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SURGECompressor surge is an unstable phenomenon where the gas flow forward and backward across the machine alternatively. This phenomenon occurs at certain conditions of flow, pressure ratio and rotational speed.

As a consequence some damages can occur:

• shaft vibration

• instability in axial thrust

• bearings seizure

• seals damage

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FLOW

TIME (sec)

PRESSURE

TIME (sec)

TEMPERATURE

TIME (sec)

MAJOR PROCESS PARAMETERS DURING SURGE

Rapid flow oscillations

Rapid pressure oscillations with process instability

Rising temperatures inside compressor

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SURGE FEATURES

• FLOW REVERSES IN 20 TO 50 MILLISECONDS

• SURGE CYCLES AT A RATE OF 0.3 sec TO 3 sec PER CYCLE

• COMPRESSOR VIBRATES

• TEMPERATURE RISE

• “WHOOSHING” NOISE OR “CLANKING” NOISE

• CONVENTIONAL INSTRUMENTS AND HUMAN OPERATORS MAY FAIL TO RECOGNIZ SURGE

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g OIL & GAS CUSTOMER TRAINING CENTER

FACTORS LEADING TO ONSET OF SURGE

• DURING NORMAL OPERATION WITH:

- High discharge pressure ( Increase of plant resistance )

- Reduction of suction pressure

- Increase of suction temperature

- Gas composition changes (Decrease molecular weight)

• START-UP

• SHUTDOWN

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g OIL & GAS CUSTOMER TRAINING CENTER

g OIL & GAS CUSTOMER TRAINING CENTER

ANTISURGE SYSTEM

An antisurge system avoids that the machine operating point falls on the surge line.

Substantially the protection consists in the opening of a control valve that put in connection the compressor discharge to the suction.

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On the right of the surge line a protection curve, called “antisurge line”, is defined.

As a consequence the “resistance curve” become the antisurge valve resistance itself.

This causes an increase in the suction flow and so the operating point moves away from the critical condition.

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g OIL & GAS CUSTOMER TRAINING CENTER

A compressor with a single stage, or, if multistage, realizing a limited compressor ratio and where the surge is determined by the first stage, can be regulated by a law of this simple type: h

PD-PS

> Kh= Inlet flow index

PD = Pressure discharge

PS = Pressure suction

K = Set-point that corresponds to the antisurge control line

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SLL FOR A COMPRESSOR WITH ONE IMPELLER

SLL FOR A COMPRESSOR WITH MORE THAN ONE IMPELLERS

ρ

Q

SLL

speed

Q

SLLρspeed

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CONTROLL ALGORITHM

2*

K

P

PPf

P

Ph

s

sd

s

sds

 The controller set-point is K2. It represents the deviation margin beteween SLL and SCL. The margin is expressed in percentage of flow. The algorithm takes in account the deltaP measured across the flow element (indicated by “hs” that is proportional to square suction flow (hs = Q2).

The controller algorithm compensates continuously the hs signal according to the suction pressure variation, taking in account the suction pressure measured and comparing it with the design suction pressure.

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THE SURGE LIMIT LINE (SLL) IS NOT A FIXED LINE.

IT CHANGES DEPENDING ON THE COMPRESSOR INLET CONDITIONS:

T P MW

THE ANTISURGE CONTROLL MUST PROVIDE A DISTANCE TO SURGE CALCULATION THAT IS INDEPENDENT OF ANY CHANGE IN INLET CONDITIONS

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PressureRatio

Suction Flow

Compressor Operating Point

Surge Limit Line

Safety Protection Line o Recicle trip line

Safety Correction Line

Surge Control Line

Speed Line

CONTROLL CURVES This curve is drawing at 2% on the right of SLL. When the OP reach this line the controller generate a “Quick-Opening” signal to the valve.

This curve is drawing at 5% on the right of SLL. When the OP reach this line the controller generate a “Step-Opening” signal to the valve.

This curve is drawing at 10% on the right of SLL. When the OP reach this line the antisurge controller is activated

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g OIL & GAS CUSTOMER TRAINING CENTER

CHOKING

When the flow rate increases beyond a certain value it may happen that in some ducts is achieved the speed of the sound.

This phenomenon determine the pressure drops due to the friction and high incidence of the flow on the fixed and mobile ducts.

As a consequence there is an enormous reduction in efficiency.

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SOME DEFINITIONS ABOUT PERFORMANCE CURVES

ACCORDING TO NORM API 617

FIELD OF OPERATION

NORMAL OPERATING POINT:

The point at which usual operation is expected and optimum efficiency is desired.

NORMAL SPEED :

The speed corresponding to the requirements of the normal operating point.

100 PERCENT SPEED: is the highest speed required for any specified operating point above the normal speed curve.

MAXIMUM CONTINUOUS SPEED:

The speed at least equal to 105 percent of the highest speed required by any of the specified operating condition.

MINIMUM ALLOWABLE SPEED:

Is the lowest speed at which the manufacturer’s design will permit continuous operation.

STABILITY: The difference in capacity (in percent of rated capacity) between the rated capacity and the surge point at rated speed.

TURNDOWN: The percentage of change in capacity (referred to rated capacity at the rated head).

COMPRESSOR RATED POINT:

The intersection on the 100 percent speed curve corresponding to the highest capacity of any specified operating point.