COMPRESSOR TRAINING

BEARINGS*

*What bearing do they have on compressors?

Most Compressor “Problems” are traced to the rotating elements or the rotor support system and are first detected at the bearings by bearing RTD’s or vibration at the bearings.

The bearings are generally more compliant that other machine elements and when there is a malfunction they tend to give first.

Whatever the cause, modification of the bearing is generally the most direct and economical means of altering the behavior.

Bearing Family Tree

S p h erica lE lip tica l

C ylin d rica lTap ered R o lle r

R o llin g E lem en t(n o t A F )

H Y D R O S TA TIC

N on -p re load~ P la in Jou rn a l~ A xia l G roove

~ P ressu re D am

P re-load ed~ E lip tica l~ L ob ed~ O ffse t

T ilt in g P ad~ fixed p ivo t

~ se lf-eq u a lized

F ixed L an d~ Tap er

~ R a le ig h

H Y D R O D Y N A M IC(se lf-g en era tin g )

F L U ID F IL M

A ttrac tive R ep u ls ive

M A G N E TIC

C O M P R E S S O R B E A R IN G S

Principal of Operation

Oil is drawn into the converging wedge by the moving bearing surface. A pressure is generated in the fluid which tends to separate the two bearing surfaces.

Behavior of the oil in the Journal bearing converging wedge is defined by the Reynolds equations:

Pressure ~ viscosity, rpm, distribution function; and (1/radial clearance)2

= eccentricity ratio = e / radial clearance

Difference between the bearing set bore radius and the journal radius is the bearing set clearance or running clearance. This is often specified as clearance ratio: typical values are 1.5 to 2 mils per inch of shaft.

True partial arc and floating bush bearings are NEVER used with Centrifugal compressors for Process services

Plain journals are SELDOM used.

Two-Axial Groove Journal Bearing

Grooves allow more oil flow and better heat removal.

Pressure Dam abrupt endstep causes a pressure differential before and after the step which creates a net down force which further loads the bottom bearing and increases stability.

RpRj

Rb

Rj = Journal Radius

Rp = Radius of max shaft to fit in pad bore

Rb = Machined pad bore radius

Cp = Rp - Rj

Cb = Rb - Rj

M = 1 - Cb/Cp = PRELOAD

PRELOAD forces the oil to converge resulting in a stronger oil wedge effect.

BEARING PRELOAD

PRELOAD

In a two-lobe (lemon bore) bearing.

Each of these bearings has specific advantages and disadvantages

There is a broad selection of available stiffness and damping properties available. Therefore some bearings will be better than others for a particular application.

Bearings “Crush”

CRUSH - dimensional interference between the bearing shell and the support housing.

TOO MUCH - hot bearing!

TOO LITTLE - loose bearing and synchronous or subsynchronous vibration.

Need 0.002 in Crush on diameter!

Stiffness and Damping Relationships

In the flexible bearing region, the shaft is stiff relative to the bearing so bearing stiffness significantly affects the critical speeds.

If the bearing is too stiff, the effectiveness of the bearing damping will be limited.

The cross-coupled stiffness can create an out-of-phase force. Under certain conditions the bearing goes unstable and OIL WHIRL will occur.

S = NLD/W (R/c)2

Tilting Pad Radial Bearings

The independent pivoting feature significantly reduces or even effectively eliminates the CROSS COUPLING stiffness.

Each pad can pivot independently to develop its own pressure profile

Tilting Pad Bearings - guidelines•Typically 4, 5, 7 or 9 pads are used.•Load On Pad increases stiffness. Load Between Pads increases damping.•L/D (Slenderness Ratio) typically varies between 0.2 and 1.0.•Bearing UNIT LOADING (Load / Brg. Length x dia.) typically between 150 and 250 psi.

A ROCKER is a line-contact pivot with little axial alignment compensation.

A SPHERICAL is a semispherical surface-contact pivot.

Types of Tilt-pad Shoe Contours

Tilting Pad Bearings - guidelines

• LOAD ON PAD– increases stiffness– favors light rotors at

high speed– assymetry promotes

resistance to cross-coupling (stability)

• LOAD BTWN PAD– increases damping

– favors heavy rotors at low speeds (low Sommerfeld No.)

Comparison of Journal Bearing Types

What can cause bearing failures?• Design Deficiencies

– film thickness too small

– inadequate heat dissipation

– insufficient lubricant

– unstable rotor/brg system

– structural deflections

– incompatible babbitt-journal material (rub)

• Mfg/assembly errors– not per design clearances

– liners misaligned

– non-cylindrical journals

• Mfg/ass’y - cont’d– rough/scratched journal– excess runout thr-collar– dirt or debris from assy– poor bond of babbitt

• Operator mistreatment– dirt/scale in oil– operation dry (no oil)– excess vibration– hot inlet oil– misalignment of driver

Under Running Conditions a fluid film bearing can only be damaged or fail from two causes:

*. Film too thin which breaks down the film and damages bearing surfaces

*. Film too hot which damages babbit surfaces directly.

