aspen compressor dynamics

2013 Aspen Technology, Inc. AspenTech, aspenONE, the Aspen leaf logo, the aspenONE logo, and OPTIMIZE are trademarks of Aspen Technology, Inc. All rights reserved.11-3979-0913

Jump Start: Compressor Modeling in Aspen HYSYS

Dynamics

A Brief Tutorial (and supplement to training and online documentation)

Nicholas Brownrigg, Product Marketing, Aspen Technology, Inc.


CVb

Jump Start: Compressor Modeling in Aspen HYSYS Dynamics

Table of ContentsIntroduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Setting up a Steady-State Compressor Model in Aspen HYSYS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Preparing a Dynamic Model from a Steady-State Compressor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Adding and Plotting Compressor Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Modifying the Surge Curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Dynamic Modeling of Compressor Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Additional Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13



1

IntroductionCompressors, generally used to increase the pressure of a gas, are an integral part of many production processes in the oil

and gas, energy, and chemicals industries. They are highly specific to each process for which they are designed. As a

result, they are mostly custom manufactured, expensive, and difficult to replace.

Compressors operate with very fast dynamics. Even a slight disturbance to a processs steady-state could lead to a

compressor surge, which is the point at which a centrifugal compressor cannot add enough energy to overcome the

systems backpressure. A compressor surge can lead to the unit malfunctioning, including rapid flow reversals and

changes in axial thrust, high vibration, and damage to rotor seals and bearings, which could necessitate equipment

replacement. Additionally, a surge is dangerous for individuals working near the compressor and can lead to a process

becoming hazardous, especially if process gas is released to the surrounding atmosphere.

To combat compressor surge, specialized control systems are designed and implemented into processes. Control systems

are enacted to maintain fluid flow to the compressor, which keeps the unit from surging.

Predicting which process conditions will cause the compressor unit to surge, and designing the subsequent control

scheme is a challenging, involved task. Aspen HYSYS Dynamics allows users to view the dynamic response of

compressors to changes in a steady-state simulation, as well as explore control strategies to best minimize the impact of

process disturbances to ensure smooth compressor operation.

Aspen HYSYS Dynamics provides performance charts to help the user visualize the process changes and compressor

performance. Tables can be inserted to the dynamic simulation to aid in the exact quantification of the changes in the

process.

In this Jump Start guide, analyzing the dynamic surge response of a compressor using Aspen HYSYS Dynamics will be

explored.

While this Jump Start guide will walk the reader through the process of using Aspen HYSYS dynamics to model

compressor performance, AspenTech recommends that a range of other resources be called upon in conjunction with this

document to give the user a comprehensive view of how to use Aspen HYSYS. These may include:

AspenTech support website (support.aspentech.com) this website has a wealth of information on the use of

AspenTech products and provides answers to frequently asked questions

AspenTech courseware available in on-line and in-person versions

AspenTech business consultants

This document assumes that the user has Aspen HYSYS V8 or higher installed on his or her computer, and has completed

a functional process design.


2



Setting up a Steady-State Compressor Model in Aspen HYSYSBefore beginning the set up for a steady-state compressor in Aspen HYSYS, process components and a fluid package

must be selected. This guide assumes that these tasks have already been completed. For help getting started in Aspen

HYSYS, please refer to the separate Aspen HYSYS Jump Start Guide available at

www.aspentech.com/JumpStart_HYSYSV8/.

Before dynamic simulation can be initialized in Aspen HYSYS, a working steady-state simulation is required. Begin by

placing a compressor block onto an Aspen HYSYS flowsheet and attach an inlet, outlet, and energy stream. In order to

have a working steady-state simulation, the inlet stream into the compressor must be fully specified and either a heat

duty, the compressors pressure increase, or the compressors pressure ratio must be defined. To enter any of the three

compressor values, double click the compressor block on the flowsheet, go to the Parameters section under the

compressors Design tab, and then type a value in the appropriate box (Figure 1). In addition, choose whether the

compressor is centrifugal or reciprocating.

Figure 1. Parameters Section to Set Up a Steady-State Compressor Model in Aspen HYSYS

Having a converged, steady-state compressor model now enables continuation to dynamic simulation to view how the

compressor will respond under disturbance.



3

Preparing a Dynamic Model from a Steady-State CompressorIn order to model a steady-state compressor dynamically in Aspen HYSYS, the appropriate dynamic control models

must be placed onto the flowsheet and set up. For more information on how to complete the preluding steps to dynamic

simulation, refer to the Jump Start with HYSYS Dynamics guide, which can be downloaded at

www.aspentech.com/HYSYSDynamicsJumpStart2/.

For the remainder of this document, an example process containing a previously solved steady-state compressor will be

used. The file DYN COMPRESSOR MODEL SAMPLE.HSC came attached to this guide and should be downloaded and

opened in Aspen HYSYS. The flowsheet for the process with dynamic control models from the example file is shown in

Figure 2. Compressor 23KA001 is connected to requisite process equipmentbefore reaching a Tee block to model a

recycle loop designed for surge protection for the compressorwhose flow is governed by VLV-100. The surge protection

loop is necessary to ensure that a continuous flow of fluid is provided to the compressor, should any process perturbations

disrupt normal steady-state operation. View any process specifics by opening a stream or process block from the

flowsheet.

