Compressor - Firdaus Sabturani Flashcards
Compressor
a mechanical device that increases the pressure
of compressible fluid e.g. natural gas, air, etc
Compressors are similar to pumps
both increase the pressure on a fluid and both can transport the fluid through a pipe.
Reciprocating Compressor Types
- Horizontal : Traditional / Free floating
- Vertical : Traditional / Labyrinth Ring
- High Speed : Traditional
- Integral with Engine
Reciprocating Compressor Capacity Control
- Variable speed control
- Unloader control
- Suction throttling
- Recirculation bypass
- On-off control
Centrifugal Compressor Types
- Single / Multi stage
- Between bearings / Overhung
- Inter cooled
- Side Load
- Horizontal / Vertical split / Barrel
Axial Compressor
- Gas flows in direction of rotating shaft
- Can be built for lower pressures only (0.7 to 6.8 Bar)
- High flow rate & Efficiency
- Not as common as centrifugal compressors
Performance Control by Blow-off or Recycle
- Most inefficient control method
- Regularly found inplant air systems
- Rare in other systems
- Not recommended
Compressor Capacity Control Can be based on controlling:
– Net flow to the user ( recycle )
– Discharge pressure
– Suction pressure
– Variable Inlet Guide Vanes
– Speed
Performance Control by Discharge Throttling
- Extremely inefficient
(approx. the same power for every load) - Rarely used
- Not recommended
Performance Control by Suction Throttling
- Common on electric motor machines
- Much more efficient than discharge throttling
- Power consumed proportional to the load
- Throttle losses are across suction valve
Performance Control by Adjustable Guide vanes
- Improved turndown
- More efficient than suction throttling
- Power consumed is proportional to the load
- Power loss on inlet throttling is eliminated
Performance Control by Speed Variation
- Most efficient (Power f(N)3
) - Steam turbine, gas turbine or
variable speed electric motor - Typically capital investment higher
than with other systems - No throttle losses
Surge Phenomena
– A break down of flow through the compressor
– Surge occurs when the machine tries to compress a gas to too high pressure at too low
flow
Surge symptoms
– Flow fluctuations and oscillations
– Temperature rise
– Compressor vibration
– Noise !
Process parameters during surge
FLOW
- Rapid flow oscillations
- Thrust reversals
- Potential damage
Process parameters during surge
PRESSURE
- Rapid pressure oscillations
with process instability
Process parameters during surge
TEMPERATURE
- Rising temperatures inside
compressor
Consequences of Surge
- Process instability
- Compressor sequentially starting with damage to seals followed by
damage to bearings, impellers and shaft with increase in severity of the
surge - Increased seal clearances and leakage
- Reduced compressor life
- Catastrophic damage leading to prolong production loss
Factors leading to onset of surge
- Non-compliance to procedures during start-up & shutdown
- Operation at high throughput during normal operation with the following
scenarios: - Trips - Power loss
- Operator errors - Process upsets
- Load changes - Gas composition changes
- Cooler problems - Filter or strainer problems
- Driver problems
Basic Anti-surge Control System
- The anti-surge controller UIC-1 protects the compressor against surge
by opening the recycle valve - Opening of the recycle valve lowers the resistance felt by the
compressor - This takes the compressor away from surge
Surge line moves closer to operating point if :
- Molecular weight of gas reduces
- Suction temperature increases
- Suction pressure reduces
- Impellers become fouled
Effects of Fouling:
- Deposits
- Polymerisation
- Performance loss
Prevention of Fouling:
- Process parameters
- Anti-fouling coatings ( Sermalon )
- Liquid Injection
– Pipe
– Compressor inlet - Off-line washing
