pressure too low • • Trap too small for load • Source: Integrated Systems Inc. 440 Steam Traps Operating Methods Steam traps are designed for a relatively constant volume, pressure, and condensate load. Operating practices should attempt to maintain these parameters as much as possible. Actual operating practices are determined by the process system, rather than the trap selected for a specific system. The operator should periodically inspect them to ensure proper operation. Special attention should be given to the drain line to ensure that the trap is properly seated when not in the bleed or vent position. Troubleshooting A common failure mode of steam traps is failure of the sealing device (i.e., plunger, disk, or valve) to return to a leak-tight seat when in its normal operating mode. Leakage during normal operation may lead to abnormal operating costs or degradation of the process system. A single 3 4  steam trap that fails to seat properly can increase operating costs by $40,000 to $50,000 per year. Traps that fail to seat properly or are constantly in an unload position should be repaired or replaced as quickly as possible. Regular inspection and adjustment programs should be included in the standard operating procedures (SOPs). Most of the failure modes that affect steam traps can be attributed to vari- ations in operating parameters or improper maintenance. Table 22.1 lists the more common causes of steam trap failures. Operation outside the trap’s design envelope results in loss of efficiency and may result in premature failure. In many cases, changes in the conden- sate load, steam pressure or temperature, and other related parameters are the root causes of poor performance or reliability problems. Careful atten- tion should be given to the actual versus design system parameters. Such deviations are often the root causes of problems under investigation. Poor maintenance practices or the lack of a regular inspection program may be the primary source of steam trap problems. It is important for steam traps to be routinely inspected and repaired to assure proper operation. 23 V-Belt Drives “Only Permanent Repairs Made Here” V-belt drives are widely used in industry and commercial applications. V-belts are utilized to transfer energy from a driver to the driven and usu- ally transfer one speed ratio to another through the use of different sheave sizes. V-belts are constructed for three basic components, which vary from maker to maker: 1 Load carrying section to transfer power. 2 Rubber compression section to expand sideways in the groove. 3 Cover of cotton or synthetic fiber to resist abrasion. Understanding the construction of V-belts assists in the understanding of belt maintenance. The standard V-belt must ride in the sheave properly. If the belt is worn or the sheave is worn, then you will have slippage of the belt and transfer of power, and speed will change resulting in a speed change to a piece of equipment. If a V-belt drive is located near oil, grease, or chemicals the V-belts could lose their capability through the deterioration of the belt material, again resulting in the reduction of energy transfer and quickly resulting in belt breakage or massive belt slippage. Introduction Belt drives are an important part of a conveyor system. They are used to transmit needed power from the drive unit to a portion of the conveyor system. This chapter will cover: 1 Various types of belts that are used to transmit power; 2 The advantages and disadvantage of using belt drives; 3 The correct installation procedure for belt drives; 442 V-Belt Drives Driven roll Drive motor Figure 23.1 Belt drive 4 How to maintain belt drives; 5 How to calculate speeds and ratios that will enable you to make corrections or adjustments to belt drive speeds; 6 How to determine belt length and sheave sizes when making speed adjustments. Belt Drives Belt drives are used to transmit power between a drive unit and a driven unit. For example, if we have an electric motor and a contact roll on a conveyor, we need a way to transmit the power from the electric motor to the roll. This can be done easily and efficiently with a belt drive unit. See Figure 23.1. Belt drives can consist of one or multiple belts, depending on the load that the unit must transmit. The belts need to be the matched with the sheave type, and they must be tight enough to prevent slippage. Examples of the different belt and sheave sizes are as follows: 1 Fractional horsepower V-belts: 2L, 3L, 4L, and 5L; 2 Conventional V-belts: A, B, A-B, C, D, and E; Conventional cogged V-belts: AX, BX, and CX; 3 Narrow V-belts: 3V, 5V, and 8V; Narrow cogged V-belts: 3VX and 5VX; 4 Power band belts: these use the same top width designations as the above belts, but the number of bands is designated by the number preceding V-Belt Drives 443 A B B A Figure 23.2 Examples of V-belts the top width designation. For example, a 3-ribbed 5V belt would be labeled 3/5V; 5 Positive-drive belts: XL, L, H, XH, and XXH. The size of the belt must match the sheave size. If they do not match, then the belt will not make proper contact with the sheave and will decrease the amount of load it can transmit. They may look something like the illustration in Figure 