The feed to the process in Figure 2 is contained inside a transfer function, TRF-1. In the model, a constant 1900 kmol/hr of

fluid is fed to the process when the transfer function is not activated. When activated, the feed is turned off, goes to 0

kmol/hr, and activates the surge protection loop.

Figure 2. Flowsheet Showing Dynamic Compressor Performance Setup


4


Adding and Plotting Compressor Curves

In order to view the dynamic response of the compressor, compressor performance curves must be added to the model.

To accomplish this, click the Rating tab in the compressor window and select the Curves option in the navigation

pane. Enter the operating speed of the compressor and choose an efficiency type for the curves. Either a linear or

quadratic extrapolation method can be used between compressor curve points by choosing the appropriate option in the

Extrapolation grouping. When finished selecting curve options, check the Enable Curves box, and select Add

Curve as seen in Figure 3 below.

Figure 3. Setup Window for Addition of Compressor Curves, Including Completed Curves

Input compressor operating data, which is usually obtained from compressor manufacturers. Compressor curves plot the

head of the compressor unit versus a volumetric fluid flow at a constant compressor operating rate. The efficiency of the

compressor for a volumetric fluid flow can also be defined. Figure 4 displays an example of data for the compressor in the

simulation, operating at 4900 rpm.



5

Figure 4. Window Enabling Compressor Curve Addition

After all compressor curve data has been entered and an operating speed for the compressor has been designated, return

to the setup window for the compressor curves, and click Plot Curves. A plot of the compressor curves for the

specified operating rates, as well as the current operating point for the steady-state compressor along the curves will be

displayed, shown in Figure 5. Additionally, a surge curve is displayed if the user enters the information. The surge curve

displays the boundaries past which the compressor would experience a surge in operation. Details on how to edit the

surge curve will be discussed in the following section.

Figure 5. Compressor Curves Plot Including Surge Curve


6


Note that this plot is a view of head versus flow at various compressor speeds. To view compressor efficiency versus flow,

choose the efficiency option under the Plot Type grouping in the top right of the compressor plot window.

Modifying the Surge Curve

A surge curve can be seen in Figure 5. This surge curve was previously inserted into the simulation. To begin with a

dynamic simulation from a new steady-state simulation or to edit the surge curve present in the example file, click the

Flow Limits option in the compressor window navigation pane under the Rating tab, as displayed in Figure 6. Then,

click Surge Curve.

Figure 6. Navigating to Edit the Surge Curve

Clicking Surge Curve opens the following window in Figure 7. The compressor speed and/or the flow at which surge

occurs can be edited to either modify existing points or generate new points for the surge curve.

Figure 7. Edit Window for Surge Curve

After ensuring that the compressor and surge curves are properly plotted, continue to the Inertia section of the Rating

tab to update parameters pertaining to the inertia of the compressor impeller and shaft, Figure 8.



7

Figure 8. Updating Inertia Parameters

Finally, continue to the Dynamics tab to edit the surge controller. Click View Surge Controller, shown below in Figure 9.

Figure 9. Editing the Surge Controller

This opens the surge controller window. Verify that the surge controller has the proper connections and objectives, and

then navigate to the Surge Control option under the Parameters tab. Here, the robustness of the surge controller can

be managed by varying the surge control parameters, as shown in Figure 10.


8


Figure 10. Modifying Surge Controller Parameters

Dynamic Modeling of Compressor PerformanceAfter setting up a flowsheet similar to that in Figure 2, with all dynamic mechanisms in place and all compressor curves

added, compressor performance can then be viewed using Aspen HYSYS Dynamics. This section will explore how to use

the example from Figure 2 to explore the compressors response to a surge.

Open the compressor curves plot, which should appear as it does in Figure 5. Then, double click the transfer function

block on the flowsheet and select the Face Plate option that appears in the subsequent window, displayed in Figure 11.

Note that the configuration and parameters for the transfer function used in the example simulation can also be seen in

this figure and can be used for reference for other simulations, if needed.

Figure 11. Opening Transfer Function Face Plate



9

The transfer function face plate will appear and display the current process value, as well as the current operating point,

shown in Figure 12.

Figure 12. Transfer Function Face Plate

Place both the transfer function face plate and the compressor curve plot on the simulation flowsheet for use in the

dynamic simulation.

Before running Aspen HYSYS Dynamics, note that the charts on the flowsheet for VLV-100, valve 23FE1015, the

compressor, and process stream 1 appear as follows:

Figure 13. Key Parameters for VLV-100, Valve 23FE1015, Compressor 23KA001, and Stream 1

The flow through the surge control valve is essentially zero, indicating that there is no surge being experienced in the

compressor. This can be verified in the compressor curve plot. As a corollary, all 1900 kmol/hr of feed are flowing to the

compressor and exits through the splitter to stream 1. The compressor operates at the speed indicated on the compressor

curve plot.

To start the dynamic simulation of compressor behavior, click the Dynamics tab in the Aspen HYSYS ribbon, and then

click Run, shown in Figure 14 below.

VLV-100

Feed Pressure 1220 kPa

Product Pressure 200.2 kPa

Molar Flow 3.600e-012 kgmole/h

23FE1015

Molar Flow 1900 kgmole/h

Pressure Drop 1.073 kPa

Feed Pressure 192.7 kPa

Percentage Open 100.00 %

CompressorSpeed 6285 rpm

Power 3725 kW

Capacity (act feed vol flow) 26159.5 ACT_m3/h


Product Pressure 1220 kPa

1

Temperature 35.00 C

Pressure 1220 kPa



10


Figure 14. Navigating to the Dynamics tab

After clicking Run, the dynamic simulator begins collecting compressor flow, head, and efficiency performance data.

The transfer function block serves to provide the compressor a disturbance, which in turn, allows the user to see how the

compressor and its surge control loop will respond. To initiate the disturbance from the transfer function block, check the

Active box on the transfer function face plate, shown in Figure 15.

Figure 15. Transfer Function Face Plate with Active Option Checked to Stop Feed to Process

For the simulation set up, clicking the Active box completely stops the feed to the compressor. It can be observed on the

compressor curves plot that the compressor experiences surge when the feed is stopped, followed by the surge protection

control loop recycling fluid, which leaves the compressor back to the inlet in order to ensure continuous fluid flow through

the unit (Figure 16).

Figure 16. Compressor Curves Showing Surge Response and Control Response



11

When the surge control loop is activated, the compressor operates at a higher rpm, has a higher head, and passes a

higher flow. This is evident when comparing the tables at steady-state in Figure 13 to the updated tables after initiating

the feed disturbance, as shown below in Figure 17.

Figure 17. Results Tables During Feed Disturbance

Product ceases to leave the process in stream 1 after the surge control loop begins, while the flow through the surge

control valve, VLV-100, jumps from zero to 2500 kmol/hr. This then makes its way back to the compressor through valve

25FE1015. An increase in compressor speed, capacity, and power can also be verified.

Unchecking the Active box on the transfer function face plate restores the feed stream to the compressor to 1900

kmol/hr. Accordingly, compressor performance will return to the original, steady-state operating point. Figure 18 shows

the compressor curves plot after returning the feed stream.

Figure 18. Compressor Performance After Restoring Feed

VLV-100




23FE1015






Power 4163 kW




1

Temperature 35.00 C

Pressure 1220 kPa

Molar Flow 0.0000 kgmole/h


12


The compressor gradually returns to the steady-state performance it began with. The following results table shows the

key process variables for the compressor at the operating point shown in Figure 18.

Figure 19. Tables of Compressor Performance After Restoring Feed

Letting the process run with the feed stream renewed, shows that 1900 kmol/hr of fluid leaves the process in stream 1, as

previously observed in steady-state operation. The flow through the surge control valve returns to zero, while the

compressors speed, power, and capacity decrease to the steady-state values.

A view of the compressors efficiency curves throughout the dynamic disturbance modeling is shown in Figure 20.

Figure 20. Compressor Efficiency Curve for Surge Response

VLV-100



Molar Flow 3.600e-012 kgmole/h

23FE1015






Power 3724 kW




1

Temperature 35.00 C

Pressure 1220 kPa




13

ConclusionControlling compressor surge is very significant for safe process operation and mitigating equipment malfunction and

subsequent replacement. Aspen HYSYS Dynamics helps engineers model the dynamic changes in performance expected

for a compressor experiencing a disturbance, as well as the response of a control loop connected to the compressor. Using

Aspen HYSYS Dynamics, many scenarios in which steady-state operation is interrupted can be considered and designed

for.

Both numerical and graphical dynamic models are generated in Aspen HYSYS Dynamics, which allows for in-depth

analysis to be performed on each completed simulation. In addition to stopping the feed to the compressor, as performed

in this guide, other compressor disturbances could be explored in Aspen HYSYS Dynamics. Control schemes and

parameters can also be modified, yielding more or less robust compressor control, depending on the users preference.

Aspen HYSYS Dynamics is a powerful tool that can be utilized to ensure safe and proper functioning of a processs

compressors.

Additional ResourcesPublic Website:

http://www.aspentech.com/products/aspen-hysys.aspx

http://www.aspentech.com/products/aspen-hysys-dynamics.aspx

Online Training:

http://www.aspentech.com/products/aspen-online-training

AspenTech YouTube Channel:

http://www.youtube.com/user/aspentechnologyinc

Worldwide Headquarters

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For a complete list of offices, please visit www.aspentech.com/locations

2013 Aspen Technology, Inc. AspenTech, aspenONE, the aspenONE logo, the Aspen leaf logo, and OPTIMIZE are trademarks of AspenTechnology, Inc. All rights reserved. All other trademarks are property of their respective owners. 11-2596-0113 2013 Aspen Technology, Inc. AspenTech, aspenONE, the Aspen leaf logo, the aspenONE logo, and OPTIMIZE are trademarks of AspenTechnology, Inc. All rights reserved. 11-3979-0913

About AspenTech

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practices for optimizing their engineering, manufacturing, and supply chain operations. As a result,

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