Root Cause Failure Analysis Part 9 pdf

232 Root Cause Failure Analysis Complete the equipment assembly, taking care when compressing the seal into the stuffing box. Seat the gland ring and gasket to the face of the stuffing box by tightening the nuts and bolts evenly and firmly. Be sure the gland ring is not cocked. Tighten the nuts and bolts only enough to form a seal at the gland ring gas- ket, usually finger tight and one half to three quarters of a turn with a wrench. Excessively tightening the gland ring nuts and bolts will cause dis- tortion that will be transmitted to the running face, resulting in leaks. If the seal’s assembly drawing is not available, the proper setting dimension for inside seals can be determined as follows: - Establish a reference mark on the shaft or sleeve flush with the face of the stuffing box. - Determine how far the face of the insert will extend into the stuffing-box bore. Take this dimension from the face of the gasket. - Determine the compressed length of the rotary unit by compressing it to the proper spring gap. - This dimension, added to the distance the insert extends into the stuffing box, gives the seal-setting dimension from the reference mark on the shaft or sleeve to the back of the seal collar. - Outside seals are set with the spring gap equal to the dimension stamped on the seal collar. Cartridge seals are set at the factory and installed as complete assemblies. These assemblies contain spacers that must be removed after being bolted into position and the sleeve collar is in place. Installation of Environmental Controls Mechanical seals often are chosen and designed to operate with environmental con- trols. If this is the case, check the seal’s assembly drawing or the equipment’s drawing to ensure that all environmental-control piping is properly installed. Seal Startup Procedures Before equipment startup, all heating and cooling lines should be operating. These lines also should remain in operation for a short period after equipment shutdown. On double-seal installations, be sure the liquid lines are connected, the pressure-control valves are properly adjusted, and the sealing-liquid system is operating before starting the equipment. Before startup, all systems should be properly vented. This is especially important on vertical installations where the stuffing box is the uppermost portion of the pressure- containing part of the equipment. The stuffing-box area must be properly vented to avoid a vapor lock in the seal area that would cause it to run dry. When starting equipment with mechanical seals, make sure the seal faces are immersed in liquid from the beginning so they will not be damaged from dry opera- Seals and Packing 233 tion. The following recommendations for seal startup apply to most types of seal installations and will improve their life if followed: Caution the electrician not to run the equipment dry while checking motor rotation. A slight turnover will not hurt the seal, but operating at full speed for several minutes under dry conditions will destroy or severely damage the rubbing faces. The stuffing box always should be vented before startup, especially with centrifugal pumps. Even if the pump has a flooded suction, it is still possible that air may be trapped in the top of the stuffing box after the pump’s initial liquid purge. Where cooling or bypass recirculation taps are incorporated in the seal gland, piping must be connected to or from these taps before startup. These specific environmental-control features must be used to protect the organic materials in the seal and to ensure proper performance. Cooling lines should be left open at all times. This is especially true when hot product passes through off-line standby equipment, commonly done so that additional product volume or equipment change can be achieved easily. often by simply pushing a button. At the end of each day when hot operational equipment is shut down, it is best to leave the cooling water on long enough for the seal area to cool below the temperature limits of the organic materials in the seal. Before startup, face-lubricated seals must be connected from the source of lubrication to the tap openings in the seal gland. For double seals, it is nec- essary for the lubrication feed lines to be connected to the proper ports before startup for both circulatory and dead-end systems. This is very important because all types of double seals depend on the controlled pres- sure and flow of the sealing fluid to function properly. Even before the shaft is rotated, the sealing liquid pressure must exceed the product pressure opposing the seal. Be sure a vapor trap does not prevent the lubricant from promptly reaching the seal face. Thorough warm-up procedures include a check of all steam piping arrange- ments to