Alex Roff, PDL's Engineering Director gives a talk to the BVAA about the use of Computational Fluid Dynamics (CFD) to assess performance and operation in extreme conditions

1. FS 90756
“The use of CFD to assess valve performance and operation in extreme conditions”
BVAA Conference
Tuesday 12th May 2015
Alex Roff – Engineering Director

2. Overview:
 Introduction.
 The use of CFD in the valve industry.
 CFD case study – Joule Thomson effect in a valve.
 Industry simulation trends.
 Questions.
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3. Introduction
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 Simulation can be used in the valve industry to gain confidence and verify the performance of equipment.
 Representing operating conditions for valves and actuators can be challenging and in some cases an impossible task.
 Where test facilities are available, testing can be expensive.
 Gives you the ability to understand what is happening within the valve itself.
 Ability to find the limit of operation of the components, as opposed to just verifying test conditions.
 Gain confidence in a design before metal is cut.
 Demonstrate an increased understanding to customers and compliance to the relevant design codes.

4.  Flow behaviour / Cv calculation
 Multi-phase flows
 Valve closure analysis
 Erosion / deposition analysis
 Extreme flow conditions
Computational Fluid Dynamics – Overview
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 Computational Fluid Dynamics – Uses in Valve Industry:

5.  4” Subsea Choke
 3 Stage Concentric Cage Trim
 ΔP across choke ≈ 100 bar
 Inlet Temp ≈ 5°C
 Mass Flow Rate ≈ 25kg/s
 What is the minimum temperature of the gas at the exit of the Choke Valve?
 Demonstrate that the downstream pipe remains within design temperature
limits.
Computational Fluid Dynamics – Flow Assurance
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 Choke Valve Thermal Assessment – Problem Statement:

6. Computational Fluid Dynamics – Flow Assurance
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 Choke Valve Thermal Assessment – Input Parameters:
ANALYSIS
PLAN
Fluid Properties Assumptions
Simplifications
Expected OutputsGeometry
Customer Review
and Approval

7.  Geometry Preparation
 Simplify Internal flow geometry.
 Split geometry into mesh regions.
 Extend inlet and outlets.
Computational Fluid Dynamics – Flow Assurance
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 Choke Valve Thermal Assessment – Geometry Simplification:

8.  Mesh:
 Inflation layers used next to the walls to resolve the boundary layers.
 Local refinements required (very different length-scales).
 Regions swept, where possible, to control element size.
 Split sizing in stream-wise & cross-flow directions.
 Reduce number of elements – large mesh, memory issues, solve times.
Computational Fluid Dynamics – Flow Assurance
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 Choke Valve Thermal Assessment – Meshing:

9.  Ideal gas law cannot be used to capture the Joule Thomson effect.
 Assumes the molecules have a negligible volume.
 Assumes there are no intermolecular forces between the molecules.
 Assumes all collision between the molecules are elastic.
 Real gases need to be represented using an alternative Equation of State.
Computational Fluid Dynamics – Flow Assurance
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 Choke Valve Thermal Assessment – Ideal Gas vs. Real Gas:

10.  Real gas effects modelled using the Peng-Robinson Equation of State.
 Model developed for hydrocarbon processes.
 Cubic equation; determines molar volume, given pressure & temperature.
 Predicts liquid and vapour properties & vapour-liquid equilibrium.
Computational Fluid Dynamics – Flow Assurance
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 Choke Valve Thermal Assessment – Equation of State:

11.  Peng-Robinson Equation of State used to calculate the properties each component.
 Real gas mixing rules implemented.
 Psuedo-critcial constants determined for the mixture.
 Also Considered: Specific Heat Capacity, Dynamic Viscosity, Thermal Conductivity.
Computational Fluid Dynamics – Flow Assurance
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 Choke Valve Thermal Assessment – Equation of State – Mixture:

12.  Choked flow conditions, means that conventional boundary conditions cannot be easily
applied.
 Flow Conditions
 Specified outlet pressure.
 Specified inlet pressure & temperature.
 Initially solved using mass flow rate to determine initial conditions.
 Boundary condition then updated.
 Mass flow then an output of the analysis.
 Adiabatic wall boundary conditions.
Computational Fluid Dynamics – Flow Assurance
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 Choke Valve Thermal Assessment – Boundary Conditions:

13.  Multiple convergence criteria monitored through out the solve process.
 Minimum fluid temperature and location determined.
 Analysis Solve time: 10 hours (750 iterations).
 Using in-house 48-core dedicated High Performance Computing Cluster.
 Solve time in excess of 7 days for desktop computer.
Computational Fluid Dynamics – Flow Assurance
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 Choke Valve Thermal Assessment – Solve Process:

14.  Mach Number for 100% open case.
 Maximum Mach number ≈ 1.1
 Maximum Velocity ≈ 300m/s
Computational Fluid Dynamics – Flow Assurance
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 Choke Valve Thermal Assessment – Results – Mach Number:

15. Computational Fluid Dynamics – Flow Assurance
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 Choke Valve Thermal Assessment – Results – Streamlines:

16.  Absolute Pressure for 100% open case.
Computational Fluid Dynamics – Flow Assurance
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 Choke Valve Thermal Assessment – Results – Absolute Pressure:

17.  Temperature for 100% open case.
 Inlet Temperature ≈ 5°C
 Average Outlet Temperature ≈ -30°C
 Absolute Minimum Temperature ≈ -60°C
Computational Fluid Dynamics – Flow Assurance
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 Choke Valve Thermal Assessment – Results – Temperature:

18.  Best verification of the model is to compare test data vs. the predicted results.
 Results of Cv flow testing were compared against the CFD model (blind).
 Verifies model set-up/geometry.
 Verifies mesh quality.
 Best verification that was possible.
Computational Fluid Dynamics – Flow Assurance
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 Choke Valve Thermal Assessment – Model Verification:

19.  Comparison of calculated fluid properties with PVT data from 3rd party review.
 Comparison of results with simple flash calculations performed by others.
 Rigorous review of final reports by end customer:
 International Oil Company (Confidential)
 EPC Contractor (Confidential) – Flow Assurance
 DNV – 3rd Party Review
Computational Fluid Dynamics – Flow Assurance
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 Choke Valve Thermal Assessment – Model Verification:

20.  Examination of the valve body temperatures.
 Inclusion of the external sea water domain.
 Determination of Icing and the subsequent effects.
 Transient analysis to determine the rate of ice build up.
Computational Fluid Dynamics – Flow Assurance
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 Choke Valve Thermal Assessment – Further Work:

21. Simulation Trends
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 More demand from end users for both CFD and FEA reports.
 The general complexity of analysis requested is increasing.
 More rigour applied to the depth and detail of the analysis requirements.
 The use of simulation to mitigate risk, shorten development timescales and reduce costs.
 Will become a mandatory requirements for some safety critical valves (API 17G (WD6)).

22. Questions?
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T: + 44 1 434 609 473 T: + 1 713 260 9635 T: + 44 1 434 609 473
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