Modes of Bearing FailureMODE OTHER NAMES

Abrasion Gouging, scoring, scratchesBond failure SpallingCavitation erosion CavitationCorrosion Chemical attackElectrical Pitting FrostingErosion Worm tracksFatigueFretting Fretting corrosionHigh chromium damage Wire-wool; black scabNon-homogeneity Blistering; porosityOverheating Mottling; anisotropy;

ratcheting; sweatingSeisureStructural damageSurface wear Black scaleTin oxide damage WearWiping Smearing; polishing

REF:

McCloskey, T., Trouble shooting Bearing and Lube Oil System Problems. Proceedings of the 24th Turbomachinery Symposium.

Zeidan/Herbage, Fluid Film Bearing Fundamentals and Failure Analysis. Proceedings of the 20the Turbomachinery Symposium

OIL TEMPERATURE AND PRESSURE

• API 617 limitations:– 180 F (82 C) Oil Temp Out– 50 F (28 C) Temp Rise– 200 F (93 C) Brg. Temp.

• Increasing oil flow / pressure seldom cures the problem

• Change to oil grade or bearing geometry is more beneficial

• 98 - 99% Copper alloyed with chromium can be backed pads (as a retrofit) and are effective in dissipating heat and lowering bearing temperature.

Bearing Babbitt

Traditional babbitt material is lead-based or tin-based.

Presently favored material is white metal - which has broad definition.

Broad use of 60 Sn - 40 Al.

Tri-metal: lead-tin-copper which is a thin (<.004”) overlay of a stronger aluminum or copper alloy backing

• Lead-based:– excellent embedibility

– excellent conformability

– sensitive to corrosion from organic acids

• Tin-based:– Excellent antiseizure

– corrosion resistant except HCl & H2S (seal oil)

– good embedability

– good conformability

– Poor Fatigue properties

Bearing Babbitt

Embedability implies thickness (20 to 50 mils)

but thickness reduces fatigue strength (resistance to fracture from vibration loads)

Improvements in oil filtration have reduced the concern for particulates.

Typical of Compressor Bearings

FMCL Air Compressor Train Bearing

This is a picture of the FAILED FMCL NH3 Plant bearing.

Final determination was a electrical discharge problem

Misalignment

Fixed Land type Thrust Bearings

(Plain grooved thrust bearings are never used on process compressors)

Typical design load < 200 psi

Tilt-pad Thrust Bearing

Unit DESIGN loads for a tilting-pad thrust bearing are typically 250 to 500 psi.

Check experience for designs using higher loads.

•Center pivot - most common and good for reverse rotation and avoids miss-installation.

•Offset pivot can lower bearing temperature.

Typical No. of pads vs speed & load

Self-aligning Thrust Bearings*

EQUALIZING LEVER

SPRING EQUALIZER SPHERICAL SEAT

*This compensates for mis-alignment of shaft or housing or dirt behind the carrier ring.

This is NOT a cure for a cocked thrust collar!

Directed Flow Bearings

• Injection Nozzle

• Groove / Slotted Edge

• Pocket

• Spray Bar

Most of the oil is needed to remove heat rather than develop a hydraulic film.

Too much oil assures turbulent flow which promotes higher fluid friction losses.

One way of beating the horsepower loses of tilting-pad bearings is by minimizing the oil used to assure laminar hydraulics. The risk is inadequate heat removal.

Directing it to the proper point (bearing hot spots) allows an economy of oil flow.

Ref: Stewart, C., Influence of Oil Injection Method on Thrust Bearig Performance. Proceedings of 28th Turbomachinery Symposium

Directed flow bearing

BN ServoFluid Control Bearing

BN ServoFluid Bearings

Supply pressure is typically 600 to 1000 psi.

Self compensating

F

Rotor

Electromagnetic forces

Clearance= 0.5 mm

Electromagnets

laminations

Radial bearing

© S2M PPT 20.1.0 RADIAL

Electromagnet

0.5 mm

Auxiliary Bearings

Position Sensors

0.2 mm

RotorRotor

Laminations

RotorRotor

F

Radial bearing

© S2M PPT 20.2.0 RADIAL 2

+/-

ElectromagnetsElectromagnets

SensorsSensors

Position informationPosition information

Controller PID,...

AmplifiersAmplifiers

SetpointSetpointCenterCenter

RotorRotor

Signal treatmentSignal treatment

Control loopControl loop

Typical compressor w/ magnetic bearings

SABIC Unit

Radial mag-bearing & thrust bearing assembly

Requires extensive interface and coordination with CSE and Electrical team members.

• NO MECHANICAL CONTACT ~ No wear• No lube oil system~ No process pollution by lubricating oil • Wide operating range (1 to 500°K, 0 to 100% speed)

• Higher rotation speed• Lower energy consumption => Best operating costs

• Maintenance free concept => Best availability

• PERMANENT ACTIVE CONTROL

• Unbalance control => low vibration level• Built-in diagnostic and monitoring• Optimal rotor dynamic performances

ADVANTAGES

Technical limitations• High temperatures over 250°C without cooling• Max. axial load / speed (ex.: 20 tons at 6,900 rpm)

• Rotor dynamic : requires careful engineering• E-ex environment : requires careful design• Current saturation leading to poor restoring forces• Considerable heat removal• Load carrying capacity

Human limitations• Training on a new technology (maintenance...)• Resistance to innovation• Higher costs

LIMITATIONS