be sure that all are connected and functioning. Products that solid- ify when cool must be fully melted before startup. It is advisable to leave all heat sources on while the system is shut down to ensure that the product remains in the liquid state. This facilitates quick startups and equipment switchovers that may be required during a production cycle. Thorough chilling procedures are necessary for some applications; for example, applications involving liquefied petroleum gas (LPG). LPG always must be kept in a liquid state in the seal area, and startup usually is the most critical time. Even during operation, the recirculation line piped to the stuffing box might need to be run through a cooler to overcome fric- tional heat generated at the seal faces. LEG requires a stuffing-box pressure greater than the vapor pressure of the product at pumping temperature. A 25 to 50 psi differential is generally desired. 234 Root Cause Failure Analysis OPERATING METHODS This section discusses operating methods for packed-stuffing boxes and simple mechanical seals. Packed-Stuffing Boxes Packed-stuffing boxes commonly are used on slow- to moderate-speed machinery where a slight amount of leakage is permissible. If the packing is allowed to operate against the shaft without adequate lubrication and cooling, frictional heat eventually will build up to the point of total packing destruction 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 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 causing the gland to leak excessively. When the sacrificial sleeve on the drive shaft becomes ridged and worn, it should be replaced as soon as possible. In effect, this is a continuing maintenance pro- gram that readily can be measured in terms of dollars and time. Uneven pressure 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 at certain locations, producing acute wear and packed-box leakage. The only effective solution to this problem is to replace the shaft sleeve or drive shaft at the earliest opportunity. Simple Mechanical Seal As with compressed packing glands, lubrication must be provided in mechanical seals. The sealing-area surfaces should be lubricated and cooled with pumped fluid (if it is clean enough) or another 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. Most packing glands have 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 and results in leakage. However, leakage is so slight Seals and Packing 235 that, if the temperature of the fluid is above its saturation point at atmospheric pres- sure, it flashes off to vapor before it can be visually detected. Friction Drive or Single-Coil Spring Seal The seal shown back in Figure 18-2 is a typical friction drive, or single-coil spring seal unit. This design is limited in use to nonlubricating fluids (e.g., water) because it relies on friction to turn the rotary unit. For use with liquids that have natural lubricat- ing properties. the seal must be mechanically locked to the drive shaft. Two drawbacks must be considered for this type of seal. Both are related to the use of a coil spring that fits over the drive shaft. Nevertheless, the simple and reliable coil spring seal has proven itself in the pumping industry and often is specified despite its drawbacks. In regulated industries, this type of seal design far exceeds the capabilities of a compressed packing ring seal. One drawback of the spring seal is the need for relatively low shaft speeds. The com- ponents have a tendency 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, which causes 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 several sizes of spare springs be kept in inventory. 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 set screws, are shown in Figure 18-1 1. In the Figure on the left, the end tabs of the spring are bent at 90" 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. In the right drawing of Figure 18-1 1, two horizontally mounted pins extend over the spring from the collar to the seal ring. Figure 18-11 Conversion of a simple seal to positive drive (Roberts 1978). Part I11 EQUIPMENT TROUBLESHOOTING GUIDE Most machine trains are prone to certain kinds of abnormal behavior and generally exhibit a finite number of recumng failure modes. In most cases, failures result from improper maintenance and operating practices that do not abide by design limits and restrictions. This part provides an overview of the more common failure modes for machinery found in integrated process plants. Troubleshooting guides are provided for: pumps. fans, blowers, fluidizers, conveyors, compressors, mixers, agitators, dust collectors. process rolls, gearboxes/reducers, steam traps, inverters, control valves. seals, and packing. 