simulation and compressor control case study

Confidential Property of Schneider Electric

Turbomachinery Simulation and Compressor Control

Case Study

Presented by: Hector Buchelly

Compressor Components and Operational Safety

Dynamic Simulation

Case Study

Application of Dynamic Simulation

Agenda

Confidential Property of Schneider Electric |

Compressor Components and Operational Safety


– Compressor

– Piping

– Heat exchangers

– Knockout drums

– Anti-surge valves

– Instrumentation

– Control system

F T

P

T

Elements are designed to maintain throughput and

keep the compressor at a safe operating point.

Compressor System Components


Surge

Surge is a condition at which the

compressor cannot raise the pressure

any higher without causing a complete

breakdown of steady flow.

Choke (Stonewall)

Because the compressor operates at a

very high mass flow rate, flow through

the compressor cannot be increased

after sonic velocity is reached.

Compressor Operational Safety


Pd

Ps

hPs

Minimum Flow Control


Pd

Ps

hPs

Maximum Pressure Control



• Defining the Operating Point

• Locating the Surge Limit

• Surge Acceleration (Curve

Shape)

• Interacting with Control Loops

• Meeting Load-Sharing

Requirements

Surge Control Application

Challenges


What is Surge?

A condition wherein the compressor is unable to

maintain a greater force than system resistance.

The Surge Cycle

1. System resistance

increases discharge

pressure required

(operating point moves

up the curve)

2. Operating point nears

the Surge Limit

3. Operating point goes

into the Surge Region Flow

4

Pressure

1

2 3

5


The Surge Cycle

4. Flow reverses as

discharge pressure drops

5. Drop in discharge

pressure re-establishes

forward flow (Compressor

resumes full flow)

Since resistance has

not changed, surge will

continue until cycle is

broken

Flow

4

Pressure

1

2 3

5



An operating point can move from stability

to surge in less than 100 milliseconds!

A complete surge cycle takes from ½ to 3 seconds,

depending on compressor size and piping volume.


• The operator sees

process upset because

of flow reversals

• The machinery person

sees damage to seals

• The plant manager sees

catastrophic compressor

failure if surge is severe

Why is surge bad?

Several viewpoints:

What is dynamic simulation?

Page 17 Confidential Property of Schneider Electric |

How does simulation work across industries?


Real Plane Virtual Plane

Real Plant Virtual Plant

How does simulation work in a plant?


• Shows components and

thermodynamics

• Includes rigorous dynamic behavior

model of equipment and pipes

• Provides a high-fidelity rendering of

instruments and controls

• Enables testing of scenarios and

alarms

• Helps train operators

What is dynamic simulation?


Safely study the integration of TMC equipment and controls

Boost system reliability

Avoid costly errors, thereby reducing expenses

Improve efficiency in real time

Reduce downtime during operation by debugging the system

Increase machinery availability

Why use simulation?


Case Study



Let’s consider a new field

unit consisting of:

• Elliot Steam Turbine

(1200 HP, 660# inlet

pressure)

• Atlas Copco Ethane

Kickback

Compressor (TP-

14/10)

Case Study

System Overview


Control Hardware:

• Two-chassis Tricon System

• Bently Nevada BN3500

Control Software (Tristation 1131):

• Turbine – Compressor Start Stop Control

• Turbine Speed Control

• System Shutdowns

• Compressor Surge Control

• Decoupling

• Lube Oil Aux Pump Control

DCS: Suction Pressure Control in DCS (Guide Vanes)

Scope of Supply


SimSci products:

• DynSim for the dynamic model

• TriSim to connect model to control program (PT2)

via Tristation Emulator

Modeling Software


• Local Operator

Panel

• Wonderware HMI

• Honeywell DCS

(represented in

model)

Operator Interface


Simulation Architecture


System: Turbine and

Process

Control

System

Application

Human

Machine

Interface

Simulation: Dynsim Trisim Tristation

Emulator

Wonderware

Recycle valve critical flow

Motor torque curves

Shaft rotating inertia WK2

Piping and KO drum volumes for accurate

transients

Performance curves

Fluid feed composition

Intercoolers

Vapor-liquid equilibrium “flash”

Dynamic Simulation: Rigorous Model


Model Flow Sheet


We start with the P&IDs, and then complete data input for each component.

Building the Model


Building the Model: Added Controls


Application of Dynamic Simulation


Process Design

Controls Configuration

Start Up

Plant Operation

Applying the Model


Before completing the Front End Engineering

Design study or beginning commissioning,

verify elements of the compressor system:

• Compressor specifications

• Anti-surge valve sizing

• Compressor trips, driver failure, loss of instrument air, etc.

