Minimum requirements for Dynamic Machine train Simulation

Status: 28. February 2015


  1. The process, which should be simulated, has to be segmented in discrete elements or modules with adequate fineness.


  1. For each discrete module at least pressure, temperature and mass flow must be calculated as function of time. It may be necessary to treat additional variables like enthalpy, entropy, mole fraction etc. in the same way.


  1. It must be possible to plot each calculated variable versus time and to store time and data vectors for further plots.


  1. Modelling must be based on sets of nonlinear differential equations for each individual module within the simulation model.


  1. Time increments for the numerical integration of the differential equations depend on the fastest part of a module. They shall be well below 1 ms to allow proper time resolution even under high speed transient conditions.


  1. Actual steady state compressor performance curves showing Head/Pressure Ratio versus inlet flow with IGV position and speed as parameter per compressor stage or stage group have to be used to calculate the compressor flow. It may be necessary to implement several maps for different molecular weights or suction temperatures.


  1. The model must allow to simulate the impact of reverse flow during surge


  1. The purchaser must have the option to specify that simulation runs are made with or without reverse flow during surge.


  1. The compressor train speed must be calculated from the torque balance of all driving and driven machines, considering bearing friction and other losses. Turbines and expanders have to be modeled similar to compressors. For electric drives the steady state torque versus speed characteristic may be used.


  1. The simulation model must allow to simulate the impact of flow turbulences at the compressor inlet flow measurement. These flow turbulences, typically called “noise” of inlet flow measurement, must be included in the simulation model. The purchaser will decide prior to the first simulation run whether this “noise” has to be included in the simulation.


  1. Valves and valve actuator need to be simulated such that nonlinear valve characteristic curves can be considered. If travel speed is not constant, e.g. through damping, this needs to be considered. Check valves are highly critical items for surge protection. Thus the simulation model must allow to simulate them in very detail (opening depends on torque balance of opening forces from gas stream which counteracts with disc and counter weight as well as bearing friction). Note that bearing friction may vary largely over time or valve opening throttle.


  1. The simulation tool must include a tool box for control system applications. Typical control functions such as PI-controller, tracking function, adaptive control etc must be included. Anti Surge Control has to be implemented according to purchaser’s description.


  1. The simulation model must allow different time steps for the model and the simulated control system. To simulate the scan time of digital controllers. If e.g. the simulation model runs with a sample time of 0.2 millisecond but the controller samples with 10 millisecond, the model must allow to simulate the control system such that all controller calculations are performed only once every e.g. 10 ms or every 100 ms and all output values are updated every e.g. 10 ms or every 100 ms as well. Sample time must be freely selectable and automatically changeable during a simulation run.  


  1. Any dead time either in logic controllers or valves or actuators needs to be simulated, similar to scan times of controller.


  1. The investigations must not be limited on the machinery system only. Upstream and downstream system has to be included as there is always a counteraction between upstream and downstream system and machine train. It may be necessary to create new sets of differential equations for upstream and downstream process elements.


  1. The simulation study vendor must be prepared to coordinate directly with different departments within purchasers organisation and collect required data from different sources in different departments.


  1. Results of the first steady state simulation results need to be issued for approval by purchaser and final end user. Purchaser and/or end user can change parameters and can provide flow/pressure/temperature data at different model points. Simulation vendor has to change model parameter to meet these requirements.


  1. Purchaser has the right to comment the final report of the simulation study and may ask for changes in wording, descriptive sections etc.

Information needed for a Dynamic Simulation




·         Inertia

·         Losses (constant, speed dependent, …)

·         Gear ratio

·         Nominal speed



Pipes, Vessels, …


Cooler, Heat Exchanger


Turbine, Expander: