Intro Predictive Maintenance 2 Part 8 pps

dis-Table 10–10c Common Failure Modes of Reciprocating Compressors THE PROBLEM THE CAUSES Discharge Pressure Below Normal Excessive Compressor V... Notethat, for a single-acting cylinder

compressors because they rely on the lubrication system to provide a uniform oil filmbetween 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 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., standingwaves) 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 toprotect other plant equipment Note, however, that most compressed-air systems donot use pulsation dampers

dis-Table 10–10c Common Failure Modes of Reciprocating Compressors

THE PROBLEM

THE CAUSES

Discharge Pressure Below Normal Excessive Compressor V

Each time the compressor discharges compressed air, the air tends to act like a pression spring Because it rapidly expands to fill the discharge piping’s availablevolume, 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:

com-Where:

l = Wavelength, feet

a= Speed of sound = 1,135 feet/second

n= Compressor speed, revolutions/minute

l =60 =2

Discharge Pressure Below Normal Excessive Compressor V

For a double-acting piston design, a compressor running at 1,200 revolutions perminute (rpm) will generate a standing wave of 28.4 feet In other words, a shock loadequivalent 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 Notethat, for a single-acting cylinder, the wavelength will be twice as long.

is a couple, or moment, caused by an offset between the axes of two or more pistons

Table 10–10e Common Failure Modes of Reciprocating Compressors

THE PROBLEM

THE CAUSES

(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)

Discharge Pressure Below Normal Excessive Compressor V

on a common crankshaft The interrelationship and magnitude of these two effectsdepend on such factors as number of cranks, longitudinal and angular arrangement,cylinder arrangement, and amount of counterbalancing possible Two significantvibration periods result, the primary at the compressor’s rotation speed (X) and thesecondary 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 forcesfor various compressor arrangements

10.5 M IXERS AND A GITATORS

Table 10–11 identifies common failure modes and their causes for mixers and tors Most of the problems that affect performance and reliability are caused byimproper installation or variations in the product’s physical properties

agita-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 settoo close to the side, corner, or bottom of the vessel, a stagnant zone will develop thatcauses both loss of mixing quality and premature damage to the equipment If thevanes 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 ofmixing 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 propertiesexpected to be encountered, applications sometimes change Such a change may result

in the use of improper equipment for a particular application

10.7 P ROCESS R OLLS

Most of the failures that cause reliability problems with process rolls can be uted to either improper installation or abnormal induced loads Table 10–14 identifiesthe common failure modes of process rolls and their causes

attrib-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 aremisaligned, either vertically or horizontally, the load imparted by the belt or carriedproduct is not uniformly spread across the roll face or to the support bearings As aresult, 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 designlimits of the roll or its support structure Operating variables, such as belt or striptension 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 andbearing-support structure These abnormal loads accelerate wear and may result inpremature failure of the bearings or roll

10.8 G EARBOXES /R EDUCERS

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 primarycauses of failure is the fact that, with few exceptions, gear sets are designed for oper-

Table 10–11 Common Failure Modes of Mixers And Agitators

Mixer/Agitator Setting Too Low  

Mixer/Agitator Shaft Too Long  Product Temperature Too Low    Rotating Element Imbalanced or Damaged      Speed Too High   

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 specificallymanufactured for bidirectional operation, the “nonpower” side of the gear’s teeth isnot finished Therefore, this side is rougher and does not provide the same tolerance

as the finished “power” side

Table 10–12 Common Failure Modes of Baghouses

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 gear in an effort to extend the gear set’s useful life Although this practice permitslonger operation times, the torsional power generated by a reversed gear set is not asuniform and consistent as when the gears are properly installed.

bull-Gear overload is another leading cause of failure In some instances, the overload isconstant, which is an indication that the gearbox is not suitable for the application Inother cases, the overload is intermittent and occurs only when the speed changes orwhen specific production demands cause a momentary spike in the torsional loadrequirement of the gearbox

Misalignment, both real and induced, is also a primary root-cause of gear failure Theonly way to ensure that gears are properly aligned is to hard blue the gears immedi-

Table 10–13 Common Failure Modes of Cyclonic Separators

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   

Source: Integrated Systems, Inc.

ately after installation After the gears have run for a short time, their wear patternshould 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 andthat the proper center-to-center distance is maintained

Induced misalignment is also a common problem with gear drives Most gearboxesare 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 willalso directly affect the gears The change in load zone caused by the misaligned drivencomponent 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 anddriven) 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 providesclues about the most likely forcing function or cause The following sections discussthe clues that can be obtained from visual inspection

Frequent Bearing Failures Abnormal Roll Face W

Table 10–14 Common Failure Modes of Process Rolls

Table 10 –15 Common Failure Modes of Gearboxes and Gear Sets

THE PROBLEM

Gear Failures V

Overheated Bearings Short Bearing Life Overload on Driver High V

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 

Source: Integrated Systems, Inc.

Abrasion creates unique wear patterns on the teeth The pattern varies depending onthe type of abrasion and its specific forcing function Figure 10–3 illustrates severeabrasive wear caused by particulates in the lubricating oil Note the score marks thatrun from the root to the tip of the gear teeth

Chemical Attack or Corrosion

Water and other foreign substances in the lubricating oil supply also cause gear dation and premature failure Figure 10–4 illustrates a typical wear pattern on gearscaused by this failure mode

degra-Figure 10–2 Normal wear pattern.

