Copyright of Shell Global Solutions (US) Inc. 2014
COMPRESSOR COAST-DOWN EVALUATION STUDY
USING DYNSIM
Presented at the 2014 Schneider Electric Software Global
Customer Conference
September 30 - October 2, 2014
Orlando, Florida, USA
Asit Mardikar, Sonia Pusha & Ashok Dewan(*)
* Speaker, To Whom Correspondence May Be Addressed:
Shell Global Solutions (US) Inc., Shell Technology Center-Houston,
3333 Highway 6 South, Houston, TX 77082-3101, USA
Copyright of Shell Global Solutions (US) Inc. 2014
DEFINITIONS & CAUTIONARY NOTE
Reserves: Our use of the term “reserves” in this presentation means SEC proved oil and gas reserves.
Resources: Our use of the term “resources” in this presentation includes quantities of oil and gas not yet classified as SEC proved oil and gas reserves. Resources are
consistent with the Society of Petroleum Engineers 2P and 2C definitions.
Organic: Our use of the term Organic includes SEC proved oil and gas reserves excluding changes resulting from acquisitions, divestments and year-average pricing
impact.
Resources plays: our use of the term ‘resources plays’ refers to tight, shale and coal bed methane oil and gas acreage.
The companies in which Royal Dutch Shell plc directly and indirectly owns investments are separate entities. In this presentation “Shell”, “Shell group” and “Royal Dutch
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also used to refer to subsidiaries in general or to those who work for them. These expressions are also used where no useful purpose is served by identifying the particular
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affect the future operations of Royal Dutch Shell and could cause those results to differ materially from those expressed in the forward-looking statements included in this
presentation, including (without limitation): (a) price fluctuations in crude oil and natural gas; (b) changes in demand for Shell’s products; (c) currency fluctuations; (d) drilling
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renegotiation of the terms of contracts with governmental entities, delays or advancements in the approval of projects and delays in the reimbursement for shared costs;
and (m) changes in trading conditions. All forward-looking statements contained in this presentation are expressly qualified in their entirety by the cautionary statements
contained or referred to in this section. Readers should not place undue reliance on forward-looking statements. Additional factors that may affect future results are
contained in Royal Dutch Shell’s 20-F for the year ended 31 December, 2013 (available at www.shell.com/investor and www.sec.gov ). These factors also should be
considered by the reader. Each forward-looking statement speaks only as of the date of this presentation, Oct 2, 2014. Neither Royal Dutch Shell nor any of its subsidiaries
undertake any obligation to publicly update or revise any forward-looking statement as a result of new information, future events or other information. In light of these risks,
results could differ materially from those stated, implied or inferred from the forward-looking statements contained in this presentation. There can be no assurance that
dividend payments will match or exceed those set out in this presentation in the future, or that they will be made at all.
We use certain terms in this presentation, such as discovery potential, that the United States Securities and Exchange Commission (SEC) guidelines strictly prohibit us from
including in filings with the SEC. U.S. Investors are urged to consider closely the disclosure in our Form 20-F, File No 1-32575, available on the SEC website www.sec.gov.
You can also obtain this form from the SEC by calling 1-800-SEC-0330.
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AGENDA
Background
Study Objectives
Methodology
Model Scope
Scenarios
Scenario Results
Analysis
Conclusions
Other Observations
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BACKGROUND (1 OF 5)
Centrifugal compressors are subject to transient events, such as
emergency shutdowns, which can cause high energy surges.
The rate at which a compressor spins down after a trip (Coast-
down rate) can have a significant impact on whether the
compressor enters a high energy surge situation or not.
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BACKGROUND (2 OF 5)
Difference in initial coastdown rate predictions
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BACKGROUND (3 OF 5)
Leads to very different compressor surge behaviour
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BACKGROUND (4 OF 5)
Fast initial coastdown prediction can result in significant
oversizing Oct 2, 2014 7
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BACKGROUND (5 OF 5)
Compressor Coastdown rate after a trip can be an important
determinant in
Anti-surge valve sizes and their open/close timings
Design issues like hot/cold gas bypass arrangements
Suction/Discharge Volumes and other parameters
Hence it was decided to evaluate DYNSIM’s capabilities in this
aspect.
The study also took the opportunity to evaluate Cascade Control
via Selector Block (multiple Master Controllers) in DYNSIM.
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STUDY OBJECTIVES
Behaviour of compressor during Coastdown Trip
Run scenarios where Compressor is tripped at given operating conditions.
Review predicted Compressor speed Coastdown rates and compare with
actual plant data.
Evaluation of Back Initialization option for cascade controllers in
DYNSIM
Run Scenarios where Cascade Controller with Selector is put in Local Auto
mode and then put back in Cascade.
