Centrifugal Compressor Surge Centrifugal Compressor Surge Avoidance System DesignAvoidance System Design

Mr. Augusto GarciaMr. Augusto Garcia--HernandezHernandezDr. Robert McKeeDr. Robert McKee

Southwest Research Institute®, SwRI®

Rotating Machinery & Measurement Technology

OutlineOutline

IntroductionCentrifugal Compressor Surge EventDynamic Modeling

Basic ModelSimulations

Case StudiesEffect of Different Recycle Valves and TimingConclusions

Introduction Introduction

Centrifugal compressors are essential to the pipeline, process, and storage industries

Limitations on centrifugal compressor operations due to surge

Surge is a flow instability

Centrifugal Compressor LimitsCentrifugal Compressor Limits

1.06

1.07

1.08

1.09

1.1

1.11

1.12

1.13

1.14

1.15

1.16

400 600 800 1000 1200 1400 1600 1800 2000Inlet Flow ACFM

Pres

sure

Rat

io

80% Sp 85% Sp 90% Sp 95% Sp100% Sp Surge Limit Surge Control Stonew all

Max Speed / MAOP / Max Pow er

Surge Limit

Stone Wall

Min Speed

Surge Should be AvoidedSurge Should be Avoided

At surge, the flow pattern in the compressor collapses

Surge at energetic conditions is very critical

Surge disrupts station flows

Surge at low energy can be acceptable

Surge Avoidance for Normal Surge Avoidance for Normal Conditions Conditions

Operate at higher flows and lower heads

Change head and flow conditions slowly

Opening of the recycle valve to maintain a minimum flow

Flow recirculation through the compressor to prevent low flow conditions

In the Event of a Trip In the Event of a Trip

Unexpected events can require a sudden trip of the unit

Surge is essentially unavoidable

Will the head at surge be high (energetic) or low (acceptable)?

Design controls to avoid energetic surge

Purpose of Proper Surge Control Purpose of Proper Surge Control

Surging at high speed and head can damage the unit

Low speed and head, surge will be mild

Determine proper conditions to reduce the head rapidly

Determine appropriate conditions to maintain high flow through the unit

Dynamic ModelingDynamic Modeling

For routine operations changes occurs slowly, so modeling is not necessary

Rapidly changes and abnormalities in operating conditions required modeling

Proper design of the anti-surge system

Modeling Modeling –– The Basics IncludedThe Basics Included

A model for surge control design must include:

All pipe elements

Compressors with performance curve

Drivers with torque / speed responses

Recycle and other valves with capacity

Valve controls/actuators - response times

Scrubbers, heat exchangers, etc

Upstream and downstream piping

Control algorithms and sequences

Special Elements Special Elements

A centrifugal compressor is represented by a specific element

Valves that open or close (recycle) have changing Cv with stroke

Other special nodes like scrubbers or filters, heat exchanges, etc.

Control Systems and Sequences Control Systems and Sequences

Controls or elements that adjust or affect the system response

A valve controller and actuator with:Delay times

Non-linear rates

Integral, proportional, and derivative responses

Control results dependent on operating conditions (pressure, flow, speed, etc.)

Driver controls (power, inertia, and accelerations rates)

SimulationSimulation

Detailed data is required to build a good compressor system model

Boundary conditions must be determined but not over specified

An operating point should be established

Define scenarios for trips of the compressor

Time step within transient response

Effects of Normal OperationsEffects of Normal Operations

Design Operating Point, Speed 4800 RPM, 4.558 m3/s, 16559 ft

Compressor Coasting Down in 72 seg withRecycle Valve Opening in 1.8 seg

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 1 2 3 4 5 6 7 8

Flow Rate [m 3/s]

Hea

d [ft

]

0

10

20

30

40

50

0 50 100 150 200 250

Flow Rate [ft 3/s]

Hea

d [k

J/K

g]

5040

87%

3120

3600

4080

4800

79%

83%

85%

One Normal Surge TransientOne Normal Surge Transient

At trip, the compressor starts to coast down based on internal load and inertia

The recycle valve is signaled to open with it maximum opening rate

The speed, head, and flow decrease in a complex manner

Decreasing head increases flow so at times speed is neg. – flow is pos. - then flow neg.

Effect of Low Flow OperationsEffect of Low Flow Operations

Near Surge Operating Point, 4800 RPM, 4.068 m3/s, 17461 ft

Com pressor Coasting Down in 72 seg with Recycle Valv e Opening in 1.8 seg

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 1 2 3 4 5 6 7 8Flow Rate [m 3/s]

Hea

d [ft

]

0

10

20

30

40

50

0 50 100 150 200 250

Flow Rate [ft3/s]

Hea

d [k

J/K

g]

79%

83%85%

87%

3120

3600

4040

4800

5040

Results in Terms of Surge EnergyResults in Terms of Surge Energy

The point at which a compressor crosses the low flow limit (surge line) is dependent on the initial operating point.