• Valve opening time

Field Hardware Verification


• Ascertain the best anti-surge &

performance control approaches

• Design and prove the value of Triconex

decoupling control algorithms

• Establish start-up and shut-down

sequencing

• Position the startup guide vane

• Position T&T valve

Control Strategy Definition


Universal Surge Line

20 40 60 80 100 0

1

2

3

4

5

Pd

Surge Line

Ps

__

h Ps

__ %

Speed

Curves


Setpoint Hover

20 40 60 80 100 0

1

2

3

4

5

Pd

Surge Line

Ps

__

h Ps

__ %

Control Line

Current Operating

Point

Setpoint Hover

Hover Setting


Control Line Recalibration

20 40 60 80 100 0

1

2

3

4

5

Pd

Surge Line

Ps

__

h Ps

__ %

Control Line

Current

Operating

Point

Recalibrated

Control Line


20 40 60 80 100 0

1

2

3

4

5

Pd

Control Line

Surge Line

Ps

h Ps

%

0%

100%

Valve Output

Surge

Override Line

Surge Override


Decoupling Capacity Control

Breakpoint is stationary if Surge

Margin is between 1% and 3%:

• Speed is not reduced further, since

this would drive the compressor into

surge

• Further decrease in Process

Controller Output will immediately

open recycle valve

Breakpoint moves to right if Surge

Margin < 1%:

• Increases speed setpoint to speed

up compressor, keeping Surge

Margin >1% Page 41 Confidential Property of Schneider Electric |

Process Design


Start Up

Plant Operation

Applying the Model


• Closed loop testing during

FAT

• Software testing independent

of hardware

• Loop tuning before

commissioning


Testing

h/P1

Max

Gain

Normal

Gain

Min

Gain

Surge

Line

Control

Line

Adaptive Tuning


Quickly responds to surge condition but doesn’t exceed response time of system

Fast Opening

Allows turbine speed controller/ process cascade controller to smoothly adjust to new process operating conditions

Slow Closing

Fast Opening, Slow Closing


Manual Recycle Valve Control

• Operator can open the recycle valve only when above the surge controller demand

• Operator cannot close the recycle valve below the surge controller demand

Partial Authority

• Operator can close and open the recycle valve with no surge protection

• Useful to stroke test recycle valve prior to startup

• Useful in some fallback scenarios

Full Authority


• Determine effectiveness of

controls

• Debug any control logic on the

system rather than on the

actual machine in field

• Establish initial controller

tuning

• Evaluate overrides that may

be necessary for particular

upsets

Pre-startup Controller Tuning


Discovery:

• Speed couldn’t be

well controlled during

startup

Why?

• Design minimum stop

on IGV meant too little

mechanical load

Fix?

• Use model to find

optimum minimum

stop for IGV

Tune Speed Controller Pre-commissioning


Process Design


Start Up

Plant Operation

Applying the Model


Shorter

Commissioning

Schedule

Error-free

Startup

Operator

Training


Benefits: Initial Startup Phase

• Final software

testing

• Controller tuning

using normal

field techniques

• Surge line

testing

Final Verification Pre-startup


Discovered:

• Even though a full FAT

was performed,

simulation allowed us to

find and correct minor

logic errors. We also

made changes to the

HMI as requested by

operations.

Result:

• Saved time during

commissioning. Normal

schedule for a unit is 4–

5 days. Actual time

spent: less than 1 day.

Pre-commissioning Logic and HMI Checkout


Process Design


Start Up

Plant Operation

Applying the Model


Operator Training

Event Analysis

Process Modifications

Control Modifications


Discussion:

How can we use the model when the system is online?


Perform Transient Tests Offline


Offline Test

With simulation, we are able to perform

multiple system upsets that would be

impossible to test in the field.

Results: we can tune the system

accurately and gain confidence in the

controls.

What concerns has simulation identified in actual situations?

• Recycle valve too small

• Recycle valve too slow

• One stage surges on shutdown

• EPC control strategy is “not optimum”

Before implementation, simulation can verify process

changes. Even with basic models, simulation enables more

effective closed loop software and HMI testing.


Identifying Concerns Via Simulation


KEY TAKEAWAYS

•Simulation saves money and time.

•It improves safety and increases availability.

•We have extensive experience with dynamic simulation modeling.

•Our model ties directly to the actual controls and HMI—a huge differentiator.

Questions?


THANK YOU.


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