Getting a Vessel Seal Certified

I don't know if "Certified" the correct term, so i'll explain the situation.

The company I work for has been selling a device that we have installed in industrial piping and reaction vessels since around 1990. We use a custom designed seal and high temperature/high strength stainless screws.We have always rated this system at 1500 PSI and 200C. We test every unit at 2000 psi @ 200C before shipping. We supply drawings to the customer on how to machine their mating piece,

Our Seal vendor has provided pressure/temperature specifications and we have calculations for the strentgh of the screws.

We have a customer that is requesting a "certification to code" that the unit is OK for his application.

For this application, we will be using our sealing method to install the device into both a 150# and a 300# flange blind flange that will be modified.

The applicatoin is in The USA.

Any thoughts/advice would be appreciated.

thanks