19 PUMPS Design, installation, and operation are the dominant factors that affect a pump’s mode of failure. This chapter identifies common modes of failure for centrifugal and posi- tive-displacement pumps. CENTRIFUGAL Centrifugal pumps are especially sensitive to variations in liquid condition (i.e., vis- cosity, specific gravity, and temperature); suction variations, such as pressure and availability of a continuous volume of fluid; and variations in demand. Table 19-1 lists common failure modes for centrifugal pumps and their causes. Mechanical failure may occur for a number of reasons. Some failures are induced by cavitation, hydraulic instability, or other system-related problems. Others are the direct result of improper maintenance. Maintenance-related problems include improper lubrication, misalignment, imbalance, seal leakage, and a variety of other situations that periodically affect machine reliability. Cavitation Cavitation in a centrifugal pump, which has a significant, negative effect on perfor- mance, is the most common failure mode. Cavitation not only degrades a pump’s per- formance but also greatly accelerates the wear on its internal components. Causes Three causes of cavitation in centrifugal pumps are change of phase, entrained air or gas, and turbulent flow. 239 240 Root Cause Failure Analysis Table 19-1 Common Failure Modes of Centrifugal Pumps THE CAUSES Bent Shaft Casing DktofW fmm Excessbe Pip Sbaln cavyatilm Leakage In Plplng, Vh, Vmla Mochanlcal Def.cfs, Worn, Rusted, D&dh Beerho8 Source: Integrated Systems, Inc. Change of Phase The formation or collapse of vapor bubbles in either the suction piping or inside the pump is one cause of cavitation. This failure mode normally occurs in applications, such as boiler feed, where the incoming liquid is at a tempera- ture near its saturation point. In this situation, a slight change in suction pressure can cause the liquid to flash into its gaseous state. In the boiler-feed example, the water flashes into steam. The reverse process also can occur. A slight increase in suction pressure can force the entrained vapor to change phase to a liquid. Pumps 241 Cavitation due to phase change seriously damages the pump’s internal components. Visual evidence of operation with phase-change cavitation is an impeller surface fin- ish like an orange peel. Prolonged operation causes small pits or holes on both the impeller shroud and vanes. Entrained Air or Gas Pumps are designed to handle gas-free liquids. If a centrifu- gal pump’s suction supply contains any appreciable quantity of gas, the pump will cavitate. In the example of cavitation due to entrainment, the liquid is reasonably sta- ble, unlike with the change of phase described in the preceding section. Nevertheless, the entrained gas has a negative effect on pump performance. While this form of cavi- tation does not seriously affect the pump’s internal components, it severely restricts its output and efficiency. The primary causes of cavitation due to entrained gas include two-phase suction sup- ply, inadequate available net positive suction head (NPSH,), and leakage in the suc- tion-supply system. In some applications, the incoming liquid may contain moderate to high concentrations of air or gas. This may result from aeration or mixing the liquid prior to reaching the pump or inadequate liquid levels in the supply reservoir. Regard- less of the reason, the pump is forced to handle two-phase flow, which was not intended in its design. nrbulent Flow The effects of turbulent flow (not a true form of cavitation) on pump performance are almost identical to those described for entrained air or gas in the preceding section. Pumps are not designed to handle incoming liquids that have no stable, laminar flow pattern. Therefore, if the flow is unstable, or turbulent, the symptoms are the same as for cavitation. Symptoms Noise (e.g., like a can of marbles being shaken) is one indication that a centrifugal pump is cavitating. Other indications are fluctuations of the pressure gauges, flow rate, and motor current, as well as changes in the vibration profile. Solutions Several design or operational changes may be necessary to stop centrifugal-pump cavitation. Increasing the available net positive suction head (NPSH,) above that required (NPSHR) is one way to stop it. The NPSH required to prevent cavitation is determined through testing by the pump manufacturer. It depends on several factors, including type of impeller inlet, impeller design, impeller rotational speed, pump flow rate, and the type of liquid being pumped. The manufacturer typically supplies curves of NPSH, as a function of flow rate for a particular liquid (usually water) in the pump’s manual. One way to increase the NPSH, is to increase the pump’s suction pressure. If a pump is fed from an enclosed tank, suction pressure can be increased by either raising the level of the liquid in the tank or increasing the pressure in the gas space above the liquid. [...]