Steady State Simulation
A system in a steady state has numerous properties (pressure, temperature, mass flow, heat rate, etc.) that are steady (not changing over time). A system can however be solved and evaluated at different steady state conditions for a specific range of properties either at operating conditions or off-design conditions.
Consider a pipe with an inlet pressure of 120 kPa and an outlet pressure of 100 kPa, the steady state result will be the mass flow generated due to the 20 kPa pressure drop across the pipe, the pipe geometry and the working fluid properties.
Advantages of Quick Steady State Simulation
- Immediate answers to system condition variation
- Determine results at specific conditions
- Quick what if, design, parametric, sensitivity and optimization studies
Dynamic/Transient Simulation
Dynamic simulation gives users the ability to model the time varying behaviour of a system. This allows users to understand and analyse the dynamic behaviour of complex systems. Simulating dynamic effects is like predicting reality, since most plants and facilities engineers design, construct, control and maintain are dynamic by nature!
Again we can consider a pipe with a inlet pressure of 120 kPa and an outlet pressure of 100 kPa, the dynamic result will now show variation in the mass flow generated if any boundary conditions are changed in the network. For example if the inlet pressure is increased or decreased at any stage in the simulation. Dynamic results will be time dependent.
We can also consider a tank which is being drained or filled with fluid as a transient simulation, because the volume of fluid contained in it and also the pressure head due to the water level in the tank changes with time.
Advantages of Transient Simulation
- Determine behaviour of plant/system over complete operating range: start up, shut down, accident scenarios, transition between different modes and states.
- Ensure correct design and material selections that can bring big savings in plant construction (avoid overly conservative approach).
- Combination of control and plant thermal hydraulic simulation allows engineers to identify if the operating problems, e.g. emissions, are caused by plant modifications or control philosophy or a combination of both.
- Effective control strategy can bring very big operational cost saving (avoid overly conservative approach – e.g. oversized pump runs 90% of the time not at its optimum efficiency, resulting in unnecessary operating cost).
- Design for and optimise components to ensure optimum system behaviour, even during off design and transient behaviour. E.g. a tank blow down through a nozzle will cause temperature drops that needs to be taken into account when materials are selected, valve positioning is considered and time restrictions on the speed with which the valves needs to open or close.
- Design and commission control systems using simulations and just fine tune on installations (shorten construction projects by months). The dynamic response of a system and the thermal inertia is sometimes difficult to determine if a simulation taking this into account is not available. One can even identify the best or ideal measurement positions on the plant for optimum plant control before the plant is fully commissioned.
- Dynamic integrated simulations will: Identify bottlenecks, Identify inefficiencies and Identify safety risks not identifiable with steady-state or segregated simulation.