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Authors: Jihong Wu - KBR, USA Jeffrey Feng - KBR, USA Surajit Dasgupta - KBR, USA Ian Keith - Woodside Energy, Australia Publication / Presented: LNG journal Date: October 2007 A REALISTIC DYNAMIC MODELING APPROACH TO SUPPORT LNG PLANT COMPRESSOR OPERATIONS

Compressor Anti Surge Dynamics

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Authors:Jihong Wu - KBR, USAJeffrey Feng - KBR, USASurajit Dasgupta - KBR, USAIan Keith - Woodside Energy, Australia

Publication / Presented:LNG journal

Date:October 2007

A REALISTIC DYNAMIC MODELING APPROACH TO SUPPORT LNG PLANT COMPRESSOR OPERATIONS

LNG journal • October 2007 • 27

ENGINEERING FORUM

In LNG plants, refrigeration compressors

are among the most critical components

in terms of both capital investments and

operational costs.

Safe, stable and sustained operation of

the refrigeration compressors represents

a key component of maximizing on-

stream time and production.

Dynamic simulation has been

increasingly used in the various stages of

the LNG process life cycle, to perform

design optimization, identify production

limiting constraints, and validating the

dynamic, or time-dependent, responses of

the process [1],[2].

To accurately and reliably predict the

dynamic behavior of real world systems,

the dynamic model has to be

supplemented with accurate input data

based on as-built equipment

performance.

Once a dynamic model is developed, it

should be validated against design and

actual operating data to ensure the

accuracy of the modeling. While model

validation in steady state is quite routine,

it is a less frequent practice to validate

the model dynamically, mainly because of

the challenge of obtaining reliable

dynamic data.

Also, the controllers used in the

dynamic simulation models for general

purpose studies are often simplified

based on basic control strategies, and

thus do not represent the functionality of

actual field controllers.

The combined challenges of dynamic

validation and the lack of full

representation of the controls, has been a

hurdle for the routine application of

dynamic simulation for field and

operations support.

These challenges also point to the

opportunities for applying dynamic

simulation to evaluate real world LNG

compressor systems that can experience

a wide range of operating conditions and

whose controls have to be designed to

handle both mild disturbances and

emergency situations.

Novel solutionThis article describes a novel solution

applied in a recent study to address these

challenges.

A dynamic study was conducted by

KBR to provide support for the operation

of the refrigeration compressors in an

LNG facility operated in Australia by

Woodside Energy Ltd. After evaluating

the accuracy requirements, Woodside and

KBR decided to use a direct control-

hardware linked simulation approach

instead of conventional software

emulation, to accurately simulate the

functionality of field-installed controllers.

During the study, an integrated

software-hardware solution was

developed by linking a rigorous plant

dynamic model to a control vendor

supplied controller emulator.

This integrated tool was validated

against dynamic data from actual plant

events and thus greatly enhanced the

ability and precision of the dynamic

simulations. In the study, plant data

collected actual transient events and was

used to validate the rigorous dynamic

model, as well as the integrated

simulation approach.

During the validation process, the

simulation results also enabled in-depth

analysis of the actual plant events.

Several critical operating scenarios were

studied to help solve compressor

operational issues at the plant and improve

control system logic to provide adequate

protection for the compressors under

extreme operating conditions or upsets

Compressor surge, which is

characterized as high-speed and high-

energy flow reversals inside the

compressor, can cause damage to

compressor internals, reduced

compressor life time, and loss of profit

due to downtime and costly repairs.

Emergency shutdown, loss of power

source and operation at reduced

throughput are some of the factors that

can lead to the onset of surge.

Surge factorsAlthough the compressors and anti-surge

systems are designed for a range of

operational conditions, actual feed

variations, changing operating conditions

and production demand, and other

operational requirements may require

the compressors to be operated under less

than ideal conditions and thus can lead

to the occurrence of undesirable

incidents.

The focus of the current study is a

Mixed Refrigeration (MR) compressor

system in the Woodside LNG facility.

Figure 1 shows a simplified schematic of

the MR circuit in the LNG liquefaction

process.

The MR compressor train consists of

an axial stage and two centrifugal stages.

The drive power is supplied by a gas

turbine and a variable speed helper

motor. High pressure vapor from the

discharge of the compressor train is

chilled against vaporizing propane

provided by a cascading Propane

Refrigeration (PR) system.

