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Topic Overview---HPB Webinar VIII—December 2012

Process Gas Compressor Failure Avoidance

In this Webinar, the presenter explains some compressor issues and maps out

practical remedies that worked in his 50-year professional experience. Remedial or

preventive actions must be taken by operators, technicians, engineers, and

managers. They are our collective audience and the Webinar will reflect an acute

sense of audience awareness.

In essence, Heinz Bloch will highlight things learned to prevent process

compressor failures. His presentation touches on centrifugal, rotary positive

displacement and reciprocating compressors. It discusses missed opportunities to

get to the bottom of compressor failures and highlights steps taken by leading

performers to operate some machines without shutdown for five, eight, and more

years.

First of Two

Compressor

Categories:

Positive

Displacement

Typical Double-Acting Cylinder: Use Syn Lubes

Positive Displacement Compressors – Rotary

Screw Types---Dry or Wet Compression Space

• Reliability near centrifugal

and better than recip. comp.

• Cost is competitive with

centrifugal to 2000+HP

• Wet screw design can lower

compression temperatures

• Pressure limited to 700+psi

• High noise, needs acoustical

treatment

• Max power 7000+HP

• Close clearances, failures

means high damage

2nd Compressor Category: Dynamic

Compressor outlet nozzle

Rotor blades

Labyrinth seals

Guide-vane actuator linkage

Compressor rotor

Compressor inlet nozzle Thrust bearing

Adjustable guide vanes

What Is A Centrifugal Compressor ?

Important components:

– Rotor ( shaft )

– Impellers and diffusers

– Thrust & journal bearings

– Seals

– Balance piston

– Thrust balance line

– Coupling

– Casing (―case‖)

Centrifugal Compressor Design Principles

• A high-speed machine that

compresses gas by

centrifugal force

• Gas in 1st stage suction

given high velocity; gas is

led into eye of next stage

• The same sequence is

continued to last stage, or to

the discharge nozzle

• Interstage exits and re-

entries may be required.

(Discuss why)

Mechanical Design of Centrifugal Compressors

• Q: Horizontal or vertical

split design?

• Q: How many stages?

• Direction of thrust

• Thrust balance drum

location

• Thrust bearing location

Horizontal Split Type Centrifugal Compressor

• Casing is split parallel

to compressor shaft

• Lower pressure type

centrifugal compressor

• Primarily used to

compress air, inert

gases, hydrocarbons,

etc.

• Typically used below

800 psig/~55 atm.

Design Nuances Must be Observed:

Horizontal Split Type With ―Down‖-Nozzle

• Nozzles under machine

• Machine needs to be elevated

to accommodate piping

• Note ―hydraulically

balanced‖ impeller

orientations

• Easy access for maintenance

of rotor, bearings and seals

• Lifting top half does not

disturb piping

Horizontal Split Type With Up-Nozzle

• Nozzles are at top of

machine

• Machine can also be at

ground level

• Here, need to remove

piping for maintenance

of rotor, bearings and

seals

Operation of

Rotors and Impellers

Top: High tip speed high–performance rotor

• Open inducer type impellers

• 30,000 cfm (51,000 m3/hr) of natural gas at about atmospheric pressure

• Best suited for continuous operation at limited range

Bottom: High flow / double flow rotor

• Closed inducer impellers

• Two inducer impellers first stage, double flow second

• Large volume, low pressure high speed service with compact design

Rotors and Impellers

Top: Moderate tip speed--

• Blast furnace applications

• Can be upwards of 190,000 cfm

(320,000 m3/hr ), 6 ft ( 2 m )

diameter rotor

• Bottom: High head back-

to-back impellers

• Typically high head at low

volumes

• 12-stages develop 170,000 ft

(510 kNm/kg) of head

Vertical Split Barrel Type Compressor

• Higher pressure

• Horizontal machines: difficult to seal split line

• Barrel is a cylindrical section between two end bells, less force on joint

• Internals differ only in means of support

Vertical Split Barrel Type: Pros & Cons

Centrifugal

compressor for low-to

moderate pressures

Q 1: How do we know

that it’s an LP?

Q 2: Is it low flow or

high flow?

Q 3: Note small thrust

bearing size. Why?

