You have got me going
(Zig zag)
It's my mind you're blowing
(Zig zag)

("Zig Zag," by Roy Orbison, 1970)

 

ROTOR BLADES AND STATOR BLADES CAUSE ZIG-ZAG FLOW
DOWN THE AXIS OF AXIAL FLOW COMPRESSORS

The nearby familiar graphic illustrates the looping, up-and-down then up-and-down again radial flow of a gas moving through a Multi-Stage Centrifugal Compressor.

As PTOA Readers and Students learned in PTOA Segment #220, the gas is spun radially outward from the tip of each Impeller and then makes a U turn in the Diffuser passageway that is formed by two adjacent Diaphragms.

The flow path though an Axial Flow Compressor is more zig-zaggy than up-and-down as well as flows more closely down the Shaft of the Axial Flow Compressor.

The below schematic is almost helpful illustrating the flow path through an Axial-Flow Compressor.  The schematic would be more helpful if it were drawn horizontal instead of vertical.  Then the Axial Flow Compressor Inlet Guide Vane would be on the left side and the Exit Guide Vane would be on the right side..

 

As PTOA Readers learned in PTOA Segment #222, each Rotor Blade-Stator Blade set is a "stage" in an Axial-Flow Compressor.

The schematic would also be more helpful if it included more enumerated left-veering zigs and right-veering zags because each zig-zag represents flow through successive sets of Rotor Blades-Stator Blades.

The schematic labels the first Rotor Blade-Stator Blade combo, as "Stage #1." The next Rotor Blade-Stator Blade combo is labelled  "n." The nomenclature "n" just means "the next one in sequence", like #2 Stage, #3 Stage, #4 Stage, etc.  Today's Axial Flow Compressors might have as many as twenty  Rotor Blade-Stator Blade "stages."

 

The Rotor Blade in the Axial Flow Compressor plays a similar role to the Impeller in a Centrifugal Compressor.

The Rotor Blade is situated on a disc that rotates with the Shaft. When the Shaft rotates rapidly, the Rotor Blade gives the compressed gas (e.g. air or natural gas) velocity and pushes the gas toward the Stator Blade.

Stator Blades are incorporated into the Compressor Casing. Stator Blades are stationary and aligned between each Rotor Blade.

When the expanding gas comes in contact with the Stator Blade, its velocity abruptly decreases and the loss of velocity is swapped into a increase in the PV Pressure. Ergo, the Stator Blade plays a similar role to the Diffuser passageway in a Centrifugal Compressor.

The flow path of the compressed gas through successive Rotor Blade-Stator Blade stages results in the zig-zag flow pattern down the axis of the Shaft of the aptly named Axial Flow Compressor.

 

LIST OF CRITICAL HARDWARE ASSOCIATED WITH AXIAL FLOW COMPRESSORS

As explained above, Axial Flow Compressors are Identified by several stages made from pairs of Rotor Blades and Stator Blades.

The nearby graphic of a 15 stage Axial Flow Compressor shows the Inlet on the left with an arrow pointing down. The Discharge is on the right with an arrow pointing up. This diagram uses the terminology "Vane" instead of "Blade."  The PTOA will continue with the terminology "Blade."

The (filtered) gas that is sucked into the Axial Flow Compressor first flows through an Inlet Guide Vane. The angle of the Inlet Guide Vane can be varied via an external linkage.

The gas then flows through a series of Rotor Blades-Stator Blade Stages. As was stated above and repeated here, the Rotor Blades are attached to Rotor Discs which rotate with the (Rotor) Shaft. The Stator Blades are non-moving and incorporated into the interior surface of the (non labelled) Casing.

The 15-stage Axial Flow Compressor  has some Adjustable Stator Blades and some Fixed (non-adjustable) Stator Blades.

Furthermore, some of the Adjustable Stator Blades are "individually adjustable" and some are "continuously variable."

Individually adjustable Stator Blades are adjusted externally using a bar linkage.  Continuously variable Stator Blades are adjusted by automation.

After the gas in the Axial Flow Compressor passes through the 15th final stage, it exits the Discharge Guide Vane and hence flows through the Discharge Diffuser just before exiting the Axial Flow Compressor.

Brilliant PTOA Readers and Students can already predict that the labelled Thrust Bearings and Radial Bearings are required to stabilize the Shaft. .Bearings were featured in PTOA Segments #182 and #183.

 

LEAKAGE PREVENTION WITH SEAL GAS AND BUFFER GAS

Brilliant PTOA Readers and Students already recognize that every Axial Flow Compressor in any process service will operate under a wide variety of PV Pressures associated with Start Up, Normal Operating Conditions, Emergency Shutdown and Planned Shutdown.

PTOA Readers and Students have already been made aware that "Seals" are devices used between interfacing rotating and stationary hardware  for the purpose of separating and minimizing leakage between areas of unequal PV Pressure.

Technologies used to prevent leakage from Rotating Equipment as well as prevent contamination from entering the Casing of Rotating Equipment were featured in PTOA Segment #184.

Depending upon its service, leak prevention strategies into and out of an Axial Flow Compressor may involve a supply of  Seal Oil, Seal Gas, and Buffer Gas.

 

The Labyrinth Seal, a Pressure Differential Device 

For the following discussion, assume the Axial Flow Compressor is a "Produced Gas Compressor." This terminology implies that the Axial Flow Compressor is being driven … perhaps by a GT.

