Stuff that works stuff that holds up
Is the kind of stuff you don't hang on the wall
Stuff that's real that stuff you feel
Is the stuff you always reach for when you fall

("Stuff That Works," by Guy Clark and Rodney Crowell, 1995)

 

MULTI-STAGE CENTRIFUGAL COMPRESSOR
ROTOR COMPONENTS AND CASING DESIGN AND ACCESS

The Multi-Stage Centrifugal Compressor Casing Design
Determines Ease of Rotor Access

The previous PTOA Segment listed the critical hardware associated with a Multi-Stage Centrifugal Compressor.

All brilliant PTOA Readers and Students***  … meaning those who have been reading the PTOA Segments in the intended, sequential order … know that a Multi-Stage Centrifugal Compressor has many crucial rotating and non-rotating pieces of hardware which must work harmoniously to increase the PV Pressure of a gas via  simultaneously creating and managing the impacts of Centrifugal Force.

Eventually, one of the hardware components in the Multi-Stage Centrifugal Compressor is going to need to be fixed or replaced, preferably during a planned maintenance interval and not during a catastrophic failure emergency.

If the Mechanic Shop were consulted during the design phase of the Multi-Stage Centrifugal Compressor, the choice of Casing style will be influenced by ease of access to the Rotor.

 

Review of Rotor Components and Auxiliary Support Systems

The Rotor of a Multi-Stage Centrifugal Compressor is similar to the Rotor of the Multi-Stage Centrifugal Pump featured in PTOA Segment #175.

The Rotor includes the Shaft, multiple Impellers, and various metal components needed for stabilizing, sealing, and lubrication within the Rotating Equipment known as a Multi-Stage Centrifugal Compressor.

 

The Impellers rotate with the Shaft, which is "coupled to" the Shaft of a Driver.

Impellers were featured in PTOA Segment #176

 

Coupling of Shafts was featured in PTOA Segment #185.

Drivers were featured in PTOA Segment #187 through PTOA Segment #201.

***Want to become a "Brilliant" PTOA Reader or Student? Start reading PTOA Segments from the beginning HERE.

 

Bearings Support and Stabilize the Rotor

PTOA Readers and Students learned about Bearings in PTOA Segments #182 and PTOA #183.

Journal Bearings (aka Plain Bearings) support the Shaft/Rotor and restrict up and down movement.

Axial Thrust Bearings reduce horizontal movement of the Rotor.

 

Seals Prevent Leakage Into or Out Of the Compressor

Since the purpose of the Multi-Stage Centrifugal Compressors is to build up the PV Pressure of a gas, brilliant PTOA Readers and Students can deduce that there are areas of varying PV Pressure within the Compressor.

For this reason Seals are used to prevent leakage into areas of unequal Pressure as well as prevent leakage from the Compressor and into the surrounding environment (which is logically at Atmospheric Pressure, 1 Atm or 14.7 psi).

PTOA Readers and Students learned about Seal technologies in PTOA Segment #184.

Multi-Stage Centrifugal Compressors might employ an oil film Seal technology called  Cone Seals.

Tilting Pad Journal Bearings must also be constantly supplied clean, circulating seal oil from an Auxiliary Oil Seal System. .

Alternatively, the Multi-Stage Centrifugal Compressor might employ  Dry Gas Seal technology. .In that case a separate Dry Gas Seal Auxiliary System will continuously supply the seal gas.

Labyrinth Seal technology is used to prevent gas leakage from the Compressor into its surroundings. A Labyrinth Seal is shown in the nearby schematic.

 

Auxiliary Lubrication System Needed for all the Rotor's Moving Metal Parts

The best PTOA Readers and Students … meaning those who are reading the PTOA Segments in the intended, sequential order … have read the PTOA Tribology Focus Study and thus already know  about the critical necessity of lubricating moving metal parts. The term "Tribology" refers to the close relationship between Friction, Wear, and the Lubrication needed to mitigate Wear caused by Friction.

The PTOA Tribology Focus Study starts with PTOA Segment #177 and ends with PTOA Segment #180.

An Auxiliary Lubrication Oil System will be needed to support the operation of every Multi-Stage Centrifugal Compressor.

 

The nearby schematic illustrates the components of a Compressor Train's Auxiliary Lube Oil System. The "train" of Multi-Stage Centrifugal Compressors is represented as the box labelled at the top of the schematic.

 

 

 

 

 

Horizontal and Radial Centrifugal Compressor Casings

A Horizontally-Split Centrifugal Compressor Casing is a Casing that allows access to the Rotor after removing the bolts that connect the top half of the Casing to the bottom half of the Casing.

Horizontally-Split Casings are easy to identify because of the many bolts that run down both sides of the Compressor.

