Dynamic simulation vs steady state simulation and PSV sizing
Dynamic simulation vs steady state simulation and PSV sizing
(OP)
We're having a lively discussion in our department where and when to apply a dynamic simulation for sizing a relieve valve. I've argued that dynamic is not necessary for a fire scenario since this is a steady-state process. It has been suggested that a dynamic simulation is an absolute necessity for PSVs sized for closed valve. The engineer arguing this case says that dynamic simulation can be used to justify a drastically smaller valve. There is also an argument to be made that the dynamic simulation won't be sanctioned by API so who cares.
Can you think of any other applications where a dynamic simulation is required?
What do you think?
Can you think of any other applications where a dynamic simulation is required?
What do you think?





RE: Dynamic simulation vs steady state simulation and PSV sizing
1) calculations of discharge loads from systems with thermal inertia and liquid hold-up. This is the typical case for the HEATER-COLUMN systems.
2) analysis of combination of multiple loads and their variations with time. For example, in a power failure scenario you'll have various loads to the flare. If you don't consider dynamic effects, your flare shall be sized for the sum of all the peak loads. With a dynamic simulation you can model the variations of the single loads vs the time and easily see that the various peaks will be at different times. At the end, the design load for the flare will be lower. This concept is widely applied in flare revamping studies.
3) calculations of discharge loads originated by emergency depressurization systems.
RE: Dynamic simulation vs steady state simulation and PSV sizing
I used to normally calculate both steady state and unsteady state.
The unsteady state calc was used to only report time duration required for the external fire to activate the safety valve.
If the duration was greater thn 30-minutes, I would ocssionally invoke risk base analysis to mitigate any facility deficiencies.
Today I only calculate steady state.
RE: Dynamic simulation vs steady state simulation and PSV sizing
SD
RE: Dynamic simulation vs steady state simulation and PSV sizing
I think you're saying your system is open during the fire. That's OK as long as there's good justification for that. With a closed system (default assumption) fire is indeed a dynamic event, unless the system contains a single component. In most cases you're dealing with a mixture, which means the relief composition changes from the beginning to the end. Of course, the relief stream will start off high in light components and then gets heavier with time.
A dynamic simulation model will give you a clear picture of this changing composition, but in most cases I personally don't find that to be worth the time. I look at the composition and choose a conservative component, or mixture of components, to use as a sizing basis. A low Hvap is generally the worst-case for sizing purposes. You can spend a great deal of time doing a dynamic simulation, and come up with an answer that's not significantly different form the answer based on your intuition.
Risk tolerance varies from one company to another, and one individual to another. I think you have to have a very high risk tolerance to say that a dynamic symulation will greatly reduce the size of the PSV. For example, let's say that the dynamic model (changing composition) shows that a P orifice is needed in the early stage of the event and an F orifice is needed at the end. Yes, you can save a lot of money by installing the F orifice. But, to me, what you're really doing is saving money by shifting your risk tolerance, rather than saving money by applying a dynamic model.
RE: Dynamic simulation vs steady state simulation and PSV sizing