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Millwright Third Year 2018 > Dynamic Compressors > Flashcards

Dynamic Compressors Flashcards

1
Q

How does a centrifugal compressor work?

A

They convert velocity in to pressure using diffusers, a volute casing or a combination.

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2
Q

Describe the single stage centrifugal compressor?

A

It is the simples type of centrifugal compressor consisting of a single impeller that attaches directly to an extended motor shaft.

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3
Q

What is used to increase efficiency and capacity of a single stage compressor?

A

It uses diffuser vanes fit to the inside of the housing. increasing the speed and also increasing the volume.

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4
Q

How many stages (impeller) can you put on a single rotor?

A

You can have as many as 12 stages present on a single stage?

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5
Q

What is an alternate design to multi-staging?

A

Is to have 4 or 5 individual impellers with their volute casing arranged around a central bull gear.

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6
Q

What are the three different casings designs for centrifugal compressor?

A
  • Sectional
  • Horizontally Split
  • Vertically Split
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7
Q

Describe sectional casings and their use?

A

Sectional casings they consist of a number of sections, each bolted to another with tie rods. They are usually on small units with low-pressure ratio and small volumes.

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8
Q

Describe horizontally split casings and their use?

A

The casing splits along the centerline of the housing and bolts together , sealing it metal to metal, limited to less than 1000 psig.

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9
Q

What is a dynamic compressor?

A

Is a non-positive displacement compressor, and it has no positive seal between inlet and discharge.

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10
Q

Describe Vertically casings and their use?

A

They are cylindrical or barrel casing with removable front cover. They are used in high-pressure applications of up to 10,000 PSIG

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11
Q

What provides the acceleration velocity for the conversion into gas pressure?

A

The impellers, they are fitted on the shaft. and the rotor is dynamically balanced.

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12
Q

What is the most important part of a centrifugal compressor?

A

The Impeller.

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13
Q

What are the two (2) major classifications of dynamic compressors?

A
  • Centrifugal Compressors

* Axial Compressors

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14
Q

How do dynamic compressors produce pressure?

A

They convert velocity in to pressure using diffusers, a volute casing or a combination.

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15
Q

What can be done to increase the efficiency of a single-stage compressor?

A

Diffusers vanes inside the housing allow you to achieve a slightly higher compression ratio.

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16
Q

What has to be done to increase the volume and overall pressure ratio of a centrifugal compressor?

A

You should add additional impellers and their individual diffuser sections to the rotor shaft.

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17
Q

Which casing design do you use for high-pressure applications (10,000 psig)?

A

The Vertically Split Design.

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18
Q

Name the three (3) styles of impellers in centrifugal compressors

A
  • Open
  • Semi-Closed/Open
  • Closed
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19
Q

Describe open impellers and their purpose?

A

Open impellers are the simplest design. they are usually used in single stage applications and they consist of vanes that radiate out from a central hub. used in rough service and dirty environments.

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20
Q

Describe semi-closed/open impellers and their purpose?

A

Semi-closed impellers have blades that can be straight radial, backward leaning, forward leaning or combination of the blades depending on compressor design and can be used on single stage or multi-stage or even as the final stage in an axial compressor.

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21
Q

Describe closed impellers?

A

They are impellers with front and back vanes closed, used in single stage and multi-stage units on up to 12 stages, as well as on axial compressors as the last stage. With these impellers you can achieve higher pressure ratios and higher efficiency due to tight clearances between impellers and diffusers.

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22
Q

What are guide elements?

A

They control and direct gas flow into the compressor or from one stage to the next. they maybe stationary or movable inlet guide vanes, or adjustable or stationary diffusers.

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23
Q

What do inlet gas vanes do?

A

Inlet gas vanes control compressor output and increase efficiency. Output is controlled by throttling the inlet gas flow.

24
Q

What is the purpose of the diaphragm?

A

It directs flow through the compressor and changes the direction and velocity of the gas stream. it separates the impellers in the compressor’s casing.

25
Q

Why do we install labyrinth seals on the diaphragm assembly?

A

To minimized internal leakage along the shaft from the progressively higher stage pressures.

