Handbook Of Shaft Alignment Episode 3 Part 4 docx

Motor A to drive shaft− 80 W Sag compensated readings − 170 Roll A to drive shaft +160 Sag compensated readings − 80 Motor B to drive shaft Roll B to drive shaft +90 − 31 +55 Drive roll

42 in 16 in.

22 in 70 in.

Motor B

Drive roll B Idler roll

View looking east

FIGURE 20.32 Side view of rolls and the drive motors

Idler roll

East North

FIGURE 20.33 Top view of rolls and the drive motors

Motor A to drive shaft

− 80 W

Sag compensated readings

− 170

Roll A to drive shaft

+160

Sag compensated readings − 80

Motor B to drive shaft Roll B to drive shaft

+90

− 31 +55

Drive roll B Idler roll

25 mils to the east Reference

North target South target

North target South target Shaft alignment information Roll alignment information

“Front side” face readings Face reading diameter = 10 in.

Motor boundary condition information Roll boundary condition information

Drive roll B Idler roll

21 Alignment Considerations for Specific Types of Machinery

Up to this point, fairly broad generalizations have been made about rotating machinery Theshaft alignment measurement methods and tools discussed in Chapter 10 through Chapter 15did not mention any specific type of machinery that the methods should be used on.Intentionally so, since most of these methods can be used on any type of machinery regardless

of their specific function The rotating equipment could have been electric motors, steamturbines, pumps, fans, compressors, or whatever It really does not matter The examples ofgraphing and modeling techniques covered in Chapter 8 through Chapter 20 had specificmachine names on the diagrams, but for all practical purposes, the names of the machinerywere not relevant, only the graphing concepts The OL2R methods discussed in Chapter 16should work on virtually any type of machinery but certain OL2R methods are better suitedfor certain situations than some of the others mentioned

This is not to suggest however that a wide variety of rotating machinery behaves the sameway or should be aligned the same way It is important to know a considerable amount ofinformation about each piece of machinery before, during, and after the alignment process.Knowledge about how it works internally, what operational function does it perform, howdoes the process affect its operational performance, and how it interacts with its frame andfoundation, and influence from external connections such as piping are important in thor-ough understanding of the behavior of the machine The example in Appendix A illustratesthe type of information that should be kept on each piece of rotating machinery

This chapter will explore some of the specific information relating to alignment on commonrotating machinery equipment Much of the information contained herein is based on actualfield measurements but should not be construed as hard and fast rules Always consult themanufacturer of the specific type of rotating machinery for information pertaining to yourparticular machine But above all, do your own investigative analysis and learn the behavior

of your machines

Most of the OL2R movement ranges indicated in each machine category are based onactual field measurements These data indicate how the centerline of rotation of the shaftmight move from OL2R (or how the inboard and outboard bearing positions change fromOL2R and vice versa) The OL2R movement amounts reflect average ranges of motion; inother words, the high end values could, and in many cases, have been in excess of the numbersindicated, sometimes by a factor of 300% and up Again, most manufacturers of rotatingmachinery do not conduct OL2R measurements at a customer’s plant site If you consult theoriginal equipment manufacturer for OL2R information, ask how the measurements weretaken, what the environmental and operational conditions were during the test, and why theyfeel these data would be indicative of the machinery you have in operation

The following information is an attempt to give you an overview of what to consider wheninstalling and aligning these different types of machinery It is therefore recommended that

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this information be used strictly as a starting point for you to initially align new rotatingmachinery systems to allow you the opportunity to safely conduct field studies on yourrotating equipment.

21.1 DRIVERS

Electric motors (AC induction or synchronous or DC motors) are perhaps the best behavedtypes of rotating machinery from an alignment standpoint Electric motors up to 500 hpfrequently are outfitted today with antifriction-type bearings so they do not pose any majorinstallation problems assuming they are mechanically and electrically sound (Figure 21.1).Medium to large electric motors are frequently outfitted with sliding-type bearings(Figure 21.3) When power is applied to motors where the armatures are supported insliding-type bearings, the electromagnetic field wants to center the rotor with respect to thestator field This phenomenon is often referred to as ‘‘magnetic center’’ and needs to be takeninto consideration for proper shaft-to-shaft spacing Many electric motors have no thrustbearing per se and rely on electromagnetic forces to center the rotor To find magnetic center,uncouple the motor and run it ‘‘solo.’’ Once the field is applied, you may notice the shaft

‘‘hunting’’ back and forth axially for a short period of time and then it will typically settle out

at one specific axial position Very carefully scribe a line on the rotating shaft with a felt tippen or soapstone near a stationary reference fixture such as the bearing seal Keep yourfingers away from keys or keyways and do not let any loose clothing, tools, rags, or otherstationary objects attached to your body or near you hit the shaft Drop the field (i.e., shut themotor off) and let the rotor stop completely It is unlikely that the shaft’s axial position isdirectly on magnetic center so after safety tagging the breaker, hand rotate the shaft pushing

or pulling it axially until the scribe mark you made lines back up with the stationary referencefixture you picked Now measure and set the shaft-to-shaft distance with the machine it isdriving

FIGURE 21.1 Typical small electric motor

If misalignment conditions are severe enough on electric motors and the shaft or armatureelastically bends a sufficient amount, the rotor to stator air gap can get out of tolerance (theaccepted tolerance for air gap eccentricity differential is +10% of the total air gap) From avibration standpoint, eccentric air gap problems will frequently exhibit a spectral peak attwice line frequency (120 Hz in North America refer to Section 2.2.3).

