I was investigating about LPG application, and the best option for this kind of product is gas dry seal, in special, if the pressure bore is close to steam pressure product.
John Crane has a 28VL seal, that is special for it. It is not necessary to use a Plan 52, maybe the inicial cost is high...
I recommend to use a dual arragment, other point that you have to consider is the SG and steam vapor, if the product is flashing, you have to avoid a leaking.