Root Cause Failure Analysis Part 10 pdf

262 Root Cause Failure Analysis pressure will be transmitted to any piping or machine connected to the discharge piping and located within 28 ft of the compressor. Note that, for a single-acting cylinder, the wavelength will be twice as long. Imbalance Compressor inertial forces may have two effects on the operating dynamics of a reciprocating compressor, affecting its balance characteristics. The first is a force in the direc- tion of the piston movement, which is displayed as impacts in a vibration profile as the piston reaches top and bottom dead-center of its stroke. The second effect is a couple, or moment, caused by an offset between the axes of two or more pistons on a common crankshaft. The interrelationship and magnitude of these two effects depend on such factors as number of cranks, longitudinal and angular arrangement, cylinder arrangement, and amount of counterbalancing possible. Two significant vibration periods result, the primary at the compressor’s rotation speed (X) and the secondary at 2X. Although the forces developed are sinusoidal, only the maximum (Le., the amplitude) is considered in the analysis. Figure 22-1 shows relative values of the inertial forces for various compressor arrangements. Compressors 263 Figure 22-1 Unbalanced inertial forces and couples for various reciprocating compressors (Gibbs 1971). 23 MIXERS AND AGITATORS Table 23-1 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. 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 also will cause a loss of efficiency and accelerated component wear. Variations in the product’s physical properties, such as viscosity, 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 expected to be encountered, applications sometimes change. Such a change may result in the use of improper equipment for a particular application. 264 Mixers and Agitators 265 Table 23-1 Common Failure Modes of Mixers and Agitators Source: Integrated Systems, Inc. DUST COLLECTORS This chapter identifies common problems and their causes for baghouse and cyclonic separator dust-collection systems. BAGHOUSES Table 24-1 lists the common failure modes for baghouses. This guide may be used for all such units that use fabric filter bags as the primary dust-collection media. CYCLONIC SEPARATORS Table 24-2 identifies the failure modes and their causes for cyclonic separators. Since cyclonic separators have no moving parts, most of the problems associated with this type of system can be attributed to variations in process parameters, such as flow rate, dust load, dust composition (Le., density, size, etc.), and ambient conditions (Le., temperature, humidity, etc.). 266 Dust Collectors 267 Table 24-1 Common Failure Modes of Baghouses r THE CAUSES Bag MBterial incam+ Fa Application Bag Plugged Beg Tom or lmpmperfy lnotalled IBlow-danl Time FaW or Damaged BWw Nonkrs Pluggad loHcdoy~l Pibt Valve Fdkd To Open (Sobnoid FaUm) Source: Integrated Systems, Inc. 268 Root Cause Failure Analysis Table 24-2 Common Failure Modes of Cyclonic Separators THE I I Source: Integrated Systems, Inc. PROCESS ROLLS Most failures that cause reliability problems with process rolls can be attributed to either improper installation or abnormal induced loads. Table 25-1 identifies the common failure modes of process rolls and their causes. Installation problems normally result from misalignment, where the roll is not per- pendicular to the travel path of the belt or transported product. If process rolls are mis- aligned, either vertically or horizontally, the load imparted by the belt or the carried product is not spread uniformly across the roll face or to the support bearings. As a result, both the roll face and bearings are subjected to abnormal wear and may fail prematurely. Operating methods may cause induced loads that are outside the acceptable design limits of the roll or its support structure. Operating variables, such as belt or strip ten- sion or tracking, may be the source of chronic reliability problems. As with misalignment, these variables apply an unequal load distribution across the roll face and bearing-support structure. These abnormal loads accelerate wear and may result in premature failure of the bearings or roll. 269 270 Root Cause Failure Analysis Table 25-1 Common Failure Modes of Process Rolls ~~ Source: Integrated Systems, Inc. 26 GEARBOXES OR REDUCERS This chapter identifies common gearbox (also called reducer) problems and their causes. Table 26-1 lists the more common gearbox failure modes. A primary cause of failure is that, with few exceptions, gear sets are designed for operation in one direc- tion only. Failure often is caused by inappropriate bidirectional operation of the gearbox or backward installation of the gear set. Unless specifically manufactured for bidirectional operation, the “nonpower” side of the gear’s teeth is not finished. There- fore, this side is rougher and does not provide the same tolerance as the finished “power” side. Note that it has become standard practice in some plants to reverse the pinion or bullgear in an effort to extend the gear set’s useful life. While this practice permits longer operation times, the torsional power generated by a reversed gear set is not as uniform and consistent as when the gears are properly installed. Gear overload is another leading cause of failure. In some instances, the overload is constant, which is an indication that the gearbox is not suitable for the application. In other cases, the overload is intermittent and occurs only when the speed changes or specific production demands cause a momentary spike in the torsional load require- ment of the gearbox. Misalignment, both real and induced, is another primary root cause of gear failure. The only way to assure that gears are properly aligned is to hard blue the gears imme- diately following installation. After the gears have run for a short time, their wear pattern should be visually inspected. If the pattern does not conform to vendor’s specifications, the alignment should be adjusted. 