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Compressor Surge and Operation Rev1

This document discusses compressor limits and surge, including: 1. Stonewall, pressure, power, and speed limits constrain compressor operation, while surge is the most critical limit as flow can reverse rapidly. 2. Surge occurs when the operating point enters the surge region of the compressor map due to increased system resistance, causing oscillating pressure and reversed flow. 3. Surge can damage compressors and must be detected quickly using methods like flow derivative to allow controllers to break the surge cycle and return to stable operation.

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KorichiKarim
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© © All Rights Reserved
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Download as PPS, PDF, TXT or read online on Scribd
218 views

Compressor Surge and Operation Rev1

This document discusses compressor limits and surge, including: 1. Stonewall, pressure, power, and speed limits constrain compressor operation, while surge is the most critical limit as flow can reverse rapidly. 2. Surge occurs when the operating point enters the surge region of the compressor map due to increased system resistance, causing oscillating pressure and reversed flow. 3. Surge can damage compressors and must be detected quickly using methods like flow derivative to allow controllers to break the surge cycle and return to stable operation.

Uploaded by

KorichiKarim
Copyright
© © All Rights Reserved
Available Formats
Download as PPS, PDF, TXT or read online on Scribd
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Compressor Operating Envelope

Pressure Limit
Surge Limit
Pressure

Speed Limit
Power Limit

Stonewall

% Flow

50 60 70 80 90 100

Compressor Limits
Usually stonewall or choke does not cause damage to
either the compressor or the process. It is the
maximum flow the compressor can handle.
Pressure, horsepower, and speed limits are imposed
through the compressor/turbine control system.
Surge limit is the most critical area of the compressor
map. Control action is only taken when the operating
point nears the surge region. Compressor flow can
reverse in less than 50 milliseconds.

COMPRESSOR SURGE

Surge Cycle
Pressure

4
5

Flow

1.

System resistance increases discharge pressure required.


Operating point moves up the curve.
2. Operating point nears the Surge Limit.
3. Operating point goes into the Surge Region.
4. Flow reverses as discharge pressure drops.
5. Drop in discharge pressure re-establishes forward flow.
Compressor resumes full flow.
Since resistance has not changed, surge will continue until cycle
is broken.

Surge Cycle (cont.)


Operating Point can move from stability
into surge in less that 50 milliseconds.
A complete surge cycle takes from 1/2 to 3
seconds, depending on the size of the
compressor.

Surge Detection (single stage)


Single Stage
Compressor

2 10

6
5

Hp

2 10

Surge
Line
1 10

4
0

SURGE

500

1000

1500

2000

2500

Inlet Flow

This data supports the expected pressure and flow changes during
a surge.
However, note that the Polytropic head changes very little, but
the flow changes wildly.

Surge Detection (multi-stage)


The controller must
discriminate between noise
& an actual surge condition.
GE employs several different
routines for detecting surge:
Flow derivative
Speed derivative
Discharge Pressure
derivative
Absolute minimum flow
value

Compressor Map during a SURGE

Hp 6 104

1
7

2
3

4
5 10

4
4 10

4
3 10

Surge Line

5000

4
1 10

2 10

SURGE DETECTION #1

Multi-Stage Compressor

4
2 10

ICMH

Breaking the Surge Cycle


In order to break the cycle, the controller must
hold open the recycle valve, even after the
operating point returns to a positive flow, then
slowly allow it to go closed and return to control.

Events Leading to Surge


Surge can occur as a result of any of the following
Occurrences:

Compressor Trip
Loss of Electrical Power to driver
Control Valves failing in the process
Intercooler failure
Changes in Molecular Weight
Process Load Changes
Startup and Shutdown
Process Upsets- either upstream or downstream of the
compressor

Process Control Near Surge Conditions


Performance Curve

Pc

Surge Line
Surge Control Line

N1

Q 1

N2

N3

Q2

P
o

As the operating point approaches the surge limit, the performance curve is flat,
resulting in a much greater change in compressor flow for the same change in
pressure ( Q 1). As the operating point approaches the SCL, there is little head
pressure change. The compressor will slide through this region easily.