The chilled MR is then fed to a high-

pressure separator where heavy MR and

light MR are separated before entering

the Main Cryogenic Heat Exchanger

(MCHE). In the MCHE, the MR is further

cooled to cryogenic conditions and used to

liquefy the natural gas feed.

In the MR compression system, the

compressors are subjected to high flow –

high-pressure ratio operating conditions.

In particular, the Low-Pressure (LP)

axial stage is operated with the highest

compression ratio and is most vulnerable

to surge.

Among various operating scenarios,

emergency shutdown, or compressor

trip, represent the high-risk cases

which can expose the compressors to

surge conditions. Other major process

upsets, such as a trip of the MCHE or

propane system, can also cause

unwanted shutdowns of the MR system

without adequate control strategies,

and impose potential risk of surge on

the compressors.

The objective of the dynamic study

was to analyze the risk of surge under

these scenarios and improve the anti-

A realistic dynamic modeling approach tosupport LNG plant compressor operationsJihong Wu, Jeffrey Feng, Surajit Dasgupta, KBR, USA, and Ian Keith, Woodside Energy, Australia

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ENGINEERING FORUM

surge control strategies to mitigate

the risk.

Two critical cases identified by plant

operation were studied. The first case

was a propane compressor trip scenario,

which in several occasions had caused

the MR compressor to trip subsequently.

The second case was the emergency

shutdown operation, where surge events

had been observed during compressor

coast down.

Control systemThe control system for the MR

compressors is supplied by Compressor

Control Corp. (CCC). The CCC control

system consists of three anti-surge

controllers, one for each stage, as shown

in Figure 2.

The locations for measuring devices

that supply analog inputs to the

controllers are also indicated in the

Figure. Based on the analog inputs, each

controller determines the position of the

compressor operating point and provides

one output to the recycle valve.

The anti-surge controllers use a

combination of closed- and open-loop

control responses to provide anti-surge

protection for the compressors [3]. The

primary anti-surge control responses

that are triggered depending on the

location of the compressor operating

point are:

1) Proportional Integral (PI) control

response: The anti-surge PI control

response is the normal first level

response serving to increase the recycle

rate when the operating point is on the

left side of the Surge Control Line (SCL),

and will reduce the recycle rate when the

operating point is on the right side of the

SCL.

2) Recycle Trip response: The anti-

surge Recycle Trip response provides a

2nd level correction to the normal PI

response by step increasing the recycle

rate when the operating point is on the

left side of the Recycle Trip Line. This is

an open loop response that is

implemented to prevent surge if the

normal PI response is not sufficient.

An illustration of the compressor

performance curve and major control

lines calculated by the CCC controller is

provided in Figure 3. The illustrated

compressor operating point is in the

stable operating region.

In case of an emergency shutdown, a

trip signal from the Central Control

System will directly initiate the opening

of all anti-surge valves.

In addition to the anti-surge valves,

the LP axial stage is also protected by a

hot gas bypass (HGBP) recycle loop. The

axial stage inter-stage bleed valve (IBV)

will also be opened to provide sufficient

flow to the compressor suction.

Dynamic Simulation Model: A rigorous

dynamic simulation model was developed

using Aspen Custom Modeler (ACM), an

advanced equation based software

licensed by AspenTech.

The simulation model covered the

entire MR circuit as shown in Figure 1,

with input data based on as-built

equipment and piping details. The

compressors were modeled on tested

compressor curves provided by the

compressor vendor.

The accuracy of the base model was

first validated in steady state against

plant operating conditions. Dynamic data

such as the axial stage Inlet Guide Vane

(IGV) closing speed, control and actuator

delay, valve stroke time, and other system

dynamics were based on available data

from the plant.

The dynamic model was further

validated dynamically against plant data

collected during actual transient events.

The dynamic validation of the simulation

results will be described in more detail.

Using the rigorous dynamic model,

compressor operating conditions were

generated according to plant operational

sequences.

Integrated Simulation Approach: To

ensure accuracy of the study results, a

CCC-supplied control emulator was used

to emulate the actual controllers.

