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Gas Turbine Start-up

Pressure regulators are to be employed at a gas-fired power station to reduce upstream gas pressures from a maximum of 15 MPa to approximately 3.5 MPa. Due to the Joule-Thompson effect, the resulting gas temperature drops could be in the region of 55 °C. The dew-point temperature of the hydrocarbons (gas) flowing through is -15 °C and the minimum ambient temperature of the area is -6 °C. Thus the regulators could potentially be subjected to gas at -61 °C at start-up. According to the valve manufacturer, temperatures as low as -20 °C can be tolerated for some time, provided that condensation does not occur.

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Natural Draft Stack

This case study demonstrates the use of Flownex® to model a natural draft exhaust stack such as those typically used in natural gas combustion processes. A basic stack compound component has been developed to assist and simplify the modeling process.

Natural draft processes rely on buoyancy effects to generate draft. However, when modeling an exhaust stack, the stack height also implies a small but significant pressure drop due to elevation. These two effects combine to drive the natural draft flow.
Additionally, the exhaust stack compound component imple-ments convenient mechanisms to specify stack geometry and losses such as the elbow between the vertical stack and the horizontal piping feeding into the stack. It allows for a unity exit loss as well as an additional loss factor that could be used for any additional losses in the stack design such as a velocity seal, a silencer or a spark arrestor.

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PSV Sizing and Reaction Force Modelling

This case study demonstrates the use of Flownex® to size properly performing pressure control valves for typical plant operation and to size and select a code compliant matching pressure safety valve, and calculate the associated relief flow reaction forces. 

Pressure control valves (PCVs), pressure regulating valves and pressure safety valves (PSVs) are an important part of any plant design in the oil and gas industry. Gas products are typically transported at very high pressures to reduce pumping costs and reduce line sizes. Pressure control valves (and the subset of pressure regulating valves) are then used to reduce the pressures to the required levels at the point of consumption. However, if a pressure control valve fails, the plant design must make provision for safety systems to prevent catastrophic events from taking place. The proper selection and installation of pressure safety valves is one option. Alternatively, two pressure control valves may be installed in series in a so called active-monitor arrangement. Furthermore, a slam-shut valve may be installed that is able to shut down the plant in a short period of time.

This paper discusses the combination of pressure control and pressure safety valves where the latter is used as the means of ensuring failure should the former fail. The usage of PSVs is commonplace in most oil and gas plants, in fact, all international design codes and standards in the oil and gas industry mandate their use at any position in the plant where pressures may exceed design parameters. Pressure increases beyond design values may occur mainly due to three causes:

  1. Process failure – a situation where faulty equipment is no longer capable of controlling the pressure to acceptable limits. This may include a failed control valve, a blocked outlet, a tube rupture, a loss of utility such as cooling medium or power, gas blow-by etc.
  2. Locked-in thermal expansion – a situation where a vessel or length of pressure piping may be locked in upstream and downstream and a resident heat source then causes the internal pressure to rise beyond design limits.
  3. External fire relief – a situation where an external fire may add heat to a vessel or pressure piping, resulting in a similar scenario to the locked-in thermal expansion case.

A side-effect of the installation of a PSV is the resulting reaction force that may be created when the PSV opens. Since most PSVs “pop” open rather quickly, very high reaction forces may result and must be checked by the design engineer.

This case study demonstrates how Flownex® can be used to size a PCV as well as a matching full-flow PSV and easily calculate the resulting reaction forces. Checks for design code compliance are also performed.

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Gas Fired Heater Insulated Pipe

A client in the petrochemical industry sought help with regards to a fired heater employed on a plant. The client wished to perform thermal fatigue and stress analysis on the heater, especially during transient operating conditions such as start-up and on-off cyclic operations, where the possibility for large temperature gradients exists.

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Combustion Modelling

The Flownex® Adiabatic Flame model is based on the NASA Glenn Chemical Equilibrium Program CEA2 and supports a large range of fuels. The development of the natural gas combustion model has only attempted to implement a combustion model for typical natural gas compositions. It would be easy to add capabilities for other fuel components, whether gaseous or liquid.
The development of this model essentially comprised of three basic fields of development. Firstly, some effort went into defining fluid tables for the selected natural gas components (listed in Table 2) in such a way that accurate interpolation would result at low partial pressures and at temperatures exceeding those expected during combustion. Secondly, a suite of compound components were developed to assist with the convenient specification and analysis of the gas components. Thirdly, a Simple Burner model was developed by wrapping the Flownex Adiabatic Flame model in a compound component together with a script to enable the specification and calculation of typical natural gas burner performance parameters.

