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Jump Start: Compressor Modelingin Aspen HYSYS® Dynamics
A Brief Tutorial (and supplement to training and online documentation)
Nicholas Brownrigg, Product Marketing, Aspen Technology, Inc.
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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
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Jump Start: Compressor Modeling in Aspen HYSYS Dynamics
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 process’s steady-state could lead to a
compressor surge, which is the point at which a centrifugal compressor cannot add enough energy to overcome the
system’s 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.
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Jump Start: Compressor Modeling in Aspen HYSYS Dynamics
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 compressor’s pressure increase, or the compressor’s 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 compressor’s ‘’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.
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Jump Start: Compressor Modeling in Aspen HYSYS Dynamics
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 equipment—before reaching a Tee block to
model a recycle loop designed for surge protection for the compressor—whose 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
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Jump Start: Compressor Modeling in Aspen HYSYS Dynamics
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.
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Jump Start: Compressor Modeling in Aspen HYSYS Dynamics
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
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Jump Start: Compressor Modeling in Aspen HYSYS Dynamics
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.
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Jump Start: Compressor Modeling in Aspen HYSYS Dynamics
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.
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Jump Start: Compressor Modeling in Aspen HYSYS Dynamics
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 compressor’s 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
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Jump Start: Compressor Modeling in Aspen HYSYS Dynamics
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 kPaMolar Flow
3.600e-012 kgmole/h
23FE1015
Molar Flow 1900 kgmole/h
Pressure Drop 1.073 kPaFeed Pressure
192.7 kPa
Percentage Open 100.00 %Compressor Speed 6285 rpm
Power 3725 kWCapacity (act feed vol flow) 26159.5 ACT_m3/h
Feed Pressure 183.6 kPa
Product Pressure 1220 kPa
1
Temperature 35.00 C
Pressure 1220 kPaMolar Flow
1900 kgmole/h
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Jump Start: Compressor Modeling in Aspen HYSYS Dynamics
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
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Jump Start: Compressor Modeling in Aspen HYSYS Dynamics
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
Feed Pressure 1220 kPa
Product Pressure 214.9 kPaMolar Flow
2502 kgmole/h
23FE1015
Molar Flow 2500 kgmole/h
Pressure Drop 1.207 kPaFeed Pressure
206.4 kPa
Percentage Open 100.00 %Compressor Speed 6431 rpm
Power 4163 kWCapacity (act feed vol flow) 27604.6 ACT_m3/h
Feed Pressure 195.8 kPa
Product Pressure 1220 kPa
1
Temperature 35.00 C
Pressure 1220 kPaMolar Flow
0.0000 kgmole/h
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Jump Start: Compressor Modeling in Aspen HYSYS Dynamics
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
compressor’s speed, power, and capacity decrease to the steady-state values.
A view of the compressor’s efficiency curves throughout the dynamic disturbance modeling is shown in Figure 20.
Figure 20. Compressor Efficiency Curve for Surge Response
VLV-100
Feed Pressure 1220 kPa
Product Pressure 200.4 kPaMolar Flow
3.600e-012 kgmole/h
23FE1015
Molar Flow 1900 kgmole/h
Pressure Drop 1.072 kPaFeed Pressure
192.8 kPa
Percentage Open 100.00 %Compressor Speed 6283 rpm
Power 3724 kWCapacity (act feed vol flow) 26147.8 ACT_m3/h
Feed Pressure 183.7 kPa
Product Pressure 1220 kPa
1
Temperature 35.00 C
Pressure 1220 kPaMolar Flow
1900 kgmole/h
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Jump Start: Compressor Modeling in Aspen HYSYS Dynamics
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 user’s preference.
Aspen HYSYS Dynamics is a powerful tool that can be utilized to ensure safe and proper functioning of a process’s
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
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About AspenTech
AspenTech is a leading supplier of software that optimizes process manufacturing—for energy,
chemicals, engineering and construction, and other industries that manufacture and produce products
from a chemical process. With integrated aspenONE® solutions, process manufacturers can implement
best practices for optimizing their engineering, manufacturing, and supply chain operations. As a result,
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more energy efficient. To see how the world’s leading process manufacturers rely on AspenTech to
achieve their operational excellence goals, visit www.aspentech.com.