... Positive-Displacement Compressors Compressors 2 59 Table 22-33 Common Failure Modes of Reciprocating, Positive-Displacement Compressors Table 22-3c Common Failure Modes of Reciprocating, Positive-Displacement Compressors 260 Root Cause Failure Analysis Table 22-3d Common Failure Modes of Reciprocating, Positive-Displacement Compressors THE PROBLEM I THE CAUSES Table 22-3e Common Failure Modes of Reciprocating, Positive-Displacement... valves used to control pumping action These valves are the most frequent source of failure In most cases, valve failure is due to fatigue The only positive way to prevent or minimize 244 Root Cause Failure Analysis T b e 19- 2 Common Failure Modes of Rohry-Type, Positive-DisplacementPumps al Source: Integrated Systems, Inc such failure is to ensure that proper maintenance is performed regularly on these components... ROTARY, POSITIVE DISPLACEMENT Table 22-2 lists the common failure modes of rotary-type, positive-displacement compressors.This type of compressor can be grouped into two types: sliding vane and rotary screw 254 Compressors Table 22-1 Common Failure Modes o j Centrqugal Compressors THE CAUSES 255 256 Root Cause Failure Analysis Table 22-2 Common Failure Modes of Rotary, Positive-DisplacementCompressors... Table 21-2 provides the more common failure modes of this type of conveyor Most of the failure modes defined in the table can be attributed directly to operating practices, changes in incoming product quality (Le., density or contamination), or maintenance practices 25 1 252 Root Cause Failure Analysis Table 21-1 Common Failure Modes of Pneumatic Conveyors I THE CAUSES Piping Configuratkn Unsuitable... 20-2 (also see Tables 19- 2 and 22-2), lists the failure modes that most often affect blowers and fluidizers In particular, blower fail- Fans, Blowers, and Fluidizers 2 49 Table 20-2 Common Failure Modes of Blowers and Fluidizers THE PI P - Source: Integrated Systems, Inc ures occur due to process instability, caused by stadstop operation and demand variations, and mechanical failures due to close tolerances... However, they still are subject to a variety of common failure modes caused directly or indirectly by the process ROt8V TVpe Rotary-type, positive-displacement pumps share many failure modes with centrifugal pumps Both types of pumps are subject to process-induced failure caused by demands that exceed the pump’s capabilities Process-induced failure also is caused by operating methods that either result in... Conversely, blowers that operate constantly in a stable environment rarely exhibit problems The major reason is the severe axial thrusting caused by the frequent variations in suction or discharge pressure caused by the stadstop operation 250 Root Cause Failure Analysis Demand Variations Variations in pressure and volume demands have a serious impact on blower reliability Since blowers are positive-displacement... 1XLCyde Magnetic Hum Source: Integrated Systems, Inc I I I@ 1 e e 247 248 Root Cause Failure Analysis its critical speed, the point where the phenomenon referred to as resonance occurs This occurs because the additional mass affects the rotor’s natural frequency Even if the fan’s speed does not change, the change in natural frequency may cause its critical speed (note that machines may have more than one)... Valve failure is the dominant failure mode for reciprocating compressors Because of their high cyclic rate, which exceeds 80 million cycles per year, inlet and discharge valves tend to work harden and crack Lubrication System Poor maintenance of lubrication-systemcomponents, such as filters and strainers, typically causes premature failure Such maintenance is crucial to reciprocating compressors because... envelope or instability in the process system Table 19- 2 lists common failure modes for rotary-type, positive-displacement pumps The most common failure modes of these pumps generally are attributed to problems with the suction supply The pumps must have a constant volume of clean liquid to function properly Reciprocating Table 19- 3 lists the common failure modes for reciprocating-type, positive-displacement . entrained air or gas, and turbulent flow. 2 39 240 Root Cause Failure Analysis Table 19- 1 Common Failure Modes of Centrifugal Pumps THE CAUSES Bent Shaft Casing DktofW fmm Excessbe. frequent source of failure. In most cases, valve failure is due to fatigue. The only positive way to prevent or minimize 244 Root Cause Failure Analysis Table 19- 2 Common Failure Modes. is the severe axial thrusting caused by the frequent variations in suction or discharge pressure caused by the stadstop operation. 250 Root Cause Failure Analysis Demand Variations Variations