centrifugal compressor

Understanding centrifugal compressor equipment interaction

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Equipment Interaction

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Equipment Interaction

The compressor is taking suction from a source,

point A at pressure P1, and discharging at

pressure P2 at point B into a process system

consisting of pipelines, valves and equipment.

The gas is transported through this system to be

delivered to point C at pressure P3. The line

pressure drop, (P2-P3), is usually denoted as p

(delta P) of the system

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System curve

Plot of pressure drop versus flow gives the

system curve. The total pressure drop has

two components, the superincumbent

pressure or static pressure difference

between Points B and C, and the dynamic

pressure drop, i.e., the drop due to friction,

velocity change.

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System curve

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Compressor Curve

The flow Q, HEAD, P, and speed N

relationship of a centrifugal compressor . It

can be seen that pressure increase as the

flow drop till reaches the surge line.

Similarly if the flow increase it reaches the

stone wall limit for the particular speed.

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Compressor Curve

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Relationship between Compressor and System Curves.

When a compressor is connected to a system, the

operating point can be obtained by superimposing

the system curve on the compressor curve, it can

be noted that although a system is quite

independent of its upstream source equipment, in

this case of compressor; operation is very much

dependent on its downstream connected system.

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Relationship between Compressor and System Curves.

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Single-speed Compressor

Simple arrangement

of a centrifugal

Single-speed

compressor drive

with

suction/discharge

pressure control

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Incoming Flow to Compressor Decreases

Suction pressure reduces

Discharge control valve closes

System curve moves to left with new operating point.

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Incoming Flow Drops Below Surge Control Point

Discharge control valve closes

Anti surge valve opens

New system curve and operating point is located

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Gas Demand Increase at the Consumer End

Flow increases

System curve shift to left

Pressure drop increase

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Summary

In summary, for all single-speed

compressors, variation in flow is achieved

only by varying the system curve position

backward and forward. The operating point

shifts along the compressor curve,

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Variable-speed Compressor Drive with Speed Controller

The speed is changed by a variable-speed

driver to achieve varying flow-pressure

requirements. The driver is often a gas or

steam turbine, although variable-speed

electric motors are also used in certain

cases.

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Variable-speed Compressor Drive With Speed Controller

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Incoming Flow to Compressor Decreases

Suction pressure

reduces

Speed reduces

New operating

point is reached

where the

discharge flow

equals the

reduced incoming

flow .

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Incoming Flow Drops Below Surge Control Point

Speed reduces to minimum

Anti surge valve open to control the flow

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Gas Demand Decrease at the Consumer End

Flow decreases

System curve shift to

left

Speed reduces till it met

the compressor curve at

reduce flow rate

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Compressor Delivers at a Fixed Pressure to Consumer, Set Pressure Changed

Static pressure moves down

Flow increases

Suction pressure drops

Speed reduced

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Summary

when a compressor is driven by a variable-speed

drive (gas turbine, steam turbine, etc.), the

operating point may shift either along the

compressor curves, the system curve, the fixed

pressure line, or a combination of them

depending on the nature of change as discussed.

This is quite different from a compressor driven

by a single-speed drive (electric motor) described

earlier:

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Gas Molecular Weight Changes

Molecular weight decrease pressure decrease

Increase speed

If on margin new curve will be formed and flow reduces.

Increases suction pressure

Molecular weight increase, pressure increase

Flow increases new curve will be formed

If gas is not available

Speed reduces

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Conclusion

Compressor-like a centrifugal pump can be

operated anywhere on its curve between surge

point (or surge control line, SCL) and stonewall

by shifting the system curve, provided the driver

is adequately sized. However, if the increase is

due to a change in MW where the compressor

curve itself shifts, this should be limited to the

specified MW variation range.

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Number of Impellers at Different Compression Ratios

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Characteristics Shape

Impeller and diffuser geometry influence compressor performance

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Blade Angle

If blade angle is zero resulting basic slope

normally shows 2 or 3 percent head rise when

going from design flow to minimum flow . If

angle is 45 impeller will typically produce a head

rise of 20 percent or more, when moving from

design flow to minimum flow. industry standard

for conventional closed impellers is represented

by the middle line, which is 25 to 35" of

backward lean.

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Blade Angle