You should check if AD Merkblatt still applies. Essentially these were specifications for pressure containing equipment and accessories such as valves. The Pressure Equipment Directive 'PED' has been the regulation since May 2002. However, some clients/users may want to still apply the requirements of AD- Merkblatt. If the installation is in a EU country, then PED is mandatory. You probably are in possession of a request for a replacement built in the ore PED days. I would check with your client.

*** Per ISO-4126, the generic term 'Safety Valve' is used regardless of application or design ***

*** 'Pressure-relief Valve' is the equivalent ASME/API term ***

In Germany the AD-Merkblätter were used pre the introduction of the PED. Nowadays we have the AD2000 Merblätter that full fill the requirements of the PED (safety, documentation,...).
PED only describes general safety requirements.-> You don't fabricate something according to the PED but to a code that full fills the requirements of the PED. This can be AD2000 or EN 12516 or even the ASME code( with additional testing) .
Part of the documentation you have to create for the valve is usually a declaration of conformity to the PED.

For AD2000 as the valve manufacturer you will need to be certified according to AD200 HP0.
Certification can be performed by a Notified Body (e.g. TÜV).

As to the requirements for the castings and forgings I replied to that in your other post here: https://www.eng-tips.com/viewthread.cfm?qid=483270