Together with configuration files

downloaded from the plant CCC system,

the emulator is able to replicate the exact

control functions of the field-installed

controllers, and thus allow in-depth

analysis of the actual compressor and

control system in the field.

Figure 4 shows a schematic view of the

software-hardware assembly used in the

dynamic study. The system includes the

CCC emulator hardware and a single PC

on which the dynamic simulation

software and emulator operating

software were operated.

Data link between the simulation

model and emulator was accomplished

through OPC server interfaces. The

values of process variables calculated by

the dynamic model, such as flows (as

pressure differentials), pressures,

temperatures, valve positions and speed,

are supplied to the CCC emulator as

inputs.

p23-30:LNG 3 12/10/2007 12:16 Page 6

LNG journal • October 2007 • 29

ENGINEERING FORUM

The emulator-calculated controller

outputs are sent back to the dynamic

model to control the position of the

recycle valves. With control parameters

downloaded from the plant CCC system,

the emulator would function the same

way as the field control system. The

emulator would also allow the tuning of

the parameters so that new control

strategies could be evaluated and new

tuning parameters could be tested.

A Visual Basic (VB) based script was

developed to externally control the

execution of the dynamic model, which

subsequently controls the execution steps

of the CCC emulator. Complete

synchronization between the dynamic

model and emulator was validated before

the integrated tool was put in use.

Simulation resultsThe integrated simulation tool was

validated against plant data collected

from a previous MCHE trip event, which

resulted in a trip of the MR compressor

train and surge of the compressors.

Dynamic data collected from the actual

event was used to validate the simulation

results.

According to the plant control logic, an

MCHE trip will initiate the following

operation sequences.

� Close the axial compressor inlet guide

vane (IGV)

� Stop heavy and light MR flows by

closing the MR flow control valves

� Anti-surge control valves open when

compressor approaches SCL

With the opening of the anti-surge valves,

the MR compressors were expected to

remain on line in stable recycle operation.

However, in the actual event, the MR

compressors became unstable after the

MCHE trip, and eventually resulted in a

shutdown of the compressor train.

Plant data showing the closing of the

axial stage IGV, initiated by the MCHE

trip signal, and the compressor speed are

provided in Figure 5. Speed decay started

at approximately 49 seconds into the

event, indicating that a train trip was

initiated.

The dynamic model was first

initialized to plant conditions prior to the

event. Dynamic tasks were activated

based on plant control sequences. To

accurately emulate the real plant control

system, the simulation time step was set

to be the same as the scan frequency of

CCC controllers in the actual plant.

Figure 6 shows some of the

comparisons of dynamic simulation

results with plant data. Two of the plots

reproduced here are a) Compressor

discharge flow (shown as pressure

differential) and b) Suction and discharge

pressures.

Overall, the simulation was able

accurately to reproduce the plant event,

both in terms of the simulated

compressor conditions and control

responses from the emulator.

During the process, the dynamic

simulation model and CCC emulator

were validated separately and combined,

and deemed to be highly accurate and

reliable for the purpose of the study.

Control issuesClose examination of the MCHE trip

event as described in the previous section

suggested that, without other preventive

measures, the disturbance caused by an

MCHE trip could develop into a situation

that was beyond the controllability of the

CCC system.

Another finding was that the first

level PI control response apparently was

not tuned to serve such fast disturbances.

The anti-surge responses mainly relied

on the second level response by step-

increasing the output.

However, a sudden increase of flow to

one stage could itself magnify the

instability of the system as it could

deprive flow for the other stages.

The CCC system is designed to handle

a number of limitations over the

compressor operating window.

Given the complexity of the multi-

staged compressor system, plus the

extreme operating conditions imposed by

the plant operation, the CCC system

alone appeared to be ineffective in

handling certain scenarios.

Adding more safety margin to the

surge control line was not considered a

practical solution as it would

significantly affect the compressor

operation flexibility. Therefore, other

control improvements became necessary

to address these operating scenarios.

A trip of the PR compressors was

found to be more critical and could drive

the MR compressors into the unstable

region even faster. The controllability of

the MR system in the event of a PR trip

is studied in the first case study below.

The objective of this case study was to

improve the control strategies to ensure

continuous operation of the MR system

after a trip of the propane compressor

system.