27 1 [...]... Common Failure Modes of Inverters Source:Integrated Systems, Inc 279 CONTROL VALVES Although there are limited common control valve failure modes, the dominant problems are usually related to leakage, speed of operation, or complete valve failure Table 29-1 lists the more common causes of these failures Special attention should be given to the valve actuator when conducting a root cause failure analysis. .. failure modes for the gearbox Abrasion Abrasion creates unique wear patterns on the teeth The pattern varies, depending on the type of abrasion and its specific forcing function Figure 2 6 2 illustrates severe abrasive wear caused by particulates in the lubricating oil Note the score marks that run from the root to the tip of the gear teeth Figure 2 6 1 Normal wear pattern 274 Root Cause Failure Analysis. .. process-control valve is jammed open or closed, it may cause failure of the valve mechanism This overtorquing of the valve’s sealing device may cause damage or failure of the seal, or it may freeze the valve stem Either failure mode results in total valve failure 280 Control Valves Table 29-1 Common Failure Modes of Control Valves I I E PROBLEM > Q) THE CAUSES 0 3 a Dirt/Debris Trapped In Valve Seat Excessive... root cause failure analysis The wear pattern or surface finish of parts may provide clues that will help the investigator resolve a problem Table 3 1-1 provides a guide to the meaning of these patterns Bolting practices and bolted joints may be a contributor to equipment reliability problems Table 3 1-2 provides troubleshooting guidelines for this common machine-train component 285 286 Root Cause Failure. .. Personal protection equipment PSM Process safety management PV Pressure-volume PVT Pressure-volume-temperature RCFA Root cause failure analysis RCRA Resource Conservation and Recovery Act rpm Rotating speed rpm? Rotating speed, squared rpm3 Rotating speed, cubed 290 Root Cause Failure Analysis SARA Superfund Amendments and Reauthorization SE Static efficiency SERC State Emergency Response Committee... Wearpattern caused by abrasives in lubricating oil Chemical Attack or Corrosion Water and other foreign substances in the lubricating oil supply also cause gear degradation and premature failure Figure 26-3 illustrates a typical wear pattern on gears caused by this failure mode Figure 26-3 Pattern caused by corrosive attack on gear teeth Gearboxes or Reducers 275 Figure 2 6 4 Pitting caused by gear... component 285 286 Root Cause Failure Analysis Table 31-1 Common Failure Modes of Wear Part Surfaces Source: Integrated Systems, Inc Others Table 31-2 Common Failure Modes o Bolted Joints f = 8 * 5 BoWJdnt SWmess Ratlo Damplng in Joint ~irectlonofBoltAxisRelathretoVibratlonAds Fillet S i and Shape Galling lmpmper Heat Treatment NutDilation PartsFlnlsh PoorFit Radius of T h m d Roots RelaxabbnEffeds Thread... operating parameters or improper maintenance Table 27-1 lists the more common causes of steam trap failures Operation outside the trap’s design envelope results in loss of efficiency and may cause premature failure In many cases, changes in the condensate load, steam pressure or temperature, and other related parameters are the root cause of poor performance or reliability problems Careful attention should...272 Root Cause Failure Analysis Table 2 6 1 Common Failure Modes of Gearboxes and Gear Sets d 0 0 0 ~ 0 0 0 0 I Source: Integrated Systems, Inc Poor maintenance practices are the primary source of real misalignment problems Proper... probability of a mechanical seal failure is extremely high Most seal tolerances are limited to no more than 0.002 in of total shaft deflection or misalignment Any deviation outside this limited range will cause catastrophic seal failure Misalignment Physical misalignment of a shaft will either cause seal damage and permit some leakage through the seal or it will result in total seal failure Therefore, it is . Inc. 268 Root Cause Failure Analysis Table 24-2 Common Failure Modes of Cyclonic Separators THE I I Source: Integrated Systems, Inc. PROCESS ROLLS Most failures that cause reliability. wear caused by particulates in the lubricating oil. Note the score marks that run from the root to the tip of the gear teeth. Figure 261 Normal wear pattern. 274 Root Cause Failure Analysis. loads accelerate wear and may result in premature failure of the bearings or roll. 269 270 Root Cause Failure Analysis Table 25-1 Common Failure Modes of Process Rolls ~~ Source: Integrated