Review whether backtracking of selected output is correctly transmitted to non-
selected controllers.
Verify whether Set Point (SP) correctly tracks Present Value (PV), for specified
controller during above transition.
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METHODOLOGY
A model of a Compressor Facility in an upstream asset was built
in DYNSIM 5.1.
This model was validated for Steady State predictions and
Dynamic behaviour against plant data and also against a
previously validated model built for engineering studies.
The DYNSIM 5.1 model was then used to run several scenarios
for compressor coastdown.
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MODEL SCOPE (1 OF 2)
The dynamic simulation study scope included the following:
Export Gas Compressor Suction Scrubber V-101
Export Gas Compressor K-101
Export Gas Compressor Discharge Cooler E-101
Export Gas Compressor Anti-surge Valve XV- 101
Export pipeline
The model also included associated piping within the compressor
platform, bridge piping, control valves, shut down valves, and
blow down valves.
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MODEL SCOPE (2 OF 2)
SCHEMATIC FOR EXPORT GAS COMPRESSOR
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SCENARIOS
The following scenarios were run and evaluated
Compressor K-101 Trip with Blow down (300 MMSCFD, 10266 rpm)
Compressor K-101 Trip without Blow down (300 MMSCFD, 10266
rpm)
Cascade Control Output (OP) Back Initialization Evaluation
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SCENARIO1: RESULT
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SCENARIO 2: RESULT
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ANALYSIS OF COASTDOWN SCENARIOS (1 OF 2)
RPM Plant Data DYNSIM - No Blow down DYNSIM - with Blow
down
Time (s) Time (s) % Abs. Error Time (s) % Abs. Error
9000 3.3 3 9.09 3 9.09
8000 5.6 5 10.71 5 10.71
7000 6.8 7 2.94 7 2.94
6000 8.1 9 11.11 9 11.11
5000 9.4 10.9 15.96 11.1 18.09
Comparison of Predicted Coastdown rates and actual plant
data
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ANALYSIS OF COASTDOWN SCENARIOS( 2 OF 2)
DYNSIM prediction of initial Coastdown prediction is close to
observed plant data
The initial coastdown rate is critical in design for surge
mitigation Oct 2, 2014 17
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SCENARIO 3: RESULT & ANALYSIS (1 OF 2)
Correct backtracking of selected output to the 2 non-selected
controllers Oct 2, 2014 18
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SCENARIO 3: RESULT & ANALYSIS (2 OF 2)
Correct SP tracking PV behaviour for FC101 during switch from
Cascade to Local Auto and back
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CONCLUSIONS (1 OF 3)
Initial Compressor Coastdown prediction in DYNSIM is close to
actual plant data (about 10% difference). Much higher differences
were observed in some studies in the past.
Coastdown rate prediction from 50% of operating speed to zero
speed is slightly slower than plant data.
Time predicted to reach zero speed is within 10-12% of plant
data.
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CONCLUSIONS (2 OF 3)
Better match of Coastdown rate at lower speeds could be
achieved if performance curves at lower speeds were available. In
their absence, the model uses Fan Laws for extrapolation. This is
a less accurate method.
However as seen before, it is the initial Coastdown rate that is
important in sizing of anti-surge valves and for decisions on
hot/cold gas bypass.
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CONCLUSIONS (3 OF 3)
For Cascade Control via a selector, the DYNSIM controllers
correctly backtracked the selected output for reference to the 2
non-selected controllers. Correct SP Tracking PV behaviour for
the Compressor Flow controller FC101 was also confirmed.
DYNSIM Shaft unit operation makes it easier to model
Compressor Inertia and Speed behaviour. It also helps in better
modelling and representation of drivers like Motors, Gas Turbines
and Steam Turbines.
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OTHER OBSERVATIONS
Anti-Reset Windup as suggested in the DYNSIM FAQ manual
results in non-selected controllers having the same output as
selected controller.
Actually non-selected controller outputs should be higher(for Low-
Select) or lower(for High-Select) than the selected output.
Implementation was modified to achieve Correct Anti-Reset
Windup behaviour.
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Q & A
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REFERENCES
Botros, K.K. and Ganesan, S.T., 2008,“Dynamic Instabilities in
Industrial Compression systems with Centrifugal Compressors,”
Proceedings of the Thirty-Seventh turbo machinery Symposium,
Turbo machinery laboratory, Texas A & M University, College
Station, Texas, pp. 119-132
Moore, J.J., Garcia-Hernandez, A., Blieske, M., Kurz, R., Brun, K.,
2009’, Transient Surge Measurements of a
Centrifugal Compressor Station during Emergency Shutdowns,
38th Turbo machinery Symposium, Houston, Texas.
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