Typical OL2R movement range of electric motors (horizontally mounted):

Vertical movement: 1 to 5 mils upward (5 to 200 hp); 3 to 30þ mils upward (200þ hp),

typically symmetrical (i.e., inboard and outboard ends move up the same amount)Lateral (sideways) movement: 0 to 4 mils (usually much less than vertical movement)Axial movement: 5 to 10 mils (5 to 200 hp); 8 to 50þ mils (200þ hp)

FIGURE 21.2 Medium size electric motor

FIGURE 21.3 Large electric motor supported in sliding bearings

21.1.1.1 Additional Information on Electric Motors

Moderate to excessive soft foot conditions have been experienced on virtually every sizemotor regardless of frame construction design Uneven air gap problems found occasionallydue to improper positioning of end bells or housing distortion due to uncorrected soft foot.Inboard (coupling end) bearings may run hotter due to misalignment conditions Excessivevibration may be due to improperly bored coupling hubs Infrared thermography surveys andmotor current signature analysis are very helpful in diagnosing problems

Steam turbines can range in output from 20 to 100,000þ hp with speeds up to 25,000þ rpm

and therefore become some of the more interesting equipment for OL2R surveys andconsequentially some (Figure 21.4 through Figure 21.6) of the more difficult equipment tomaintain and operate properly Steam pressures can range from 200 to 4000þ psig and

temperatures from 4008F to 11008F Due to the fact that a high-temperature gas is used topropel blades for shaft rotation, extensive frame and casing design considerations concerning

FIGURE 21.4 Small steam turbine with upper casing removed

FIGURE 21.5 Small steam turbine

casing and rotor expansion and contraction are taken into account to minimize excessivepositional change of the rotor during operation However, movement of the shaft invariablyoccurs from OL2R conditions that can range considerably from unit to unit In addition,rotor expansion must be taken into consideration when selecting a flexible coupling toprevent thrust transfer from one rotor to another, causing premature bearing or couplingfailure On several occasions, the condensing end of the steam turbine has been observed tomove downward during operation The cooler temperatures and the ‘‘vacuum draw down’’effect of the condenser may actually move the condenser end opposite of what one mightexpect Again, since there is such a wide variety of equipment in existence, it is always best toconsult with your equipment manufacturer for initial installation, design modification, over-haul, or operational problems with these units Thank them for their input, but always doyour own research.

Typical OL2R movement range of steam turbines (horizontally mounted):

Vertical movement: –10þ to 25 mils upward (5 to 500 hp); 5 to 40þ mils upward (500þ hp),

typically asymmetrical (i.e., inboard and outboard ends do not move up the same amount)Lateral (sideways) movement: 0 to 40þ mils (can be as much or considerably more than the

vertical movement)

Axial movement: 10 to 100þ mils (5 to 200 hp); 20 to 250þ mils (200þ hp)

21.1.2.1 Additional Information on Steam Turbines

Moderate to excessive off-line soft foot conditions have been experienced on virtually every sizesteam turbine regardless of frame construction design Frequently, on small- to medium-sized steam turbines, one end of the casing is rigidly bolted to the frame and a ‘‘sway bar’’ orflexible support is mounted at the other end to allow for axial expansion to occur to preventcasing warpage during operation Sometimes on larger steam turbines, the casing is keyed at thecasing centerline and the hold-down bolts are not tightened to lock the casing against the framesupport but are kept loose to allow for symmetric lateral and axial casing expansion to occur.The lateral movement that occurs is often directly related to the expansion and contraction ofthe steam piping connected to the steam turbine casing and proper design and installation of thepiping system is imperative to minimize static (off-line) and dynamic (running) nozzle loads

FIGURE 21.6 Large steam turbine

Most steam turbines are supported in sliding-type bearings and therefore exhibit a certainamount of axial clearance between the thrust runner and the active–inactive thrust bearings(often referred to as thrust float) When setting the machinery axial positions off-line, seat thethrust runner against the active thrust bearing before measuring and adjusting the shaft-to-shaft distance Bear in mind that the axial movement amounts mentioned above are for thecasing and housing The shaft may expand more than that and may influence how you shouldset the off-line shaft end to shaft end distances.

