Anti Surge Valves For Copressor

8/9/2019 Anti Surge Valves For Copressor

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Compressor Anti Compressor Anti - - Surge Surge

V V alve alve

V.E.S – TOYO India


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Agenda Agenda Agenda

Surge Mechanism

Surge Impacts

Surge Control

Anti-Surge Valve Issues

Fisher – CCC Agreement

Invitation for Jemima ShowInvitation for Jemima Show


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Surge is an inherent phenomenon in compressors

Surge limit is determined from compressor performance

curve - a plot between discharge pressure and flowthrough the compressor at a given speed for a particulargas at an inlet temperature

There is a curve plotted for each speed and their

compilation is called “wheel map”

Surge Mechanism Surge Mechanism Surge Mechanism


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Impeller has to be coupled to theload

– This depends upon a flowthrough the impeller

– While flow is sufficientimpeller keeps “grip” on the

gas – There is a transfer of energyfrom impeller to gas

– This needs a minimum flow

rate


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Surge is characterized by fast flow reversals through acompressor, and is caused by a large scale breakdown offlow patterns within the compressor.

It occurs when flow decreases below a certain minimumpoint.

The flow pattern in the compressor becomes unstable

and fluid move back through the compressor from highpressure side to the low pressure side.

Flow reversals are only temporary and forward flowquickly re-established.



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Forward

Compressor Discharge Press

XXX RPM

XXX RPM

XXX RPM

XXX RPM

XXX RPM

Compressor Inlet Flow

Compressor “Wheel Map”

Design point


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Backward Forward

Compressor discharge press


Impeller is at constant speedHead (discharge press) varies with Downstream load (flow)

Normaloperating point


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Backward Forward

• Sudden decrease in gas demand• Increases head (discharge press)• Flow decreases

Operating pointCompressor discharge press



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Backward Forward

• As flow continues to decrease• Head increases until it reaches max for that speed

Max pressure heador Surge pointCompressor discharge press



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Backward Forward

Operating point

Flow is a minimum where the impeller can maintain its “grip” on the gas. Impeller becomes decoupled from load

Gas in the impeller is no longer pushed forward and spins with the impeller

Gas stops moving and operating point moves to zero flow




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Backward Forward

Operating point

Downstream compressed gas leaks back around the impeller Downstream head reverses gas flow

Flow moves into negative flow regime




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Backward Forward

Operating point

This flow reversal pushes impeller to opposite side of the housing

The head bleeds to the minimum maintained by the impeller bycentrifugal force




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Backward Forward

Operating point

The impeller picks up the load, pushing the gas forward to newoperating point

If line flow remains low for the speed, the cycle repeats Cycle time approx 1 sec

Causes catastrophic damage to compressor




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Backward Forward XXX RPM

XXX RPM

XXX RPM

XXX RPM

XXX RPM

Compressor Surge LineCompressor discharge press


“Surge Point” is a point on each curve where dischargepressure is maximum as flow is reduced progressively

A line joining surge points from each curve is called “SurgeLine”


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Can lead to damage of seals, bearings, impellers, shaft, casing,pedestal, etc.

– Cost of replacing Seals in a 3,000 hp compressor can easilyexceed US$ 20,000

– Bearings – US$ 10,000

– New rotor assembly – US$250,000

Reduce compressor life span

– Typical life span is 30 years or more if operated andmaintained properly

To prevent surge from happening it is required to ensure that theflow through the compressor never falls below the surge line

Surge Impacts Surge Impacts Surge Impacts


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Surge Control Surge Control Surge Control

Not easy and not efficient to adjust compressor speedto control surges

Surge control system opens recycle valve – Opening recycle valve too soon will result in

wastage of fuel / energy and capacity

– Therefore valve should be opened only when theoperating point is very close to surge and closedimmediately after it exceeds the surge limit


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Surge prevention is achieved by a dedicated controlsystem, separate from compressor’s other controlsbecause of high speed of response requirements

A surge relief valve, also referred as anti-surge valve, isthe final control element in the surge control

The valve function is to open very quickly when

demanded and re-circulate the gas from discharge tosuction and ensure that minimum flow goes through thecompressor when it is running

Surge Control Surge Control Surge Control


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Antisurge Valve Issues Antisurge Antisurge Valve Issues Valve Issues

Protect the Compressor

– Reliability (3 years between maintenance cycles)

– Fast Stroke Ability (< 2 seconds closed to open)

– Accurate Step Response (Fast acting, no hunting,minimal hysterisis)

– Ease of Tuning (Ability to easily tune in the field) – Eliminate Noise & Vibration


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Antisurge Valve Performance - Fisher

& CCC agreement

Antisurge Antisurge Valve PerformanceValve Performance - - FisherFisher

& CCC agreement & CCC agreement


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CCC Guidelines for Antisurge Valves CCC Guidelines forCCC Guidelines for Antisurge Antisurge Valves Valves

Valve Actuation

- Stroking time with Solenoid Trip = Less than 1 second

- Stroking time with Positioner Control = Less than 2 seconds- Positioner should be used

- Stroking time should be roughly equal in the opening andclosing directions

- Quick exhaust valves should be avoided. Volume boostersshould be used instead.

Valve Sizing

- Linear valve trim should be used

- 1.8 ~ 2.2 x (Cv @ max perf. curve and surge limit line)


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Anti Surge Valves For Copressor

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