The plant operation sequences in case

of a propane trip are similar to the

MCHE trip described earlier. However, in

the event of a loss of the PR system, the

reduction of MR flow to the MR

compressor train can occur in a faster

manner and thus would require more

prompt anti-surge control actions.

Recycle valvesThe results indicate that the disturbance

caused by a propane trip could be more

severe as compared to an MCHE trip.

Although a series of step-increase control

responses were triggered, the MR

compressors moving rapidly into the

unstable region, and showed high

possibility of surge.

The simulation also took into account

the actuator performance in the field. The

recycle valves are shown to have a one-

second delay and a 3 percent dead band

before starting to open.

To prevent the foreseen surge event

from occurring, simulations were

performed to combine a feed forward

strategy with the CCC control algorithm.

The feed forward logic was applied to

ramp open the recycle valves at the

initiation of a propane trip. To avoid

upsetting the compressors, the

appropriate ramp target and ramp rate

of the recycle valves were tuned using the

simulation.

The pre-determined recycle valve

opening and ramp rate were insufficient to

keep the MR compressors in the stable

region. The second level control responses

from the CCC system were triggered before

the end of the feed forward ramp period.

The compressor system was unstable and

showed the tendency of surge.

With the final settings of the feed

forward parameters, the feed forward

logic was activated by the PR trip signal,

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ENGINEERING FORUM

and the recycle valves were ramped open

to 50 percent and 30 percent for the low,

medium and high-pressure stage

respectively over 10 seconds.

With these settings, the recycle flow

rates appeared to be sufficient to

maintain the MR compressor operating

points in the stable region.

The adequacy of the feed forward

setting was verified for the full

compressor operating range. Once the

system is stabilized, the imposed feed

forward logic can be released and the

CCC controller will take over the control.

Based on the study results, it was

concluded that a feed forward strategy

combined with the CCC control system

would provide a predictable and reliable

way to achieve continuous operation of

the MR compressors.

The use of the controls emulator in the

study allowed fine-tuning of the feed

forward settings realistic to the actual

plant.

Train tripThe objective of this case study was to

investigate the existing system and

improve anti-surge strategies to protect

the compressors from surge during coast

down. The plant history has also

identified that emergency shutdown, or

compressor trip, represents a high-risk

case for possible surge of the

compressors, in particular, the LP stage.

The plant control sequence of

operation in the event of a train trip is as

follows:

� Close the axial stage IGV

immediately

� Trip-open the anti-surge recycle

valves and hot gas bypass valve

� Open the axial stage bleed valve

� Stop heavy and light MR flow by

closing the MR flow control valves

In this case study, the dynamic behavior

and effectiveness of anti-surge elements

were thoroughly reviewed. In addition to

reviewing the size and flow

characteristics of the recycle valves, the

impact of various dynamic variables such

as control signal and actuator delays,

valve stroke time, IGV closing speed and

the timing for tripping the turbine driver

were investigated.

In this scenario, the CCC calculation

algorithm is bypassed upon the initiation

of a train trip. Therefore, the dynamic

study was performed without using the

CCC control emulator. The LP axial stage

was modeled as two stages with flow

take-off from between stages to the inter-

stage bleed valve.

The possible operating range for each

dynamic variable and study results are

summarized in Table 1.

The simulation results revealed two

important variables that might have

contributed to the surge problem. One

was the LP stage IGV closing speed. Field

data from actual events indicated that

the IGV closing time was in the range of

7 to 10 seconds.

The other important variable is the

opening time of the LP stage bleed valve.

Analysis of plant data indicated that the

stroke time of this valve could possibly be

as fast as 2 seconds.

Conceptually, fast responses of the

IGV and inter-stage bleed valve might be

considered desirable as they help unload

the compressor quickly. However, results

from the dynamic simulations indicated

that the fast closing of the IGV as

observed in the plant could actually drive

the compressor toward surge.

Based on the simulation results, it is

suggested that the IGV stroke time be

modified to close the IGV at a moderate

rate over 25 to 35 seconds. As for the

inter-stage bleed valve, an optimum

window for the opening time appeared to

exist to avoid surge in either section of

the axial stage.

A typical logic is to open the bleed valve

when speed is reduced down to 95 percent,

which in this case is equivalent to a delay

of approximately 1 second. Overall, a valve

stroke time between 5 to 10 seconds

appeared to be the optimum range.