Derivative and PID control is difficult near the SCL

Surge Effects on Process


Flow vs. Time

Discharge Pressure vs. Time

Temperature
vs.
Time

Surge Characteristics
Oscillation of flow and discharge
pressure
Increased vibration
Thrust reversal- axial vibration
Rapid increase in discharge temperature
Loud noises of gas flow through piping
and compressor

Compressor Damage from Surge


Opens internal clearances - wiping impeller seals
- wiping balance piston
labys
Damage to compressor shaft end seals
Damage to compressor thrust bearing
Damage to compressor radial bearings
Impellers rub against stationary diaphragm
Shaft coupling failure
Sheering of drive shaft

Consequences of Compressor
Surge
Unstable process flow and pressure
Damage to impeller and balance piston labyrinths,
increasing clearances and internal recycle, thus
reducing compressor efficiency
Large reversals of thrust and damage to thrust bearing
Increased vibration and damage to radial bearings
Rotating component rub against stationary
component
Torsional stress on rotor causing possible shaft or
coupling failure

Impeller Seals
P2

Labyrinth
Seals

P3
P1

SHAFT

Balance Piston Seals


Balance Piston Line - Internal Recycle
To Suction
P1
P1

P2

P3

P4

Net Impeller Thrust

Balancing Thrust

Log of Surge Event (1)

Log of Surge Event (2)

Compressor Performance
Hp

Q2 (ICFM)

There is one and only one Surge Point for the compressor (at a
given speed and compressor geometry) when performance is
expressed in terms of Polytropic Head and Flow. All of the inlet
variables are incorporated into the Head equation, which makes
this performance map valid for all set of inlet conditions.

Performance Curve Expressions


Careful attention must be given to the actual performance curve used to
define the surge limit line. Compressor Performance Curves are commonly
expressed using the following terms;

Y-Axis: Discharge Pressure (P2)- for a constant MW, P1 , and T1


Pressure Ratio (P2 /P1 ) -for a constant MW and T1
Pressure Rise (P2 - P1 ) -for a constant MW and T1
Polytropic Head- invariant to changes in inlet conditions
Adiabatic Head- invariant to changes in inlet conditions
X-Axis: ACFM (ICFM) - Actual Cubic feet per minute
MMSCFD - Million Std. Cubic feet per day- Volume at
standard conditions of 60oF, 14.7 psia.
SCFM - Standard Cubic Feet per minute- Volume at
standard conditions of 60oF, 14.7 psia.
For gases with variable inlet conditions, compressor performance
should be expressed as ICFM vs. Polytropic Head.

Effect of Speed
Hp

N1

N2

N3

Q2 ICFM

Compressors with variable speed must adjust the surge point


as a function of the speed of the compressor.

Effect of Adjustable IGVs


Hp

Closed Vanes

Open Vanes

Q2 (ICFM)

Compressors with adjustable inlet guide vanes must adjust the surge
point as a function of the vane angle. A position transducer for
guide vane angle is required as an input to the controller.

Variable Inlet Conditions


This graph illustrates the effects of variable inlet gas density. At a
constant speed, the pressure rise across the compressor changes with
inlet gas conditions.
The normal operating point is shown as A, which is the intersection of
the system resistance curve with the compressor curve.
Pc

Operating Point goes to A' with increase

in gas density; <P1, <T1


Operating Point goes to A'' with decrease
in gas density; >P1, >T1
Surge Point also moves

Q2
However, for Hp vs. Q , the surge point is fixed.

Compressor Performance Map


Surge Limit Line

Surge Control Line

Hp

Surge Limit Line

Surge Control Line

Hp
N1

N2

N3

Q2 (ICFM)

Q2 (ICFM)

Variable Speed Compressors

Constant Speed Compressors

Process Control Loop provides a speed


setpoint to Speed Governor to meet
process demand.

Process Control Loop provides output


to Throttle Valve (Suction or Discharge)
which throttles compressor flow to meet
process demand.

Compressor Operating Point


R

Pc

N1

N2 N3

Qs
The Compressor Operating Point is determined by
the intersection of the System Resistance Line with
the Speed Curve of the compressor performance map.
The Compressor reacts to changes in the system
resistance as well as to changes in inlet gas density.

Sample Surge
Map (1)

Sample
Surge
Map (2)

Sample Surge Map


(3)

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