Compressors (Bojoe)

Front 1

What is the general function of a compressor?

Back 1

Compressors add energy to gas in the form of pressure to generate flow.

Front 2

How does a centrifugal compressor work?

Back 2

Increase gas pressure by increasing the velocity of gas molecules.

Front 3

What is the relationship between centrifugal compressor head and capacity?

Back 3

They are inversely related.

Front 4

What is surge?

Back 4

Surge is defined as the operating point at which the compressor peak head capability and minimum flow limit are reached.

Front 5

What are the consequences of surge?

Back 5

• Axial thrust on rotor can damage bearings or cause rub
• Discharge check valve will slam shut shaking piping
• Overheating of compressor internals may occur
• Can cause process upset

Front 6

Where will you find an axial compressor in the refinery?

Back 6

In the FCC unit

Front 7

What is choke or stonewall?

Back 7

when the inlet flow rate of the compressor is at maximum

Front 8

What is the purpose of the impellers?

Back 8

Impart velocity to the gas entering through the eye.

Front 9

What is the purpose of diffusers?

Back 9

Increase flow area for the gas that leaves the impeller
Gas velocity is decreased with increase in pressure

Front 10

What is the purpose of a volute?

Back 10

further decreases velocity with increased pressure

Front 11

What is the purpose of a balance piston or balance drum?

Back 11

used to reduce thrust created by the compressor rotor’s impellers

Front 12

How is leakage prevented between the impellers of a multistage centrifugal compressor?

Back 12

Labyrinth seals

Front 13

How can a centrifugal compressor capacity be controlled?

Back 13

• Speed variation
• Suction throttling
• Discharge throttling
• Recycling (using anti-surge systems)

Front 14

How does a recip compressor work?

Back 14

In a reciprocating piston compressor, mechanical movement of a piston creates a low pressure area in the cylinder to draw gas in, and then reverses the direction to reduce volume and increase gas pressure as it forces gas out of discharge

Front 15

List and describe the four steps of the compression cycle.(1)

Back 15

 Compression
• Piston first begins to move into the cylinder
• Volume is reduced
• Pressure begins to increase

Front 16

List and describe the four steps of the compression cycle.(2)

Back 16

 Discharge
• Cylinder pressure > cylinder discharge piping pressure
• Discharge valve opens (spring-loaded check valve)
• Pressurized gas flows out of cylinder and into discharge piping

Front 17

List and describe the four steps of the compression cycle.(3)

Back 17

 Expansion
• Piston reverses direction at the end of the discharge stage
• Remaining gas in cylinder (clearance volume) expands
• Expansion results in decrease of pressure
• Spring-loaded discharge valve closes

Front 18

List and describe the four steps of the compression cycle.(4)

Back 18

 Intake
• Cylinder pressure < inlet piping pressure
• Spring-loaded suction valve opens
• Gas enters the cylinder

Front 19

What is a double acting cylinder?

Back 19

Inlet and discharge valves on different ends of the cylinder
As the piston moves back and forth:
Compression and discharge occurs on one side of the cylinder, while expansion and intake occur on the opposite side as the piston moves back, the discharge and intake sides reverse

Front 20

What is the purpose of a cross head?

Back 20

Convert the combined motion of the connecting rod into pure reciprocating motion
Guide the piston rods in a straight path - in and out of the cylinder

Front 21

What is “non rod reversal” and what causes it?

Back 21

Inadequate lubrication of the crosshead pin due to improper loading. You must load the crank side first and unload the head side first.

Front 22

What prevents leaking from the cylinder at the piston rod penetration?

Back 22

Pressure packing

Front 23

What is the purpose of the distance piece?

Back 23

Isolates the crankcase from the cylinder to provide containment of any:
Leaking process gas
Contaminated oil from the pressure packing

Front 24

How is recip compressor capacity controlled?

Back 24

 Recycling
 Variable clearance pockets
 Suction valve unloaders
 Stepless capacity control – stepless unloaders

Front 25

How does an unloader work?

Back 25

Prevent compression in a cylinder by physically holding the suction valve open

Front 26

What are the two lubrication systems associated with most large recip compressors?

Back 26

Crankcase oil
Cylinder lubrication

Front 27

How does a liquid film seal work?

Back 27

These seals rely on oil flow in a direction opposite to normal gas flow.

Front 28

What is the purpose of a seal oil tank?

Back 28

to control the differential pressure between the seal oil and the compressor-gas sealing pressure.

Front 29

What is the purpose of a rundown tank?

Back 29

serves as an emergency source of oil pressure in case of low lube oil pressure trip

Front 30

What is the function of unloaders?

Back 30

Prevent compression in a cylinder by physically holding the suction valve open (finger unloaders) or by opening a separate port back to suction

Front 31

What is the function of a pulsation dampener?

Back 31

Absorb energy from the pulsations and prevent excessive vibration
Magnitude of pulsations is greatly reduced with the use of pulsation dampeners

Front 32

What is a clearance pocket and how does it work?

Back 32

Make up part of the cylinder volume, used to vary the compressor capacity
For a small capacity adjustment they open to decrease the cylinder's volumetric efficiency

Front 33

What is tempered water used for in a cooling system?

Back 33

Ensures cylinder temperature is higher than the suction gas temperature to prevent condensation of gas

Front 34

What component is used to collect oil from the seals?

Back 34

Sour oil trap

Front 35

What is the purpose of a coastdown/rundown tank?

Back 35

Provides lubrication in the event of a trip.

Front 36

What are the hazards of a compressor in a facility?

Back 36

Noise
Heat
Rotating equipment
Liquid in the compressor supply
Hazardous gas

Front 37

What are the 2 types of dynamic compressors in the refinery?

Back 37

Axial & Centrifugal

Front 38

Why aren’t labyrinth seals used as a casing seal?

Back 38

They don’t provide a complete seal

Front 39

What is the purpose of a buffer gas?

Back 39

To clean and cool the seal in a dry gas seal

Front 40

What side of the seals is the contaminated oil discharged on?

Back 40

Impeller side