A Radially-Split Centrifugal Compressor Casing is also known as a Barrel Casing.  There is no "top half" or "bottom half" of a Radially-Split Casing (aka Barrel Casing).

Therefore access to the Rotor can only be made via disassembling  the front of the Compressor. For this reason, the Radially-Split Casing (aka Barrel Casing) has a ring of bolts surrounding its front face.

Note that the Rotor for a Radially-Split Casing (aka Barrel Casing) is enclosed in a separate Horizontally-Split Casing which is then slid into the outer Casing of the Radially-Split Compressor Casing.

The :"Casing within a Casing" architecture of the Radially-Split Centrifugal Compressor design allows significantly greater Discharge PV Pressures.

The Compressor Operating Range graph cited below apparently does not describe a Barrel Casing design. The chart indicates that the greatest  PV Discharge Pressure attained by a Multi-Stage Centrifugal Compressor is approximately 10,000 psi.

A Horizontally-Split turbo charged Centrifugal Compressor can attain a  PV Discharge Pressure of 1450 psi (100 bar). However, a turbo charged  Radially-Split (aka Barrel) Centrifugal Compressor can attain 15,000 psi (1034 bar) PV Discharge Pressure.

PTOA Readers and Students were made aware of the Diaphragm design differences between Horizontally-Split and Radially-Split Casings in PTOA Segment #220. The nearby graphic is a reminder that the Diaphragm for a Horizontally Split Casing is incorporated into the upper and lower halves of a Horizontally-Split Multi-Stage Centrifugal Compressor Casing. Alternatively, circular Diaphragms are incorporated into the Rotor of a Radially-Split Centrifugal Compressor Casing.

 

 

THE OPERATING RANGE OF MULTI-STAGE CENTRIFUGAL COMPRESSORS 

In PTOA Segment #219, PTOA Readers and Students were introduced to the nearby graph and guided to focus on the Operating Range for a Single-Stage Centrifugal Compressor.

The nearby graph also describes the Operating Range of a Multi-Stage Centrifugal Compressor.

Find the label "Multi-Stage Centrifugal" and note the sort-of trapezoidal shape that it points to.

Adding Impellers significantly increases both the PV Discharge Pressure and Capacity of Centrifugal Compressors.

A mere glance at the graph instantly relates that the Inlet Flow of a Multi-Stage Centrifugal Compressor is much farther to the right (hence, greater) than the Inlet Flow range for a Single-Stage Centrifugal Compressor. The Multi-Stage Centrifugal Compressor Inlet Flow ranges from approximately 7,000 ACFM to 133,000 ACFM.

Ditto the greatly increased PV Discharge Pressure attainable by a Multi-Stage Centrifugal Compressor. The vertical height of the Multi-Stage Centrifugal Compressor "trapezoid" indicates the PV Discharge Pressure includes a maximum range of  approximately 10,000 psig at an Inlet Flow Capacity of approximately 750 ACFM. As the Inlet Flow Capacity increases to 10,000 ACFM, the PV Discharge Pressure decreases to 8000 psig.

Here's another way to compare the Operating Ranges of a Multi-Stage Centrifugal Compressor and a Single-Stage Centrifugal Compressor:

The graph illustrates that both the Multi-Stage Centrifugal Compressor and Single-Stage Centrifugal Compressor can attain a 1000 psig PV Discharge Pressure. The maximum Inlet Flow Capacity of the Single-Stage Centrifugal Compressor is approximately 1,200 ACFM; however the maximum Inlet Flow of the Multi-Stage Centrifugal Compressor is approximately 80,000 ACFM.

Next Up?  Dynamic Axial Compressors!

TAKE HOME MESSAGES: The Rotor of a Multi-Stage Centrifugal Compressor s the Shaft and its rotating component hardware including: 

• Impellers (spun by the rotational movement of the Shaft)
• Bearings (for Shaft Support and Stabilization)
• Seals (prevent leakage out of and into the Compressor)

 

Auxiliary systems that support Multi-Stage Compressor hardware include:

• Lube Oil and Seal Oil System
• Gas Seal System

 

The two Multi-Stage Compressor Casing designs are:

• Horizontally-Split Casing
• Radially-Split Casing

 

Adding Impellers to the Shaft of a Multi-Stage Compressor makes it possible to increase the PV Discharge Pressure and the Capacity (aka Inlet flow rate) of the gas that is flowing through the Compressor.

©2021 PTOA Segment 0221

PTOA PV PRESSURE FOCUS STUDY AREA
PTOA ROTATING EQUIPMENT AREA - DYNAMIC AND POSITIVE DISPLACEMENT COMPRESSOR

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