surge in compressor

1. A phenomenon seen in the gas turbine engines.

2. Involved in the sudden decrease of supply pressure to the compressor inlet.

3. The rate of pressure fall is slower in the downstream of the compressor than that in the upstream of the compressor.

4. So, air flows in the reverse direction .

5. A vibration is created throughout the machine (engine).

6. This phenomenon is known as surge.

What is surge ?

Generation of Surge:

1.By DiffuserVanes:

a. The diffuser vanes retards the airflow due to frictional force.

b. The reason behind this is that it is very difficult to split the air uniformly so that themass flow of air is the same in each passage.

c. As the number of diffuser increases the tendency of surge occurrence also increases.

2.By Rotating Stall:

a. The non uniformity of air flow to the geometry of the channel between two bladescreates a breakdown of flow .

b. This increases angle of incidence in the upper blade (as shown in the figure 4.9) anddecreases angle of incidence in the lower blade .So, the upper blade stalls but themiddle blade pick up the flow again.

c. So, the stall transfers from one channel to another.d. At the impeller eye the flow rotates in the direction opposite to that of the impeller.

Compressor characteristics

Compressor characteristic is the curve to show the behavior of fluid like change in pressure, temperature,

entropy, flow rate etc. as it passes through the compressor at different compressor speeds.

Some phenomena of compressor Surge process

Surge cycle.

Surge point

Surge line

Surge control line

Surge margin

Choke/Stone wall point

Choke line

Constant efficiency lines

Maximum allowable speed

Minimum required speed

.

.

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Effects of surge in centrifugal compressor

Occurrence of surge in centrifugal compressor: The point at which the compressor cannot add enough energy to overcome the system resistance or backpressure is surge.

Damages in centrifugal compressor due to surge: This causes a rapid flow reversal (i.e. surge). As a result, high vibration, temperature increases, and rapid changes in

axial thrust can occur. These occurrences can damage the rotor seals, rotor bearings, the

compressor driver and cycle operation. Most turbomachines are designed to easily withstand occasional surging.

However, if the machine is forced to surge repeatedly for a long period of time, or if it is poorly designed, repeated surges can result in a catastrophic failure.

Of particular interest, is that while turbomachines may be very durable, the cycles/processes that they are used within can be far less robust.

Effects of surge in axial compressor

In the plot of pressure-flow rate the line separating graph between two regions- unstable and stable is known as the surge line. This line is

formed by joining surge points at different rpms.

Some surge consequences• Unstable flow and pressure

• Damage in sequence with increasing severity to seals, bearings,

impellers, shaft

• Increased seal clearances and leakage

• Lower energy efficiency

• Reduced compressor life

• Reduced compressor output

Causes of surge:The following are some of the usual causes of surge that are not

related to machine design.

Restriction in suction or discharge of system.

Process changes in pressure, temperatures of gas composition.

Internal plugging of flow passage of compressor.

Inadvertent loss of speed

Instrument on control valve malfunction

Malfunction of variable inlet guide vanes

Operator errors

Misdistribution of load in compressor

Mispositioning of rotor

Compressor stall

• A compressor stall is a local disruption of the airflow in a gas turbine or turbocharger compressor. It is related to compressor surge which is a complete disruption of the flow through the compressor.

• Modern compressors are carefully designed and controlled to avoid or limit stall within an engine's operating range. Stall was a common problem on early jet engines with simple aerodynamics and manual or mechanical fuel control units, but has been virtually eliminated by better design and the use of hydro-mechanical and electronic control systems such as Full Authority Digital Engine Controls(FADEC).

There are two types of compressor stall:

1. Rotating stall

2.Axi-symmetric stall or compressor surge

Notable stall occurrences

On January 15, 2009 US Airways Flight 1549, an Airbus A320, floating in

the Hudson River after bird strikes caused compressor stalls and complete

failure of both engines.

Prevention of surge:

Fig. Typical Anti-surge control system

∆P= Ch + b

Where, P=calculated compressor

differential signal

C= Control line slop (ratio signal)

h= inlet orifice differential signal

measured by FT

b=Control Line Bias

Fig. : Surge Control Line

When the calculated ∆P is greater than the measured ∆P, the compressor is operating to the right of the control line. When the calculated ∆P is equal to or less than the measured ∆P, the compressor is on or to the left of the control line and surge occurs.