In this process service application the compressed gas is not air but rather natural gas.

Use of an Axial Flow Compressor for the "Produced Gas Compressor" service implies that a large quantity of natural gas must be provided at a significantly elevated PV Pressure, perhaps for the purpose of being reinjected into a well used for natural gas and crude oil production.

Unlike air, natural gas is flammable and leakage from and into the Axial Flow Compressor must be reduced to a minimum.

One technology used to prevent leakage of the compressed natural gas from the Axial Flow Compressor is a Labyrinth Seal.

Note that the nearby drawing of a 15-stage Axial Flow Compressor features a labelled Labyrinth Seal which would be located in the vicinity of the labelled Thrust Bearing.

Brilliant PTOA Readers and Students learned in PTOA Segment #158 that no fluid will flow without a "Pressure Differential" aka "ΔP" providing a driving force.

The Labyrinth Seal is a "Pressure Differential" device; a fluid … like gas … is forced to flow through its maze of nooks and crannies all the while losing so much much ΔP that hardly any gas can leak from the device.

In an Axial Flow Compressor, the gas that flows into the Labyrinth Seal can be called "Seal Gas."

Seal Oil is pumped to the other side of the Labyrinth Seal from either a dedicated, circulating Seal Oil System or from a supply that is drawn from the circulating Lubrication OIl System.

Whichever way it is supplied,  the PV Pressure of the Seal Oil is intentionally set at a magnitude that is greater than the PV Pressure of the Seal Gas that is trying to escape from the Labyrinth Seal..

 

The Buffer Gas and Seal Oil Interface

Brilliant PTOA Readers and Students learned in PTOA Segment #179 that Seal Oil is commonly used in Centrifugal Pumps to prevent process liquid from leaking into the atmosphere.

The PV Pressure of circulating Seal Oil would have a tougher time keeping pace with the rapid PV Pressure changes that can occur in the Axial Flow Compressor.

In Axial Flow Compressors, Buffer Gas is used to keep the circulating Seal Oil from flowing into areas of the Axial Flow Compressor where it does not belong. .

 

Buffer Gas Defined

In an Axial Flow Compressor,  Buffer Gas is high PV Pressure gas which is diverted from the Axial Flow Compressor Discharge and piped to where a Buffer Gas/Seal Oil interface is needed.

Axial Flow Compressors are manufactured to maintain and use a PV Pressure Differential (ΔP) between the Buffer Gas and Seal Oil to prevent leakage as the PV Pressure profile of the compressor changes.

When the compressed gas tries to escape the confines of the Casing, the Buffer Gas pushes the lower PV Pressure compressed gas back into the Axial Flow Compressor.

And when the Seal Oil tries to squeeze into the Casing, the Buffer Gas pushes it out, too!

Pretty nifty how the difference in PV Pressure (ΔP) is used to control leakage from the Axial Flow Compressor, eh?

 

The Crucial Role of Process Operators  with respect to Leak Prevention

And all of the above explanation and description is to bring home this important message for GT Operators/Technicians:

Some of the Seal Gas is going to dissolve in and contaminate the Seal Oil.

 

Some of the Buffer Gas  is going to dissolve in and contaminate the Seal Oil.

 

Seal Oil that is contaminated with gas is called "Sour Seal Oil." Seal Oil that is contaminated with gas will destroy Bearings.

 

Gas Compressor Operators/Technicians must make certain that the gas is removed from the Seal Oil in a device known as a "DeGasser." Degassing Seal Oil is a unit operation in the Lubrication Oil/Seal Oil loop..

TAKE HOME MESSAGES: The hardware associated with Axial Flow Compressors includes:

• Inlet Guide Vane
• Rotor Blades (might be called Vanes) mounted on Rotor Discs
• Stator Blades (might be called Vanes) attached to the interior of the ...
• Compressor Casing.
• Discharge Guide Vane
• Discharge Diffuser

 

The angle of the Inlet Guide Vane can be adjusted.

Each Rotor Blade-Stator Blade pair is an Axial Fkiw Compressor stage.

Rotor Blades rotate with the Shaft. Stator Blades are stationary and attached to the interior of the Compressor Casing.

Some Stator Blades can be adjusted individually. Some Stator Blades can be adjusted by automation. And some Stator Blades are fixed.

The Rotor Blade increases the velocity of the compressed gas. The Stator Blade decreases the velocity of the gas which causes the kinetic energy of the gas to be converted into the PV Pressure.

The flow of the gas through successive Rotor-Blades and Stator-Blades is zig zaggy down the axis of the Axial Flow Compressor.

The rapid PV Pressure changes within the Compressor Casing technologically challenges leak prevention. Labyrinth Seals and Dry Gas Seal techniques using Seal Gas and Buffer gas are commonly used in Axial Flow Compressors.

PTOA Readers and Students were reminded that maintaining a target Pressure Differential (ΔP) is used in industry to keep flow going in the desired direction. Labyrinth Seals and Seal Gas Systems operate on the basis of achieving a desired ΔP., hence fluid flowrate.

Process Operators must insure the gas that gets into Seal Oil is properly removed in a DeGasser. Otherwise contaminated "Sour Seal Oil" can destroy internal Compressor hardware.

©2021 PTOA Segment 0223

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

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