There are several packages on the market for transient flow analysis for gasses. These will do fine with flare systems - even though they do not aim specifically for flare systems.
One such package is Flowmaster, another is Pipenet and so forth.
The vary in price and ease of use - but i think they will be cheaper than a complete HYSYS/Flarenet instalation.
However, you may want to look for a consultant if its a one-off job.
Best regards
Morten