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Pressure Piping Thickness and Flange Rating Calculation

In the oil and gas industry high pressure applications are mostly the norm rather than the exception. Process engineers may employ Flownex® to model liquid or gas piping systems as part of their heat and mass balance, and pipe sizing calculations. However, the process engineer often has to rely on others to determine the required pipe schedules (wall thickness) and flange ratings.
Flownex® has a very powerful facility in terms of its scripting capability. Combined with the Generic 4D chart library, all the tools required are available to implement pressure piping calculations according to any design standard. In fact, any data table oriented calculation procedure may be implemented using this approach. This case study demonstrates the implementation of three such international standards – ASME B31.3, AS 1210 and AS 4041 – in a simple script. It also further demonstrates how to use the Generic 4D charts as a material property library to be used by the script.

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A basic immersion firetube Flownex model

This case study demonstrates the implementation of a basic immersion firetube model in Flownex and presents natural draft and forced draft examples.

Challenge: The main challenge is to model an immersion firetube in Flownex. Immersion firetubes are widely used in industry, most commonly in indirect heating applications where either gas or oil burners are used as a heat source.

Benefits: Flownex allows the user to model combustion, heat transfer and fluid flow processes in an elegant and easy to understand way.

Solution: Using Flownex’s compound component and scripting capabilities, a simple immersion firetube model has been developed and is presented in this case study. Furthermore, examples of natural draft and forced draft design cases are presented.

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Coalescing filter sizing and life cycle analysis using rated and specified pressure loss components

This case study discusses the sizing of a coalescer filter and demonstrates its fouling life cycle analysis using a Flownex® model which implements two new pressure loss components: 

  • A rated pressure loss component. 
  • A specified pressure loss component. 

Challenge: The main challenge is the sizing and life cycle analysis of a typical coalescing filter. To simplify the Flownex model and assist with the analysis of the system performance, two new pressure loss components have been developed and are also presented in this case study. 

Benefits: Although not overly complicated, the design and lifecycle analysis of a filter system has a few interesting aspects that need to be highlighted. The two new components specifically developed to assist with this analysis should prove useful to other Flownex users by simplifying the specification of typical pressure losses in complex networks. 

Solution: A complete filter life cycle analysis is presented which may be applied to other similar filtration systems in Flownex networks. Two simple compound components have been developed and are discussed and demonstrated in this case study. 

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Flow measurement orifice plate sizing and uncertainty analysis

This case study demonstrates the use of some of Flownex’s power features – the Designer and the Sensitivity Analysis capability – during the design and uncertainty evaluation of flow measurement using an orifice plate connected to a pressure transducer and transmitter. 

Challenge: The main challenge for this case study is the application of Flownex to: 

  • Size an orifice plate to be used in conjunction with a flow transmitter to serve as an accurate flow meter for natural gas. 
  • Evaluate the measurement uncertainty of the flow meter with variations in operating conditions and manufacturing tolerances. 

Benefits: Flownex is an ideal tool to design gas flow systems, including piping, valving and most other components that are typically found in the oil and gas industry. Not only is Flownex also the ideal tool to design accurate flow measurement orifice plates, but it also has the capability to evaluate the uncertainty of the flow meter in general and the orifice plate in particular with the inevitable variations in conditions and manufacturing tolerances. 

Solution: Using the Designer and the Sensitivity Analysis features built into Flownex, the orifice plate can be designed and its operational uncertainty evaluated when functioning in combination with the pressure transducer/flow transmitter. 

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Fired Heater Design

The design of a fired heater (or similar) package typically starts with a heat and mass balance. This first step is necessary to determine the process parameters which determine most of the sizing of the package. A fired heater package heat and mass balance includes the modelling of the combustion process and the modelling of the heat transfer and fluid flow processes. Modelling the combustion process typically involves the specification of the fuel gas composition, the combustion air composition, the air-fuel ratio and the fuel flow rate. The heat transfer from the fired heater combustion process into the process fluid may be specified in terms of an overall heater thermal efficiency. Typical heat and mass balance calculations do not provide estimations of the physical size of the heater or even the ducting and other components such as combustion air fans, neither do they enable the calculation of system pressure losses or heat losses. They are also incapable of providing insight into tube wall and process fluid film temperatures which are very important in the oil and gas industry. Flownex® enables the user to perform all these tasks easily and quickly in a single calculation.

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