__Minimum requirements for Dynamic Machine train Simulation__

__Status: 28. February 2015 __

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

- 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.

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

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

- 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.

- 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.

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

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

- 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.

- 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.

- 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.

- 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.

- 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.

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

- 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.

- 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.

- 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.

- 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

- What are the questions which shall be
answered by the simulation study
- PID or flow diagram of system to be
simulated
- Material and Heat balance
- Define model boundaries. Where can
pressure, temperature, flow be considered independent from the
compressor/turbine performance?
- Steady state or dynamic relation of pressure,
flow, temperature at model boundary
- Gas properties (Gas composition, Mol,
kappa, z, …)
- Which units shall be used (SI,
imperial, …)

Train:

·
Inertia

·
Losses
(constant, speed dependent, …)

·
Gear
ratio

·
Nominal
speed

Compressor:

- Steady state performance maps head
versus flow
- Surge line as function of speed for
given IGV (to calculate rundown surge)
- Efficiency map
- Compressor inner volume

Valves:

- Cv value
- Open and close time for control and
trip
- Ccharacteristic (linear, ep…), if
necessary give curve

Pipes, Vessels, …

- Gas volume
- Pressure drop at given flow
- Process function, e.g. separation of
liquids

Cooler, Heat Exchanger

- Gas volume
- Pressure drop at given flow
- Heat load
- Design data of pressure, temperature
and flow for inlet and outlet of gas and cooling/heating flows
- Logarithmic mean temperature
difference

Motor/Generator:

- Nominal speed
- Nominal load
- Start and normal operation characteristics
(torque versus speed curve)
- Start up time (if available)

Turbine, Expander:

- Steady state design data for
pressure, temperature for in- and outlet and head, power and flow at various operating
points,
- Inertia (part of train inertia)
- volume