Why Engineers worry about rapid transients in Gas and Steam Piping systems? What are the concerns or issues can arise if these transient events happen in the piping system?
How should an engineer understand and analyze these transients? What could be the appropriate solution or mitigation strategies to these events?
These are the typical questions which may arise in an Engineer’s mind when any work starts on designing piping systems conveying high pressure fluids. Sometimes we also refer to this analysis with term Dynamic simulation. Changes which are happening in the systems in very small fraction of time (milliseconds to seconds) are called instantaneous hydraulic transients or fast transients. These changes may happen due to –
- Rapid valve closure or opening
- Pump / Compressor speed variation; start and trip events.
- Emergency operations (ESD Valves, Deluge Valves etc.)
- Relief valve cracking
- System Pressurization and De-pressurization
- Periodic pressure & flow conditions
Dynamic analysis of piping system includes the study of hydraulic transients, their effects on systems in terms of pressure and unbalanced forces; followed by adopting the strategies to mitigate transients. Dynamic analysis for incompressible flow (water, oil, cryogens, hydrocarbons) are comparatively less complex due to no effect on density changes with time & pressure variation.
However, complexity of calculations increases multifold when we have the compressible fluids (gases and steam). Major parameters like Pressure, Flow Rate, Density changes frequently at every time step of dynamic analysis along with the changes in critical parameters including Mach Number, Enthalpy and Temperature of the fluid flowing through the pipe. These parameters vary not only at inlet and outlet; however, they should be calculated in between pipe also at various number of stations.
One of the most popular method to solve the equations for dynamic analysis is Method of Characteristics (MOC). It is an analytical procedure for transforming a set of hyperbolic partial differential equations into a set of ordinary differential equations.
Typical dynamic analysis involves solving for governing equations –
- Conservation of mass
- Conservation of Momentum
- Conservation of Energy
- Equation of State
- Enthalpy state equation.
Complexity reaches to next level with inclusion of Heat transfer through pipes. To address all these concerns and to perform these transient calculations accurately, Engineers at Applied Flow Technology, USA developed a robust tool named AFT xStream, launched in April this year.
What is AFT xStream?
AFT xStream is a visual platform for modelling compressible fluid transients in pipe networks. xStream uses Method of Characteristics for transient along with an inbuilt Steady state solver. xStream can capture the propagation of pressure wave and flow based on wave speed of fluid in pipe. Unlike most of the software tools available in the market which oversimplifies the assumptions for compressible flow like Isothermal flow, xStream precisely calculates all the parameters by solving all governing equations.
Typical Applications of AFT xStream include –
- Transient pressure and pipe forces during steam and gas turbine trips & startups.
- Parallel operations of Turbines, Compressors, and disrupted delivery conditions during trips.
- Pipe forces during relief events
- Tank Blow down and charging.
- Gas pipeline transients
- Flow delivery timing
- Heat exchanger tube ruptures
- Initiate transients based on time or events in the system.
If you have already used AFT Fathom / Arrow / Impulse software earlier, then you must be aware of different input windows for System (Fluid) Properties; Steady State Solution control (Tolerance, relaxation, solution method & matrix method); Transient Control; Pipe sectioning; Pipe Forces and other General Inputs. AFT xStream is has brough all these things to one common place named ‘Analysis Setup’ window which gives better understanding and control over input settings. This is one of the best updates, I came across and it is expected to come up in upcoming versions of Fathom, Arrow, and Impulse also.
Sample Case Scenario
Let’s discuss about below high-pressure steam system piping having two turbines which are simultaneously valved shut at individual headers. Initially flow rate of 62.75 Kg/sec was flowing to both turbines from the source at 16.75 MPa (167 Bar) and 570 ‘C. Which was brought down to 0 Kg/sec in 0.25 and0.5 seconds (2 scenarios) at both turbines simultaneously.
Figure 1 : Steam System in AFT xStream with Turbine transient data
Figure 2: Force Vs. Time on P5 & P6 with 0.25 Sec, 0.5 Sec Valve shut
For 0.25 second shut, maximum force was observed is around 20 kN while in case of 0.5 second shut maximum force was observed around 10 kN.
By performing transient analysis engineers can find out the minimum time required for valve closure, this will prevent rapid pressure surges and transient forces on piping. From AFT xStream these forces can be further exported to Pipe Stress Analysis software tool like Hexagon CAESAR-II and other for dynamic stress analysis (time history and spectral analysis). There is an interface available in both AFT xStream as well as CAESAR-II for exporting forces to CAESAR-II and reading the forces file from AFT Impulse & xStream.
Pulsation Frequency Analysis (PFA)
Taking it to one step further, there is an add-on module also developed ‘Pulsation Frequency Analysis (PFA) for AFT xStream, which helps identify and avoid resonant frequencies in systems, especially in those caused by reciprocating compressors.
About the author:-
Rachit Jain is a Mechanical Engineer & MBA (Energy Management) with years of experience in Training, technical support and pre-Sales presentation for Fluid Flow & Pipe Network Analysis, Flare Radiation analysis and 2D-3D CAD Modeling. With a profound technical knowledge and typical industrial application for world’s leading software tools AFT Fathom, Arrow, Impulse, Flaresim, CAESAR-II, InstruCalc, CADWorx P&ID-Plant. Rachit also extends his arm to engineering services teams on Steady State Hydraulics and Surge Analysis projects for piping systems.