26
Q

On which direction does thrust load occurs?

A

Thrust load occurs in the direction of the suction end of the compressor.

27
Q

What are the methods of thrust control?

A

Balance Pistons or Drums, Thrust Bearings, And Impeller Alignment.

28
Q

Where are thrust collars usually installed?

A

At the suction end of the compressor.

29
Q

What type of bearing are usually used?

A

The tilting pad, also known as the Kingsbury Bearing.

30
Q

How are impellers arranged to control thrust load?

A

In a back-to-back arrangement to create opposing flows which reduces axial thrust.

31
Q

Where are carbon ring seals used?

A

Where there are low pressure differentials, in conjunction with labyrinth seals as the final sealing element. And are limited to smaller shaft diameters.

32
Q

Where are mechanical seals used?

A

They are used to seal shafts where the shaft speed is low enough to permit their use. Friction and heat generated by contact between the seal faces limits the rotational shaft.

33
Q

What do seals prevent leakage from?

A
  • Stage to Stage
  • Compressor to Atmosphere
  • Atmosphere to compressor
  • Oil to Compressor
34
Q

What are the different seal types and designs?

A
  • Labyrinth Seals
  • Carbon Ring Seals
  • Mechanical seals
  • Dry Gas Seals
35
Q

Which seals are the most commonly used?

A

Labyrinth Seals

36
Q

What determines the length of a labyrinth seal?

A

The higher the pressure drop the longer the seal must be.

37
Q

What is a problem with carbon ring seals?

A

They are self lubricating, but have a tendency to corrode the shaft when compressors shut down and are limited to smaller shaft diameter.

38
Q

Describe a dry gas seal?

A

It consist of a stationary ring, held against a rotating ring with spring pressure.

39
Q

what kind of bearings are used on larger multi-stage compressors?

A

Plain journal bearings or magnetic bearings with dry gas seals.

40
Q

How are prime movers selected?

A

Their speed range, available power, and horse power requirements.

41
Q

Describe an axial compressor?

A

Axial flow compressor flows in an axial direction through a series of rotating blades (rotor discs) and stationary vanes (stators) that attach to the casing. pressure rise is small, so they required multi-staging.

42
Q

What is the best and simplest way to control the axial compressor?

A

By varying the speed or VFD.

43
Q

What are the main components in an axial compressor?

A
  • The Rotors
  • The Stators
  • The Casing/Housing
44
Q

What are the two types of rotors?

A
  • Drum Rotors

* Disc Rotors

45
Q

Describe drum rotors?

A

Drum rotors consist of discs that are flanged to fit each other and are held together with tie bolts and stub shafts.

46
Q

Describe disc rotors?

A

Disc rotors consist of individual rotor disc that mount onto the rotor shaft.

47
Q

What is the benefit of bolted disc rotors?

A

They are lighter weight, stronger and less likely to bend.

48
Q

Describe the stator?

A

The stator has a series of blades that attach to the casing wall. They can be fixed or adjustable, solid or hollow.

49
Q

What are adjustable stator blades?

A

Adjustable stator blades are blades that can be individually adjusted and set during shutdown.

50
Q

What is a combination flow compressor?

A

Is an axial flow compressor that includes centrifugal impellers as the final stage to increase the compression ratio.

51
Q

What is blade fouling?

A

When the stator blades get debris stuck to them, mainly from dirty gases.

52
Q

What are blade fouling symptoms?

A

Slightly reduction of gas flow.

53
Q

what is a borescope?

A

Is an optical device that you can insert through a small orifice and enables the inspection of internal parts of various machines.

54
Q

At what point does the compressor runs in a stable condition?

A

Between the surge point, and the stonewall point.

55
Q

What is surge?

A

Surge is when low flow causes flow reversal and compressed gas rushes back from the discharge to the inlet, which causes a pressure drop at the discharge.

56
Q

What is Stonewall or Shocked flow?

A

Is the point along the compressor curve where maximum flow has been reached. Pressure increases as gas flow through the compressor decreases.

57
Q

what is used to control surge?

A
  • Recycle Valve
  • Bypass Valve
  • VFD

Millwright Third Year 2018 (15 decks)

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