Figure 10–3 Wear pattern caused by abrasives in lubricating oil.

The wear patterns generated by excessive gear loading vary, but all share similar ponents Figure 10–5 illustrates pitting caused by excessive torsional loading The pitsare created by the implosion of lubricating oil Other wear patterns, such as spallingand burning, can also help identify specific forcing functions or root-causes of gearfailure

com-Figure 10–4 Pattern caused by corrosive attack on gear teeth.

Figure 10–5 Pitting caused by gear overloading.

10.9 S TEAM T RAPS

Most of the failure modes that affect steam traps can be attributed to variations inoperating parameters or improper maintenance Table 10–16 lists the more commoncauses 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 condensate load, steam pressure

or temperature, and other related parameters are the root-cause of poor performance

or reliability problems Careful attention should be given to the actual versus designsystem 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 theprimary source of steam trap problems It is important for steam traps to be routinelyinspected and repaired to ensure proper operation

10.10 I NVERTERS

Table 10–17 lists the common symptoms and causes of inverter problems Most ofthese problems can be attributed to improper selection for a particular application.Others are caused by improper operation When evaluating inverter problems, carefulattention should be given to recommendations found in the vendor’s operations andmaintenance manual These recommendations are often extremely helpful in isolatingthe true root-cause of a problem

10.11 C ONTROL V ALVES

Although there are limited common control valve failure modes, the dominant lems are usually related to leakage, speed of operation, or complete valve failure Table10–18 lists the more common causes of these failures

prob-Special attention should be given to the valve actuator when conducting a cause failure analysis Many of the problems associated with both process and fluid-power control valves are really actuator problems In particular, remotely con-trolled valves that use pneumatic, hydraulic, or electrical actuators are subject to actuator failure In many cases, these failures are the reason a valve fails to properlyopen, close, or seal Even with manually controlled valves, the true root-cause can

root-be traced to an actuator problem For example, when a manually operated 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 theseal, or it may freeze the valve stem Either of these failure modes results in total valvefailure

process-T Will Not Shut-of

Continuously Blows Steam Capacity Suddenly Falls Of

Condensate Will Not Drain Not Enough Steam Heat T Back Flow in Return Line

Table 10–16 Common Failure Modes of Steam Traps

THE PROBLEM

THE CAUSES

Back-Pressure Too High 

Boiler Foaming or Priming   Boiler Gauge Reads Low 

Bypass Open or Leaking  

Condensate Load Greater Than Design 

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.

10.12 S EALS AND P ACKING

Failure modes that affect shaft seals are normally limited to excessive leakage andpremature failure of the mechanical seal or packing Table 10–19 lists the commonfailure modes for both mechanical seals and packed boxes As the table indicates, most

of these failure modes can be directly attributed to misapplication, improper tion, or poor maintenance practices

Table 10 –17 Common Failure Modes of Inverters

THE PROBLEM

THE CAUSES

Accel/Decel Time Too Short   Acceleration Rate Too High   Ambient Temperature Too High  Control Power Source Too Low 

Cooling Fan Failure or Improper Operation  Deceleration Time Too Short   Excessive Braking Required 

Improper or Damaged Power Supply Wiring  

Improper or Damaged Wiring in Inverter-Motor 

Incorrect Line Voltage   

Main Circuit DC Voltage Too Low 

Motor Coil Resistance Too Low  

Motor Insulation Damage  

Pre-Charge Contactor Open 

Process Load Exceeds Motor Rating   Process Load Variations Exceed System Capabilities  Source: Integrated Systems, Inc.

Main Circuit Undervoltage Control Circuit Undervoltage Momentary Power Loss Overcurrent Ground Fault Overvoltage Load Short-Circuit Heat-Sink Overheat Motor/Inverter Overload Frequent Speed Deviations

Physical misalignment of a shaft will either cause seal damage or permit some leakagethrough the seal, or it will result in total seal failure Therefore, it is imperative thatgood alignment practices be followed for all shafts that have an installed mechanicalseal

Table 10 –18 Common Failure Modes of Control Valves

Not Packed Properly 

Packed Box Too Loose 

Packing Too Tight  

Threads/Lever Damaged  

Valve Stem Bound  

Valve Undersized   Dirt/Debris Trapped in Valve Seat   

Mechanical Damage (Seals, Seat)   

Pilot Port Blocked/Plugged   

Pilot Pressure Too High   Pilot Pressure Too Low    Corrosion   

Dirt/Debris Trapped in Valve Seat   

Line Pressure Too High      Mechanical Damage   

Solenoid Failure  

Solenoid Wiring Defective  

Wrong Type of Valve (N-O, N-C)  

Source: Integrated Systems, Inc.

Excessive Leakage Continuous Stream of Liquid No Leakage Shaft Hard to

Shaft Damage Under Packing Frequent Replacement Required Bellows Spring Failure Seal Face Failure

Table 10–19 Common Failure Modes of Packing and Mechanical Seals

Packing Gland Too Loose  

Packing Gland Too Tight      Cut End of Packing Not Staggered 

Line Pressure Too High 

Mechanical Damage (Seals, Seat)     Noncompatible Packing   

Packing Gland Too Loose 

Packing Gland Too Tight    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 

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.