23.2. Usually a set of numbers will follow the belt designation. These numbers represent the actual length of the belt in inches. On conventional belts, the length is given for the inside length of the belt, and on narrow belts it is given for the outside length. An example of this would be a 5V750 belt; the size of the belt gives it the 5V and the outside length of 75.0" gives it the 750. More information about the specific belt dimensions can be found in the Goodyear Power Transmission Belt Drives manual. Belt Selection V-Belts V-belts are best suited for transmitting light loads between short range sheaves. They are excellent at absorbing shock. When an overload occurs, they will act as an overload device and slip, thereby protecting 444 V-Belt Drives Figure 23.3 Standard V-belt Figure 23.4 Cogged belt valuable equipment. They are also much quieter than other power trans- mission devices such as chains. Because of their design, they are easier to install and maintain than other belt types. Other than an occasional retensioning, V-belts are virtually main- tenance free. When properly installed and maintained, V-belts will provide years of trouble-free operation. For an example, see Figure 23.3. Cogged Belts Cogged belts provide even longer life than conventional V-belts. Because of their design, they run cooler than conventional belts, thereby increasing the overall life of the belt. For an example, see Figure 23.4. Joined Belts Joined or power band belts provide a good alternative in pulsating drives where standard V-belts have a tendency to turn over. They function like a V-Belt Drives 445 Figure 23.5 Joined belt (VX type) Figure 23.6 Positive drive belt standard V-belt, with the exception that they are joined by the top fabric of the belt. These belts can be used with the standard V-belt sheaves, making selection and installation easy. For an example, see Figure 23.5. Positive-Drive Belts Positive-drive belts are sometimes called timing belts because they are often used in operations when timing a piece of equipment is critical. However, they are also used in applications where heavy loads cause standard V-belts to slip. They are flexible and provide the same benefit as standard V-belts, but their alignment is more critical. For an example, see Figure 23.6. Sheaves Sheaves are wheels with a grooved rim on which the belt rides. Sheaves are manufactured in various widths and diameters. Some have spokes, and some do not. For an example, see Figure 23.7. Sheaves are made of cast steel for heavy-duty applications. For lighter appli- cations, they are forged out of steel plate. Cast-iron sheaves are always used in applications where fluctuating loads are present. They provide a flywheel effect that minimizes the effects of fluctuating loads. 446 V-Belt Drives Figure 23.7 Positive drive belt When they are mounted to a shaft, sheaves should be straight and have little or no wobble. For drives where the belt enters the sheave at an angle, deep- groove sheaves are available. These are especially useful when the belts must turn or twist. Deep-groove sheaves can be used anywhere belt stability is a problem. In some cases, one drive shaft drives more than one driven shaft. When this occurs, more than one sheave can be mounted on one shaft. This is nec- essary only when sheaves of more than one size are needed. If the drive sheaves are the same size, one multibelt sheave can be used. Most sheaves are balanced and capable of belt speeds of 6,000 feet per minute or less. If you note excessive vibration during operation or excessive bearing wear, you may need to balance or replace the sheaves. Power Train Formulas Shaft Speed The size of the sheaves in a belt drive system determines the speed relation- ship between the drive and driven sheaves. For example, if the drive sheave has the same size sheave as the driven, then the speed will be equal. See Figure 23.8. If we change the size of the driven sheave, then the speed of the shaft will also change. We know what the speed is of the electric motor and the size V-Belt Drives 447 6" 6" Driven Drive 1800 rpm 1800 rpm Figure 23.8 Shaft speed 12" 6" Driven Drive 1800 rpmrpm Figure 23.9 Belt drive speed ratio of the sheaves, and now we can calculate the speed of the driven shaft by using the following formula (see Figure 23.9): Driven shaft rpm = Drive sheave diameter in inches × drive shaft rpm Driven sheave diameter Driven shaft rpm = 6 × 1800 12 900 = 6 × 1800 12 Now we understand how changing the size of a sheave will also change the shaft speed. Knowing this, we could also assume that to change the shaft rpm we must change the sheave size. The problem is, how do we know the exact size sheave that we need in order to reach the desired speed? Use the 448 V-Belt Drives 6" 6" Driven Drive 1800 rpm 1800 rpm Figure 23.10 Speed ratio same formula that was used to calculate shaft speed, only switch the location of the driven shaft speed and the driven sheave diameter. Driven shaft rpm = Drive sheave diameter in inches × drive shaft rpm Driven sheave diameter Let’s change the problem to look like this: Driven sheave diameter = Drive sheave diameter in inches × drive shaft rpm Driven shaft rpm Let’s say that we have a problem similar to the ones that we just did, but we want to change the shaft speed of the driven unit. If we know the speed we are looking for, we can use the formula above to calculate the sheave size required. See Figure 23.10 Let’s change the speed of the driven shaft to 900 rpm (see Figure 23.11): Driven shaft rpm = 6 × 1800 900 12 = 6 × 1800 900 Belt Length Many times when a mechanic has to change out belts, the numbers on the belts cannot be read. So what should be done? Take a tape measure and wrap it around the sheaves to get the belt length? This is not a very accurate [...]