Q 4: Why we recommend

staggered vanes in

3rd stage

Lubes for Positive Displacement

Compressors Merit Special Attention

• Synthesized hydrocarbons (―synthetics‖) with appropriate additives used in rotary’s compression space and in reciprocating compressor cylinders

• Working with highly qualified lube manufacturers/ formulators is very much recommended

• Qualified manufactures/formulators have application engineers that can explain benefit-to-cost ratios and user experience background

• Turbine oils (high grade mineral oils) in dynamic compressors---our second category

Compressor discharge valve

after 6 months’ service with a

diester

Compressor discharge valve

after 4 months’ service with

petroleum base lubricant

PLAIN DIESTER CYLINDER LUBRICANT From a

Top Provider With Application Engineering Know-How

High Performance Plastics

Selection Factors

Minimizing Capital Costs Axial Cent Recip Rotary

1. Bare compressor , minimum controls F G F E

2. Controls for part load operation F F G G

3. Controls for parallel operation F P G G

4. Foundation and installation E G P E

5. Driver G G P E

6. Ability to accommodate side steam on

one frame

P E E P

7. Ability to accommodate different

services on one frame

P P E P

Selection Factors Environmental Considerations Axial Cent Recip Rotary

Screw

1. Resistance to damage by entrained

solids

P F G E

2. Resistance to damage by entrained

liquid droplets

P F G E

3. Capability to deliver oil free gas E E E E

4. Ability to limit discharge temperature

by internal cooling

P F F E

5. Freedom from vibration E E P G

6. Freedom from noise F F G P

7. Freedom from piping pulsations E G P F

8. Ability to compress dirty, wet gas P P F G

9. Gas tightness G E G G

Selection Factors

Operating Flexibility Axial Cent Recip Rotary

1. Ability to accommodate multiple

operating points at good efficiency

P G E F

2. Ability to reduce volume flow at fixed

speed *

F F E G

3. Ability to reduce volume flow by speed

control *

P P E E

4. Ability to accommodate reduction in

molecular weight*

F F E G

5. Ability to accommodate increase in

pressure ratio*

F P G G

* With changes in speed or using control devices. All other operating

parameters fixed

Selection Factors

Minimize Maintenance Cost Axial Cent Recip Rotary

Screw

1. Number of items to be routinely

inspected or replaced

G E P E

2. Ability to operate reliably without

outage

G E F G

3. Length of time between scheduled

overhauls

G E F G

Selection Factors

Efficiency, Power Required Axial Cent Recip Rotary

Screw

1. Efficiency level at design point E G G F

2. Flatness of efficiency characteristic at

constant speed, changing volume flow

P G E F

3. Simplicity of indirect intercooling P E E P

Subsets and overlaps exist throughout. These

issues are important enough to be considered

on a case-by-case basis. REMEMBER OUR

TOPIC IS HOW TO AVOID FAILURES

Packing Box Oversights Note lube entry and vent ports; end cup gasket

Seals are the parts that fail most often

Experience-Based Recommendations for Selecting Compressor Seal _________________________________________________________________

Application Service Inlet Pressure Seal Type

kPa (psia)

------------------------------------------------------------------------------------------------------------

Air Compressor Atmospheric Air Any Labyrinth

Gas Compressor Non-corrosive Any Labyrinth

Non-hazardous

Non-fouling

Low value

Gas Compressor Non-corrosive or 69 to 172 Labyrinth with

corrosive (10 to 20) injection and/or

Non-hazardous or ejection using

hazardous gas being

Non-fouling or fouling compressed as

motive gas

Gas Compressor Non-corrosive to 25,000 Gas Seal

Non-hazardous or (to 3,600) Tandem Style

Hazardous preferred

Non-fouling

Gas Compressor Corrosive Any Oil Seal,

Non-hazardous or Double Gas Seal

hazardous

Fouling

Dry gas seals for centrifugal compressors

• Many different unidirectional

(top image) and bi-directional

(lower image) styles are

available, but patterns must

always be optimized

• Single, double and tandem seals

in use on thousands of machines

• Gas quality (purity, non-

condensing) critically important

Why assigning total responsibility to one

commercial entity is critically important

• Compressor manufacturer

• Original DGS (―dry gas seal‖) manufacturer

• Support system designer

• Support system manufacturer (fabricator)

• On existing compressors, dry gas seal options

include a UK manufacturer capable of fully

rebuilding/testing (or supplying its own) DGS

Specifications of Failure-Prone Mechanical

Components:

Dry Gas Seals

• Tandem style typical process industry

• Primary and secondary seal within single cartridge

• In normal operation primary absorbs total pressure drop to vent system, secondary acts as back-up

• Primary is on process or high pressure side, and secondary is on bearing or low pressure side

• ‖O‖ rings are not shown, but will be at stationary-to-stationary part interfaces

Courtesy: Dresser Rand Insights V5 #1

Tandem-style dry gas seal (AESSEAL)

So, consult good texts!

• Start with reflecting cost of reliable machines in cost estimating manual or budgetary records

• MQA—Machinery Quality Assessment

• Refers to up-front $$$ investment in defining qualified bidders, use of solid specifications, staying with project through S/U and beyond