compressors because they rely on the lubrication system to provide a uniform oil film between closely fitting parts (e.g., piston rings and the cylinder wall). Partial or com- plete failure of the lube system results in catastrophic failure of the compressor. Pulsation Reciprocating compressors generate pulses of compressed air or gas that are dis- charged into the piping that transports the air or gas to its point(s) of use. This pulsa- tion often generates resonance in the piping system, and pulse impact (i.e., standing waves) can severely damage other machinery connected to the compressed-air system. Although this behavior does not cause the compressor to fail, it must be prevented to protect other plant equipment. Note, however, that most compressed-air systems do not use pulsation dampers. Process Parameters 237 Table 10–10c Common Failure Modes of Reciprocating Compressors THE PROBLEM THE CAUSES Location Too Humid and Damp ᭹ Low Oil Pressure Relay Open ᭹ Lubrication Inadequate ᭹᭹᭹ ᭹᭹᭹᭹᭹ Motor Overload Relay Tripped ᭹ Motor Rotor Loose on Shaft ᭹᭹ Motor Too Small ᭹᭹ New Valve on Worn Seat ᭹ “Off” Time Insufficient ᭹᭹ ᭹ Oil Feed Excessive ᭹᭹ ᭹ ᭹ Oil Filter or Strainer Clogged ᭹ Oil Level Too High ᭹᭹ ᭹᭹ ᭹ Oil Level Too Low ᭹᭹ Oil Relief Valve Defective ᭹ Oil Viscosity Incorrect ᭹ ᭹᭹᭹ ᭹᭹ ᭹᭹ ᭹ Oil Wrong Type ᭹ Packing Rings Worn, Stuck, Broken ᭹ Piping Improperly Supported ᭹ Piston or Piston Nut Loose ᭹ Piston or Ring Drain Hole Clogged ᭹ Piston Ring Gaps Not Staggered ᭹ Piston Rings Worn, Broken, or Stuck ᭹᭹ ᭹᭹ ᭹᭹᭹ ᭹H ᭹L ᭹H ᭹L ᭹᭹ ᭹H ᭹H Piston-to-Head Clearance Too Small ᭹ Air Discharge Temperature Above Normal Carbonaceous Deposits Abnormal Compressor Fails to Start Compressor Fails to Unioad Compressor Noisy or Knocks Compressor Parts Overheat Crankcase Oil Pressure Low Crankcase Water Accumulation Delivery Less Than Rated Capacity Discharge Pressure Below Normal Excessive Compressor Vibration Intercooler Pressure Above Normal Intercooler Pressure Below Normal Intercooler Safety Valve Pops Motor Over-Heating Oil Pumping Excessive (Single-Acting Compressor) Operating Cycle Abnormality Long Outlet Water Temperature Above Normal Piston Ring, Piston, Cylinder Wear Excessive Piston Rod or Packing Wear Excessive Receiver Pressure Above Normal Receiver Safety Valve Pops Starts Too Often Valve Wear and Breakage Normal Each time the compressor discharges compressed air, the air tends to act like a com- pression spring. Because it rapidly expands to fill the discharge piping’s available volume, the pulse of high-pressure air can cause serious damage. The pulsation wave- length, l, from a compressor with a double-acting piston design can be determined by: Where: l = Wavelength, feet a = Speed of sound = 1,135 feet/second n = Compressor speed, revolutions/minute l == 60 2 34 050a nn , 238 An Introduction to Predictive Maintenance Table 10–10d Common Failure Modes of Reciprocating Compressors THE PROBLEM THE CAUSES Pulley or Flywheel Loose ᭹᭹ Receiver, Drain More Often ᭹ Receiver Too Small ᭹ Regulation Piping Clogged ᭹ Resonant Pulsation (Inlet or Discharge) ᭹᭹᭹᭹ ᭹ Rod Packing Leaks ᭹᭹᭹᭹᭹ Rod Packing Too Tight ᭹ Rod Scored, Pitted, Worn ᭹ Rotation Wrong ᭹᭹᭹ Runs Too Little (2) ᭹ Safety Valve Defective ᭹᭹ ᭹ Safety Valve Leeks ᭹᭹᭹᭹᭹᭹ Safety Valve Set Too Low ᭹᭹ Speed Demands Exceed Rating ᭹ Speed Lower Than Rating ᭹᭹ Speed Too High ᭹᭹ ᭹ ᭹ ᭹ ᭹ Springs Broken ᭹ System Demand Exceeds Rating ᭹᭹᭹᭹᭹᭹᭹ System Leakage Excessive ᭹᭹᭹᭹᭹᭹᭹ ᭹ Tank Ringing Noise ᭹ Unloader Running Time Too Long (1) ᭹ Unloader or Control Defective ᭹᭹᭹᭹᭹᭹ ᭹᭹᭹᭹᭹᭹᭹ ᭹ ᭹᭹᭹᭹᭹᭹ Air Discharge Temperature Above Normal Carbonaceous Deposits Abnormal Compressor Fails to Start Compressor Fails to Unload Compressor Noisy or Knocks Compressor Parts Overheat Crankcase Oil Pressure Low Crankcase Water Accumulation Delivery Less Than Rated Capacity Discharge Pressure Below Normal Excessive Compressor Vibration Intercooler Pressure Above Normal Intercooler Pressure Below Normal Intercooler Safety Valve Pops Motor Over-Heating Oil Pumping