EPC-School 2012
M for Mechanical
Topic - Compressor
M for Mechanical
Topic - Compressor
1. What is the working principle of rotodynamic/ positive
displacement compressors?
3. In a reciprocating compressor what is the function of an
unloader and how does it work?
4. In a double acting reciprocating compressor cylinder how
is gas prevented from escaping from one side of piston to another?
5. Why is lubrication needed in the cylinder?
6. What is the function of rider ring?
7. Why do we use double acting reciprocating compressor?
8. What kind of sealing system is used for piston rods in
recip machines?
9. What is the principle of operation of suction and
discharge valves in a reciprocating compressor?
10. What is a labyrinth compressor?
11. What is the difference between fans, blowers and
compressors?
12. In case of centrifugal compressors what types of seals
are used?
13. Why are intercoolers used?
14. Where
is the head developed in a centrifugal compressor?
15. What type of seals is used for air compressor?
16. How does a centrifugal compressor performance vary with molecular
weight for a fixed volume flow rate?
17. How are critical speeds avoided in case of flexible rotors?
18. What would be the effect of closing the discharge valve in a
centrifugal compressor?
19. Why do we go for reverse flow arrangement in a
centrifugal compressor?
20. What is surging?
21. Under what condition does the compressor surge?
22. What precautions can be taken to prevent surging?
23. What are the types of screw compressors?
24. Does a screw compressor develop pulsating or continuous flow?
25. What is the need for timing gears in dry screw compressors?
26. Which rotor (male/female) is the drive shaft in a screw compressor?
27. What are the benefits of screw compressors over reciprocating?
28. What are the various protection devices on centrifugal
compressors?
29. What are the various protection devices on reciprocating
compressors?
30. What is the working principle of balancing drum and why
is it used?
31. What is the method of capacity control for centrifugal
compressors?
32. What is the method of capacity control for reciprocating
compressors?
33. What is the method of capacity control for screw
compressors?
34. What are the various auxiliary systems for a centrifugal
compressor?
35. What are the various auxiliary systems for a
reciprocating compressor?
36. Why are rundown tanks required?
37. What are the symptoms of surging?
Answers to Above Questions-
1. Rotodynamic type compressors are machines in which air or
gas is compressed by the dynamic action of rotating vanes or impellers
imparting velocity and to the flowing gas. The velocity head is converted into
pressure, partially in the rotating element and mostly in the stationary
diffusers or blades.
Positive displacement type compressors are machines in which
successive volumes of air or gas are confined within a closed space. The
pressure is increased as the volume of the closed space is decreased.
2. Isothermal Process – a process in which temperature is
constant.
Adiabatic Process – a process during which there is no
external exchange of heat.
PVk
= Constant
Polytropic Process – a process for perfect gases, following
the law
PVn
= Constant n=1 for
isothermal process.
3. Unloader is a form of capacity control. It unloads by
keeping the suction valve forcibly open and thereby avoiding compression by
recirculating gas back to suction line. It is also used to start the machine in
no load condition
4. Piston rings acts as pressure breakers and prevent gas
from escaping
5. Prevent wear of cylinder and rider ring and to take away the heat of friction between
rider ring and cylinder and
6. To take the load of
piston and safe guarding the piston and also guide along the cylinder
7. To smoothen the forces on the crank shaft and reduce the
no of cylinders
8. PTFE, carbon packing rings
9. Differential pressure across them.
10. These are vertical type reciprocating machines. In this
type of compressor, rider rings and piston rings are not used as in case of
horizontal type. In labyrinth piston compressors, an extremely large number of
throttling points provide the sealing effect around pistons and piston
rods.
11. Discharge pressure generated
Fans <
0.5 psi
Blowers 0.5 >P< 1.65 psi
Compressor >
1.65 psi
12. Labyrinth seal ( incase of air compressor)
Carbon rings (incase of air compressor)
Wet mechanical seal, dry gas seals (incase of process gas
compressor)
13. To reduce the work required for compression inturn
increasing the efficiency of compressor
To avoid using higher metallurgy which increases cost and
thickness (needed for high temperature applications)
14. Velocity is imparted to gas in the impeller and the
velocity head is converted into pressure, partially in the rotating impeller
and partially in the stationary diffusers
15. Labyrinth seal and Carbon rings
16. For the same volume flow rates power consumption
increases for higher molecular weight gases
17. By quickly ramping up through critical speed range.
Ramping windows are provided in governors
18. Reduce the flow and increase the head hence operating
point is shifted to the left in characteristic curve, compressor may start to
surge
19. To counter the axial thrust generated in the compressor
20. When the compressor flow goes below certain value
reversal of flow take place, this phenomenon is called as surging
21. Low flow, high back pressure
22. Antisurge system is provided, having recycle (spillback)
valve, as soon as compressor approaches the surge control line, this valve
opens and increases the flow through compressor thereby prevent surging, for
Air Compressors blow off / vent valves are used to prevent surging
23. Dry screw compressor & Wet screw compressor
24. Pulsating flow of very high frequency
25. Screw compressors
have two rotors male and female. Both the rotors are synchronized by timing
gear, so that constant gap is always maintained between the rotors and prevent
them from contact
26. Male rotor
27. More flow, less pulsation, more reliability, low
maintenance cost, high compression efficiency compared to reciprocating
compressor, can handle change in molecular weights, can handle liquid, gas and
solids to an extent
28. Antisurge system, Alarm & trips given on certain
parameters such as High bearing temperature, high shaft vibration, low lube oil
pressure, axial displacement of shaft, overspeed trips and process related
trips.
29. Alarms & trips on High bearing temperature, Piston
rod drop, stuffing box temperature, process related trips.
30. Balancing drum is used to reduce the axial thrust load.
One side of balancing drum see the discharge pressure and other side it see the
suction pressure, so axial thrust produced by the pressure difference across
the impeller is balanced by the axial thrust across the balancing drum.
31. Suction throttling
Variable speed
Inlet guide vanes
Spill back valve
32. Unloaders on suction valves.
Clearance volume
Variable speed
33. Spill back valve
Slide valve opening
Speed control
34. Lube oil system
Seal oil/gas system
Surge control
system
Cooling water
system
Machine protection
system
35. Lube oil system
Cooling water
system
Machine
protection system
Sealing system
36. Basically to supply lube oil during emergency
shutdown/trip and for safe coast down
37. High vibration, noise, flow reversal in compressor,
fluctuations in suction/discharge pressures.
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