The exact behavior is dependent on the details of the compressor, recycle valve, attached piping, and control times.

A small difference in the head at which the compressor crosses the surge line can be 5 to 10 percent less total power (energy).

Effect of Less Coast Down Time Effect of Less Coast Down Time

Much less coast down timeInitial conditions: 4800 RPM, 5.071 m3/s, 15649 ft

Com pressor Coasting Down in 36 seg with Recycle Valv e Opening in 1.8 seg

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 1 2 3 4 5 6 7 8Flow Rate [m 3/s]

Hea

d [ft

]

0

10

20

30

40

50

0 50 100 150 200 250Flow Rate [ft3/s]

Hea

d [k

J/K

g]

79%

83%85%

87%

3120

3600

4040

4800

5040

Effects of Recycle Valve Effects of Recycle Valve Size and SpeedSize and Speed

The main control for rapidly reducing head is the recycle valve size and speed

The trade off for recycle systems is a larger valve is better and a faster valve

There are times when a smaller fast valve is better than a larger slow valve but transient simulations are needed to determine this

Effect of a Larger Recycle ValveEffect of a Larger Recycle Valve

A large recycle valve for a large compressorInitial conditions: 4800 RPM, 7.244 m3/s, 14647 ft

Com pressor Coasting Down in 72 seg with Recycle Valv e Opening in 1.8 seg

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

0 1 2 3 4 5 6 7 8 9 10 11Flow R ate [m3/s]

Hea

d [ft

]

0

10

20

30

40

50

0 50 100 150 200 250 300 350 400

Flow Rate [ft3/s]

Hea

d [K

J/K

g]

3120

3600

4080

4800

5040

3360

3840

4320

4560

Results of Recycle Valve SizeResults of Recycle Valve Size

A recycle valve with a larger capacity and same opening time will allow a lower energy emergency shutdown.

In some cases it may be necessary to have both:

A fast small (hot gas) recycle valve

A large (slower) full capacity recycle valve

Transient modeling allows parametric studies of recycle sizes, times and options.

Effect of Different Recycle ValvesEffect of Different Recycle Valves

A large recycle valve can make a big difference

Vent Valve UsageVent Valve Usage

Vent valves on a compressor discharge

Vent valves:Smaller and faster

Remove a lot of gas from the discharge to flare

Short time (1 to 2 sec) reduce the head and energy of surge

Applications in multistage compressor

Effect of Vent Valve Open Time Effect of Vent Valve Open Time

Uses of a Discharge Vent Valve for a Short Time

Power Trip (Speed) DelaysPower Trip (Speed) Delays

A short time delay in tripping allows recycle and vent valves to open before speed drops.

Some alarms (such as high temps or vibrations) allow a short delay before tripping.

With extra discharge gas from large volumes or later stages, the head will remain high.

Check valves on discharge can isolate the volumes and allow head to be reduced.

Effect of Power (Speed) Effect of Power (Speed) Trip DelayTrip Delay

A short delay in removing power is positive

0 sec

1 sec

2 sec

Methods for Reducing Methods for Reducing Stored EnergyStored Energy

The stored energy in discharge gas volume can be reduced by:

The use of large fast recycle valves

The use and timing of vent valves

The timing of driver power changes and speed reductions

The use and location of check valves and discharge volumes

The control system tuning (timing and gains) and sequence of responses

ConclusionsConclusions

Surge is an instability and a collapse of flow through a compressor that occurs at low flows.

Surge at high energy and speed is a violent, damaging, and potentially dangerous event.

Transient flow solvers are used for dynamic simulations that aid the design of new or existing installations.

Larger and faster opening recycle valves will lower the energy of surge during a trip.

Location on the compressor map affects the time and energy of surge during a trip.

Conclusions Conclusions -- continuedcontinued

A more rapid speed reduction will results in a more energetic shutdown surge.

In multi-section compressors, vent valves can remove the high pressure discharge gas energy.

A delay in speed reduction can allow a head reduction before a low flow surge is reached.

Discharge check valves can isolated upstream high pressure gas to allow a lower energy surge.

Transient simulations of compressor trips are necessary and the best way to design surge control systems to avoid energetic surges.

Questions ?Questions ?

Thank you for attention!Thank you for attention!