Excessive (Single-Acting Compressor) Operating Cycle Abnormally Long Outlet Water Temperature Above Normal Piston Ring, Piston, Cylinder Wear Excessive Piston Rod or Packing Wear Excessive Receiver Pressure Above Normal Receiver Safety Valve Pops Starts Too Often Valve Wear and Breakage Normal For a double-acting piston design, a compressor running at 1,200 revolutions per minute (rpm) will generate a standing wave of 28.4 feet. In other words, a shock load equivalent to the discharge pressure will be transmitted to any piping or machine connected to the discharge piping and located within 28 feet 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 rec- iprocating compressor, affecting its balance characteristics. The first effect is a force in the direction 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 Process Parameters 239 Table 10–10e Common Failure Modes of Reciprocating Compressors THE PROBLEM THE CAUSES Unloader Parts Worn or Dirty ᭹ Unloader Setting Incorrect ᭹᭹᭹᭹᭹ ᭹᭹᭹᭹᭹᭹᭹᭹᭹᭹᭹᭹ V-Belt or Other Misalignment ᭹᭹ ᭹ Valves Dirty ᭹᭹ ᭹ ᭹᭹ ᭹ Valves Incorrectly Located ᭹᭹ ᭹᭹ ᭹᭹ ᭹H ᭹L ᭹H ᭹L ᭹᭹H ᭹H Valves Not Seated in Cylinder ᭹᭹ ᭹᭹ ᭹᭹ ᭹H ᭹L ᭹H ᭹L ᭹᭹H ᭹H Valves Worn or Broken ᭹᭹ ᭹᭹ ᭹᭹ ᭹H ᭹L ᭹H ᭹L ᭹H ᭹H ᭹H ᭹H Ventilation Poor ᭹᭹ ᭹ ᭹ Voltage Abnormally Low ᭹᭹ Water Inlet Temperature Too High ᭹᭹ ᭹᭹᭹ ᭹ Water Jacket or Cooler Dirty ᭹᭹ Water Jackets or Intercooler Dirty ᭹᭹᭹ Water Quantity Insufficient ᭹ ᭹᭹᭹ ᭹ Wiring Incorrect ᭹ Worn Valve on Good Seat ᭹ Wrong Oil Type ᭹ ᭹᭹ (1) Use Automatic Start/Stop Control (2) Use Constant Speed Control (3) Change to Non-Detergent Oil H (in High Pressure Cylinder) L (in Low Pressure Cylinder) Air Discharge Temperature Above Normal Carbonaceous Deposits Abnormal Compressor Fails to Start Compressor Fails to Unload Compressor Noisy or Knocks Compressor Parts Overheat Crankcase Oil Pressure Low Crankcase Water Accumulation Delivery Less Than Rated Capacity Discharge Pressure Below Normal Excessive Compressor Vibration Intercooler Pressure Above Normal Intercooler Pressure Below Normal Intercooler Safety Valve Pops Motor Over-Heating Oil Pumping Excessive (Single-Acting Compressor) Operating Cycle Abnormally Long Outlet Water Temperature Above Normal Piston Ring, Piston, Cylinder Wear Excessive Piston Rod or Packing Wear Excessive Receiver Pressure Above Normal Receiver Safety Valve Pops Starts Too Often Valve Wear and Breakage Normal 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 (i.e., the amplitude) is considered in the analysis. Figure 10–1 shows relative values of the inertial forces for various compressor arrangements. 10.5. MIXERS AND AGITATORS Table 10–11 identifies common failure modes and their causes for mixers and agita- tors. 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 causes a loss of efficiency and accelerated component wear. Variations in the product’s physical properties, such as viscosity, also cause loss of mixing efficiency and premature wear of mixer components. Although the initial selec- tion 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. 10.6 DUST COLLECTORS This section identifies common problems and their causes for baghouse and cyclonic separator dust-collection systems. 10.6.1 Baghouses Table 10–12 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. 10.6.2 Cyclonic Separators Table 10–13 identifies the failure modes and their causes for cyclonic separators. Because there are no moving parts within a cyclone, most of the problems associated with this type of system can be attributed to variations in process parameters, such as flowrate, dust load, dust composition (e.g., density, size), and ambient conditions (e.g., temperature, humidity). 