The anti-surge recycle valves and the

hot gas bypass valve are required to open

as quickly as possible to reduce the risk of

surge during coast down.

Typical requirements for these valves

are less than two seconds [4]. In the

actual plant, the response time of these

valves had limitations for significant

improvements.

To compensate for any delay in the

responsiveness of these valves, the effect

of introducing a delay on turbine trip was

studied. Based on the simulation results,

it was concluded that delaying the

turbine trip by 2 to 3 seconds would

provide an additional safety factor to

avoid the surge of the compressors during

coast down.

ImplementationThe feed forward strategy studied in

Case Study 1 was incorporated into the

plant control logic and the benefits have

been observed in several plant events

since the implementation.

To solve the surge on coast down,

which had been previously considered

unsolvable by the vendor, field

modifications were made based on the

findings from Case Study 2. Plant

operation has demonstrated safe

compressor coast downs since the

implementation and proved the accuracy

of the dynamic simulation results.

ConclusionsIt was only through the use of rigorous

dynamic simulations that the

permutations and combinations of

various system variables could be safely

tested to derive the solution.

The studies proved the accuracy and

effectiveness of such modeling and

provided useful results to diagnose and

improve field operation.

The combination of the dynamic

simulation modeling with control vendor

supplied hardware significantly enhanced

the precision, capability and credibility to

develop realistic and reliable solutions for

the actual plant system. �

AcknowledgmentThe authors wish to express sincere

thanks to Nikhil Dukle and Wayne

Jacobson of Compressor Control Corp. for

technical support and review of control

strategies, Martyn Blanchard of

AspenTech for valuable comments during

the course of the study, and Ming Yan of

KBR for technical assistance with the

assembly of software-hardware used in

the study.

Jihong Wu is a senior process engineerwith KBR in Houston, Texas, specializedin dynamic simulation. Her experiencealso includes process design andoptimization of LNG, olefins and otherlarge-scale processing facilities. Shegraduated from Tokyo University with aPhD in Chemical Engineering.

Jeffrey Feng is a process leader with KBR,in Houston, Texas. He has been with KBRsince 1995 after graduating from TexasA&M University with a PhD in ChemicalEngineering. He is responsible for thetechnical execution of dynamicsimulation and other transient analysisin LNG, refining, olefins, ammonia andoffshore in domestic and internationalprojects.

Surajit Dasgupta is the Manager ofChemical Engineering Technology andAdvanced Process Automation at KBR, inHouston, Texas. He supervises allprojects in the area of dynamicsimulation, advanced process control,operator-training simulators and real-time optimization. He graduated fromColumbia University, NY, in 1977 with aDoctoral degree in Engineering andScience.

Ian Keith is the Chief Process ControlEngineer at Karratha Gas plant inAustralia. He has been with Woodside andShell since 1998 and was the SeniorProcess Engineer for Karratha LNG plantprior to his current position.

Table 1: Dynamic Parameters for Compressor Trip-Coast Down

References[1] Omori H., Konishi, H., Ray, S., de

la Vega, F. and Durr, C., “A New Tool-Efficient and Accurate for LNG Plant Design and Debottlenecking”, 13th International Conference & Exibition on Liquefied Natural Gas, May, 2001.

[2] Valappll J., Mehrotra, V., Messersmith D and Bruner, P., “Virtual Simulation of LNG Plant”, LNG Journal,

January/February, 2004. [3] Compressor Control Corporation,

Series 5 Antisurge Control Application for Centrifugal and Axial Compressors, Publication UM5411, October 2005.

[4] Wilson, J. and Sheldon, A., “Matching Antisurge Control Valve Performance with Integrated Turbomachinery Control Systems”, Hydrocarbon Processing, August 2006

Study Range Desired Operating Range

Delay Stroke time based on study

Opening of anti-surge valves 0 – 2 sec 1 – 3 sec As fast as possible

Opening of hot gas bypass valve 0 – 2 sec 1 – 3 sec As fast as possible

Closing of axial compressor IGV 0 sec 7 – 35 sec No delay, 25 – 35 sec

closing

Opening of inter-stage bleed valve 0 – 2 sec 2 – 10 sec 5 - 10 sec

Trip of gas turbine driver 0 – 3 sec - 2 – 3 sec

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