Anti surge control system The anti-surge control system should maintain a minimum volume of flow

through the compressor so that the surge condition is never encountered.

This is achieved by bleeding flow from the discharge of the compressor to maintain a minimum inlet flow.

This flow can either be dumped to atmosphere or recirculated back into the inlet

of the compressor with the help of anti-surge valve. In the recirculation case, the

flow must be cooled to the normal inlet temperature.

For most applications, a simple control based on a flow differential is adequate for

this function. However, on compressor where the speed or the gas conditions are

variable, the anti-surge control may have to be more sophisticated to insure proper

operation under all conditions. This is frequently achieved by modulating the anti-

surge control with a signal for pressure, temperature, speed, or a combination of

parameters.

A description of the function of each component is as follows:

FE:- The flow element is usually an orifice located in the compressor suction, although it can be a venturi or calibrated inlet such as those used in axial compressors. Its purpose is to cause a temporary pressure drop in the flowing medium in order to determine the flow rate by measuring the difference of static pressures before and after the flow-measuring element.

FT:- The flow transmitter is a differential pressure transmitter which measures the pressure drop across the flow element and transmits a signal that is proportional to flow squared.

DPT:- The differential pressure transmitter measures the differential pressure across the compressor and transmits an output signal that is proportional to the measured pressure differential.

FX :—The ratio station receives the signal from the flow transmitter and

multiplies the signal by a constant. This constant is the slope of the control

line.

FZ :—The bias station receives the signal from FX, the ratio station, and biases

the surge control line.

• The ratio station must have both ratio and bias adjustment to enable the

control line to be placed as parallel to the compressor surge line as possible

(see Fig. )

∆P= Ch + b

Where, P=calculated compressor differential signal

C= Control line slop (ratio signal)

h= inlet orifice differential signal measured by FT

b=Control Line Bias

Fig. : Surge Control Line

FIC :-The surge controller is a flow control device which compares the calculated output

of FZ to the measured ∆P output of the DPT with ∆P as defined above.

When the calculated ∆P is greater than the measured ∆P, the compressor is operating to

the right of the control line. When the calculated ∆P is equal to or less than the measured

∆P, the compressor is on or to the left of the control line, and the surge controller

functions as a flow controller and opens the anti-surge valve as necessary to maintain

operation of the compressor on this surge control line.

For rapid flow changes, the response of the control system must be rapid to prevent

surge.

LAG :-This device functions to enable the surge controller to open the recycle valve

quickly, while providing a slow closure rate. This feature provides stability control system

and recycle valve.

LX:— The low signal selector is set up for two inputs and one output. The inputs are a

100% signal valve and the surge controller output signal. The output of the low selector is

sent to the recycle valve as well as back to the surge controller in the form of a feedback

signal. This prevents the surge controller from winding up. Windup of the controller

penalizes the reaction time of the anti-surge control system.

FCV:— The anti-surge recycle valve functions to prevent surge by recycling flow from the

compressor discharge back to the compressor inlet. Sizing of the anti-surge valve should be at 1.05% of design flow at design pressure rise.

Anti surge control system types:

1. Pressure oriented anti surge control system

2. Flow oriented anti surge control system

Pressure Oriented Anti-surge System

Flow Oriented Anti-surge System

Surge VS Stall

STALL LOCAL PHENOMENON

CAUSED BY FLOW SEPARATION

TEMPORARY INSTABILITY

ROTATING IN NATURE

MAY LEAD TO SURGE

MECHANICAL DAMAGE MAY OR

MAY NOT OCCUR

SURGE GLOBAL PHENOMENON

CAUSED BY FLOW REVERSAL

PERMANENT INSTABILITY

AXIAL IN NATURE

STALL MAY BE THE CAUSE, [TOTAL

FLOW BREAK DOWN]

DAMAGE TO THE COMPRESSOR /

TURBNE INESCAPABLE

ConclusionWhat we have presented so far in this presentation is that what is surge and

it’s generation. In this presentation we discussed characteristics of

compressor, phenomenon of surge like surge process, surge cycle, surge point,

surge line, surge control line, surge margin etc. We also mentioned here

causes of surge in centrifugal and axial compressor and it’s effect and

consequence in axial and centrifugal compressor and also the prevention of

surge occurrence(Anti-surge system).

ANY QUERY ?