Ductile Iron is not allowed in Refinery services on flammable Hydrocarbon (See API 610 10th - Annex G. The suitable selection is Carbon Steel (A216 WCB)as a minimum
About the sealing system solution I suggest the plan 53B.
It is suitable in case high suction pressure, since the nitrogen in confined inside the elastomeric bladder of barrier fluid accumulator. Moreover the consumption of Nitrogen is limitated at the volume necessary to charge the bladder.