240 An Introduction to Predictive Maintenance 10.7 PROCESS ROLLS Most of the failures that cause reliability problems with process rolls can be attrib- uted to either improper installation or abnormal induced loads. Table 10–14 identifies the common failure modes of process rolls and their causes. Process Parameters 241 Figure 10–1 Unbalanced inertial forces and couples for various reciprocating compressors. Installation problems are normally the result of misalignment where the roll is not perpendicular to the travel path of the belt or transported product. If process rolls are misaligned, either vertically or horizontally, the load imparted by the belt or carried product is not uniformly spread 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 prematurely fail. 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 tension or tracking, may be the source of chronic reliability problems. As with mis- alignment, 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. 10.8 GEARBOXES/REDUCERS This section identifies common gearbox (also called a reducer) problems and their causes. Table 10–15 lists the more common gearbox failure modes. One of the primary causes of failure is the fact that, with few exceptions, gear sets are designed for oper- 242 An Introduction to Predictive Maintenance Table 10–11 Common Failure Modes of Mixers And Agitators THE PROBLEM Surface Vortex Visible Incomplete Mixing of Product Excessive Vibration Excessive Wear Motor Overheats Excessive Power Demand Excessive Bearing Failures THE CAUSES 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 ᭹᭹᭹ Source: Integrated Systems, Inc. ation in one direction only. Failure is often 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. Therefore, this side is rougher and does not provide the same tolerance as the finished “power” side. Process Parameters 243 Table 10–12 Common Failure Modes of Baghouses THE PROBLEM Continuous Release of Dust-Laden Air Intermittent Release of Dust-Laden Air Loss of Plant Air Pressure Blow-Down Ineffective Insufficient Capacity Excessive Differential Pressure Fan/Blower Motor Trips Fan Has High Vibration Premature Bag Failures Differential Pressure Too Low Chronic Plugging of Bags THE CAUSES Bag Material Incompatible for Application ᭹᭹ Bag Plugged ᭹᭹᭹ Bag Torn or Improperly Installed ᭹ ᭹᭹᭹ Baghouse Undersized ᭹᭹ ᭹ Blow-Down Cycle Interval Too Long ᭹᭹ Blow-Down Cycle Time Failed or Damaged ᭹᭹ Blow-Down Nozzles Plugged ᭹ Blow-Down Pilot Valve Failed to Open (Solenoid Failure) ᭹᭹ Dust Load Exceeds Capacity ᭹ Excessive Demand ᭹ Fan/Blower Not Operating Properly ᭹ Improper or Inadequate Lubrication ᭹ Leaks in Ductwork or Baghouse ᭹᭹ Misalignment of Fan and Motor ᭹ Moisture Content Too High ᭹ Not Enough Blow-Down Air (Pressure and Volume) ᭹᭹ ᭹ Not Enough Dust Layer on Filter Bags ᭹᭹ ᭹ ᭹ Piping/Valve Leaks ᭹ Plate-Out (Dust Build-up on Fan’s Rotor) ᭹ Plenum Cracked or Seal Defective ᭹᭹ ᭹ Rotor Imbalanced ᭹ Ruptured Blow-Down Diaphrams ᭹᭹ ᭹ Suction Ductwork Blocked or Plugged ᭹ Source: Integrated Systems, Inc. Note that it has become standard practice in some plants to reverse the pinion or bull- gear in an effort to extend the gear set’s useful life. Although 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 when specific production demands cause a momentary spike in the torsional load requirement of the gearbox. Misalignment, both real and induced, is also a primary root-cause of gear failure. The only way to ensure that gears are properly aligned is to hard blue the gears immedi- 244 An Introduction to Predictive Maintenance Table 10–13 Common Failure Modes of Cyclonic Separators THE PROBLEM Continuous Release of Dust-Laden Air Intermittent Release of Dust-Laden Air Cyclone Plugs in Inlet Chamber Cyclone Plugs in Dust Removal Section Rotor-Lock Valve Fails to Turn Excessive Differential Pressure Differential Pressure Too Low Rotor-Lock Valve Leaks Fan Has High Vibration THE CAUSES Clearance Set Wrong ᭹ Density and Size Distribution of Dust Too High ᭹᭹᭹ ᭹ Density and Size Distribution of Dust Too Low ᭹᭹ Dust Load Exceeds Capacity ᭹᭹ ᭹ ᭹ Excessive Moisture in Incoming Air ᭹ Foreign Object Lodged in Valve ᭹ Improper Drive-Train Adjustments ᭹ Improper Lubrication ᭹ Incoming Air Velocity Too High ᭹ Incoming Air Velocity Too Low ᭹᭹᭹ ᭹ Internal Wear or Damage ᭹ Large Contaminates in Incoming Air Stream ᭹᭹ Prime Mover (Fan, Blower) Malfunctioning ᭹᭹ ᭹᭹ ᭹ Rotor-Lock Valve Turning Too Slow ᭹᭹ ᭹ Seals Damaged ᭹ Source: Integrated Systems, Inc. ately after 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 specifica- tions, alignment should be adjusted. Poor maintenance practices are the primary source of real misalignment problems. Proper alignment of gear sets, especially large ones, is not an easy task. Gearbox man- ufacturers do not provide an easy, positive means to ensure that shafts are parallel and that the proper center-to-center distance is maintained. Induced misalignment is also a common problem with gear drives. Most gearboxes are used to drive other system components, such as bridle or process rolls. If mis- alignment is present in the driven members (either real or process induced), it will also directly affect the gears. The change in load zone caused by the misaligned driven component will induce misalignment in the gear set. The effect is identical to real misalignment within the gearbox or between the gearbox and mated (i.e., driver and driven) components. Visual inspection of gears provides a positive means to isolate the potential root-cause of gear damage or failures. The wear pattern or deformation of gear teeth provides clues about the most likely forcing function or cause. The following sections discuss the clues that can be obtained from visual inspection. Process Parameters 245 Frequent Bearing Failures Abnormal Roll Face Wear Roll Neck Damage or Failure Abnormal Product Tracking Motor Overheats Excessive Power Demand High Vibration Product Quality Poor Table 10–14 Common Failure Modes of Process Rolls THE PROBLEM THE CAUSES Defective or Damaged Roll Bearings ᭹ Excessive Product Tension ᭹᭹᭹᭹᭹᭹ ᭹ Excessive Load ᭹᭹ Misaligned Roll ᭹᭹᭹᭹᭹᭹᭹᭹ Poor Roll Grinding Practices ᭹ Product Tension Too Loose ᭹ Product Tension/Tracking Problem ᭹᭹ ᭹ Roll Face Damage ᭹᭹᭹ ᭹ Speed Coincides with Roll’s Natural Frequency ᭹᭹᭹᭹ Speed Coincides with Structural Natural Frequency ᭹᭹ ᭹᭹ Source: Integrated Systems, Inc. 10.8.1 Normal Wear Figure 10–2 illustrates a gear that has a normal wear pattern. Note that the entire surface of each tooth is uniformly smooth above and below the pitch line. 10.8.2 Abnormal Wear Figures 10–3 through 10–5 illustrate common abnormal wear patterns found in gear sets. Each of these wear patterns suggests one or more potential failure modes for the gearbox. 246 An Introduction to Predictive Maintenance Table 10–15 Common Failure Modes of Gearboxes and Gear Sets THE PROBLEM Gear Failures Variations in Torsional Power Insufficient Power Output Overheated Bearings Short Bearing Life Overload on Driver High Vibration High Noise Levels Motor Trips THE CAUSES Bent Shaft ᭹᭹᭹᭹ Broken or Loose Bolts or Setscrews ᭹᭹ Damaged Motor ᭹᭹ ᭹ Eliptical Gears ᭹᭹ ᭹᭹ Exceeds Motor’s Brake Horsepower Rating ᭹᭹ Excessive or Too Little Backlash ᭹᭹ Excessive Torsional Loading ᭹᭹᭹᭹᭹᭹ ᭹ Foreign Object in Gearbox ᭹᭹᭹᭹ Gear Set Not Suitable for Application ᭹᭹ ᭹᭹ Gears Mounted Backward on Shafts ᭹᭹᭹ Incorrect Center-to-Center Distance Between Shafts ᭹᭹ Incorrect Direction of Rotation ᭹᭹᭹ Lack of or Improper Lubrication ᭹᭹ ᭹᭹ ᭹᭹᭹ Misalignment of Gears or Gearbox ᭹᭹ ᭹᭹ ᭹᭹ Overload ᭹᭹᭹᭹᭹ Process Induced Misalignment ᭹᭹ ᭹᭹ Unstable Foundation ᭹᭹ ᭹᭹ Water or Chemicals in Gearbox ᭹ Worn Bearings ᭹᭹ Worn Coupling ᭹ Source: Integrated Systems, Inc. [...]... viewed locations and allow people to make dual use of their time 12. 