... Center-to-center distance between the shafts Now use the following formula to solve the equation: Belt length = drive diameter × 3 .14 driven diameter × 3 .14 + 2 2 + center to center × 2 Use the formula above to find the belt length 6" × 3 .14 12" × 3 .14 + + 35" × 2 2 2 6" × 3 .14 12" × 3 .14 + + 35" × 2 98.26" or 98" = 2 2 Belt length = 450 V-Belt Drives Driven Drive 12" 6" 35" Figure 23.12 Belt length example... than ideal conditions, and therefore, equipment and operators for maintenance welding should be the best Besides making quick, on-the-spot repairs of broken machinery parts, welding offers the maintenance department a means of making many items needed to meet a particular demand promptly Broken castings, when new ones are no longer available, can be replaced with steel weldments fashioned out of standard... sheave With this information, we can use the following formula: FPM = diameter × 3 .14 × rpm 12 Use this formula to find the speed of the following belt (see Figure 23.13): diameter × 3 .14 × rpm 12 6" × 3 14 × 1800 2826 = 1" FPM = V-Belt Drives 451 Driven Drive 12" 6" 900 rpm 1800 rpm Figure 23.13 Belt speed calculation Figure 23 .14 Belt maintenance Belt Maintenance Routine maintenance is essential if a belt... Belts will slip (even though you may not hear the slippage), thus resulting in equipment not operating to specifications 24 Maintenance Welding Introduction An important use of arc welding is the repair of plant machinery and equipment In this respect, welding is an indispensable tool without which production operations would soon shut down Fortunately, welding machines and electrodes have been developed... parallel This is done by measuring at different points on the shaft and adjusting the shafts until they are an equal distance apart Make sure that the shafts are pulled in as close as possible before performing this procedure Then you can use the jacking bolts to move the shafts apart evenly after the belt is installed See Figure 23.19 WARNING: Before installing a set of used sheaves, verify the size and... maintenance department as either permanent tooling or as temporary tooling for a trial lot The almost infinite variety of this type of welding makes it impossible to do more than suggest what can be done Figures 24.2 through 24.5 provide just a few examples of the imaginative applications of welding technology achieved by some maintenance technicians The welding involved should present no particular problems... temperatures as high as 13,000◦ F, melts the 464 Maintenance Welding Section YϪY Y Y C Milled C (A) (B) (C) 16 W 40 F 14 W 68 F PL 15ϫ1ϫ1'Ϫ5 7 Bar 6ϫ — 16 3 – 5 8 — 16 – PL.8ϫ1 ϫ8 2 1 – 4 – – PL.8ϫ3 ϫ1'Ϫ3 1 8 2 1 – 4 33 W 220 F 1 – 2 5 — 16 – – PL.6ϫ3 ϫ2'Ϫ6 3 4 4 – – PL.15 3 ϫ1 1 ϫ1'Ϫ5 4 4 16 W 40 F 14 W 136 F Figure 24.6 electrode and the surface of the work adjacent to the arc Tiny globules of molten metal... belt of a multibelt drive, it will be tighter than the others See Figure 23.15 A belt that is tighter than the others in a set will pull all the load Store the old belts as a set You may be able to use part of the set on a drive requiring fewer belts Figure 23.15 Belt tensioning V-Belt Drives 453 Figure 23.16 Belt tension gauges Sheave and Belt Installation Proper tools must be selected (These must be... life of the sheaves and belts Belt-drive maintenance requires little time, but it must be done regularly Keeping the belts clean and free of oil and grease will help ensure long belt life See Figure 23 .14 When you replace a belt, always check the tension immediately after installation Check it again after 24 hours of operation 452 V-Belt Drives Never force a V-belt onto a sheave There have been a number... Before installing a set of used sheaves, verify the size and condition of the sheaves with a sheave gauge Select the proper gauge for the size of sheave For example, if you have a 5V sheave that measures 14. 4", use the 40-deg gauge Insert the gauge into the sheave groove; if you can see light on either side, the sheave is worn Sheave gauges are also useful when the size of the sheave cannot be found stamped . length = drive diameter × 3 .14 2 + driven diameter × 3 .14 2 + center to center × 2 Use the formula above to find the belt length. Belt length = 6" × 3 .14 2 + 12" × 3 .14 2 + 35" × 2 98.26". formula: FPM = diameter × 3 .14 × rpm 12 Use this formula to find the speed of the following belt (see Figure 23.13): FPM = diameter × 3 .14 × rpm 12 2826 = 6" × 3. 14 ×1800 1" V-Belt Drives. into low-pressure return • Incorrect fittings or connectors • • Internal parts of trap broken or damaged • • • • Internal parts of trap plugged • • Kettles or other units increasing condensate