Industrial gas and power turbine drivers are used in a wide variety of applications rangingfrom compression of gases and electrical generation to propulsion systems for ships(Figure 21.7 and Figure 21.8) The Brayton cycle (i.e., a gas turbine) compresses air via a

FIGURE 21.7 Gas turbine

FIGURE 21.8 Gas turbine driving an electric generator

centrifugal or axial flow compressor where the compressed air is mixed with fuel (liquid jetfuel or natural gas) and burned The hot, high-velocity gas then impinges on a series of severalstages of curved blade sets (power turbine) that is used to rotate the driven machinery.Frequently, the gas and power turbines, although separate rotors supported in their ownbearings, share a common casing and frame The residual high-velocity gas is then ventedthrough ductwork that sometimes houses a heat exchanger for a closed loop system or for use

in heating liquids for other purposes

The gas turbine produces a tremendous amount of forward thrust in reaction to the velocity gas escaping out of the tail end of the machine A considerable amount of heat isgenerated in the cycle and a twisting or torsional counter reaction occurs in the frame duringoperation These factors all contribute to some of the most radical OL2R machinery move-ment in any type of driver used today

high-Typical OL2R movement range of gas or power turbines:

Vertical movement: Intake end—10þ mils downward to 10þ mils upward; exhaust end—5

to 80þ mils upward

Lateral (sideways) movement: Intake end—2 to 20þ mils; exhaust end—2 to 60þ mils

Axial movement: See additional information

21.1.3.1 Additional Information on Gas Turbines

Moderate to excessive off-line soft foot conditions have been experienced on virtually everysize gas and power turbine regardless of frame construction design Movement in the axialdirection from OL2R conditions can also be excessive Forward movement of gas turbines(i.e., toward the intake end) has been observed to translate 180þ mils Gear- or diaphragm-

type couplings have been employed at the output shaft to drive the equipment If the coupling

is a diaphragm-type (or any flexible disk-type) and there is movement toward the intake end,damage could occur to the coupling and the thrust forces can be transmitted to the drivenmachine The shaft-to-shaft distance between the power turbine and the driven equipmentshaft is usually 40þ in in an attempt to minimize the effect from large amounts of OL2R

movement and to minimize any heat transfer from the exhaust duct work to the drivenmachine Bear in mind that the axial movement amounts mentioned above are for the casingand housing The shaft may expand more than that and may influence how you should set theoff-line shaft end to shaft end distances

Very few field studies have been conducted (or at least published) on how internal combustionengines move from OL2R conditions (Figure 21.9) Diesel engines, for example, are frequentlyused to drive backup electrical generators, fire pumps, and portable air compressors In thewastewater treatment industry, biogas engines can be used to drive the air compressors Thecrankshaft is typically set very low in the casing and engine mounts can be found below, at, orslightly above the centerline of rotation of the crankshaft The relatively few studies that havebeen done have still shown OL2R machinery movement regardless of the casing supportmounting location Flexible coupling design is somewhat critical since variations in torqueoccur as each piston delivers rotational force at varying intervals

Typical OL2R movement range of internal combustion engines:

Vertical movement: 1 to 5 mils upward (5 to 200 hp); 2 to 20þ mils upward (200þ hp),

typically symmetrical (i.e., inboard and outboard ends move up the same amount)

Lateral (sideways) movement: 0 to 4 mils (usually much less than any vertical movement)Axial movement: Unknown

21.1.4.1 Additional Information on Internal Combustion Engines

Moderate to excessive off-line soft foot conditions have been experienced on virtually everysize internal combustion engine regardless of frame construction design On medium andlarge engines, distortion of the engine frame during installation is a concern To insure thatthe crankshaft bearings are not distorted, web deflection tests are conducted as shown inFigure 21.11 and Figure 21.12 A web deflection test determines if the distance between thecrank webs is changing when the crankshaft is rotated If the bearings are misaligned due tocasing distortion, or there is an excessive amount of shaft misalignment with the couplingengaged, the distance between the crank webs will vary when the crankshaft is rotated If thegap variation between the web is excessive, shims must be added between the engine and thesoleplates to relieve the distortion of the casing

Without a doubt, one of the most common drive systems in virtually every industry is amotor-driven, horizontally mounted, centrifugal pump (Figure 21.13 through Figure 21.15).There are several hundred designs of centrifugal pumps and it would be difficult to coverevery characteristic of each design used in industry Their purpose is basically to move anincompressible fluid from point A to point B The temperature of the fluid conveyed has agreat effect on the OL2R conditions of the pump As discussed in Chapter 5, the pipingattached to the pump can have a tremendous influence on obtaining and maintaining accuratealignment, so that many people are unwilling to even try to reposition pumps, henceforthdeclaring them the ‘‘stationary’’ machine when aligning them

Typical OL2R movement range of centrifugal pumps:

Vertical movement: 0 to 80þ mils upward typically asymmetrical (i.e., inboard and

out-board ends do not move up the same amount)

FIGURE 21.9 Sixteen cylinder biogas engine coupled to a gearbox and compressor