1 .2 Sensors Because humans are not continually alert or sensitive to small changes and cannot get inside small spaces, especially when operating, it is necessary to use sensors that 26 2 An Introduction to Predictive Maintenance Figure 12 1 “Go/no-go” standards Figure 12 2 Accelerometer to measure vibration of rotating shaft measure conditions... program can greatly improve benefits to the company Predictive maintenance technologies can, and should, be used as a total-plant performance tool When used correctly, these tools can provide the means to eliminate most of the factors that limit plant performance To achieve this expanded role, the predic267 26 8 An Introduction to Predictive Maintenance tive maintenance program must be developed with clear... probability of a mechanical seal failure is extremely high Most seal tolerances are limited to no more than 0.0 02 inches of total shaft deflection or misalignment Any deviation outside of this limited range will cause catastrophic seal failure 25 2 An Introduction to Predictive Maintenance Table 10– 18 Common Failure Modes of Control Valves Galling ᭹ ᭹ ᭹ Mechanical Damage ᭹ ᭹ ᭹ ᭹ Not Packed Properly ᭹ Packed... ultrasonics to monitor bearing condition is not recommended  12 VISUAL INSPECTION Regular visual inspection of the machinery and systems in a plant is a necessary part of any predictive maintenance program In many cases, visual inspection will detect potential problems that will be missed using the other predictive maintenance techniques Even with the predictive techniques discussed, many potentially serious... and not be limited to the maintenance function Every function within the plant affects equipment reliability and performance, and the predictive maintenance program must address all of these influences Vibration monitoring and analysis is the most common of the predictive maintenance technologies It is also the most underutilized of these tools Most vibration-based predictive maintenance programs use... The distinction between operational controls and maintenance controls is not important because the result is a reduced Visual Inspection 26 5 Figure 12 4 Control chart warning of possible failure before it occurs Figure 12 5 A simple manometer to warn of inadequate airflow need for maintenance and notification that a problem is building up to a point where maintenance should be scheduled when convenient... proportional electrical signal into user-selected engineering units, they are in fact multimeters that can be used as part of a comprehensive process performance analysis program 13.1 .2 Limitation to Maintenance Issues From its inception, predictive maintenance has been perceived as a maintenance improvement tool Its sole purpose was, and is, to prevent catastrophic failure of plant equipment Although... influences The predictive maintenance program should evaluate existing operating practices; quantify their impact on equipment reliability, effectiveness, and costs; and provide recommended modifications to these practices that will improve overall performance of the production system 13.1.4 Training Limitations In general, predictive maintenance analysts receive between 5 and 25 days of training as part of... Testing Ultrasonics has been, and continues to be, a primary test methodology for materials testing Typical test frequencies start at 25 0 kiloHertz (kHz), or 25 0,000 cycles per second (cps), up to 25 MegaHertz (MHz), or 25 million cps Testing materials generally consist of introducing an energy source into the material to be tested and recording the response characteristics using ultrasonic instruments... enables them to do a very good job of predicting when maintenance should be done The aircraft maintenance techniques that required complete teardown of propeller-driven aircraft every 1,000 hours, whether they needed it or not, are rapidly vanishing in that industry Many manufacturing plants can gain improvements through the same maintenance techniques 12. 2 THRESHOLDS Now that instrumentation is becoming . sound = 1,135 feet/second n = Compressor speed, revolutions/minute l == 60 2 34 050a nn , 23 8 An Introduction to Predictive Maintenance Table 10–10d Common Failure Modes of Reciprocating Compressors THE. be followed for all shafts that have an installed mechanical seal. 25 2 An Introduction to Predictive Maintenance Table 10– 18 Common Failure Modes of Control Valves THE PROBLEM Valve Fails to. can also help identify specific forcing functions or root-causes of gear failure. 24 8 An Introduction to Predictive Maintenance Figure 10–4 Pattern caused by corrosive attack on gear teeth. Figure