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Topic Overview---HPB Webinar VIII — December December 2012 Process Gas Compressor Failure Avoidance In this Webinar, the presenter explains some compressor issues and maps out practical remedies remedies that worked worked in his 50-year professional professional experience. Remedial or preventive actions actions must 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 missed opportunities to get to the bottom of compressor failures and highlights steps taken by leading performers to operate some machines machines without shutdown shutdown for five, eight, and more years.
Topic Overview---HPB Webinar VIII — December December 2012 Process Gas Compressor Failure Avoidance In this Webinar, the presenter explains some compressor issues and maps out practical remedies remedies that worked worked in his 50-year professional professional experience. Remedial or preventive actions actions must 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 missed opportunities to get to the bottom of compressor failures and highlights steps taken by leading performers to operate some machines machines without shutdown 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 Guide-vane actuator linkage
Labyrinth seals
Compressor rotor
Rotor blades
Adjustable guide vanes Compressor inlet nozzle
Thrust bearing
Compressor outlet nozzle
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 reentries 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 backto-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
PLAIN DIESTER CYLINDER LUBRICANT From a Top Provider With Application Engineering Know-How
Compressor discharge valve after 6 months’ service with a diester
Compressor discharge valve after 4 months’ service with petroleum base lubricant
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
P
E
E
P
P
P
E
P
one frame 7. Ability to accommodate different services on one frame
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 E x p e r i en c e - B as e d R e c o m m e n d a t i o n s f o r S e le c t i n g C o m p r e s s o r S ea l _________________________________________________________________ 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
Gas Compressor
Gas Compressor
Non-corrosive or corrosive Non-hazardous or hazardous Non-fouling or fouling Non-corrosive Non-hazardous or Hazardous Non-fouling Corrosive Non-hazardous or hazardous Fouling
69 to 172 (10 to 20)
to 25,000 (to 3,600)
Any
Labyrinth with injection and/or ejection using gas being compressed as motive gas Gas Seal Tandem Style preferred Oil Seal, Double Gas Seal
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, noncondensing) 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 • • •
•
•
Courtesy: Dresser Rand Insights V5 #1
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
Tandem-style dry gas seal (AESSEAL)
