Safety and Relief Valves are completely governed by codes and regulations. The two major codes are ASME (USA) and PED (Europe), and both are laws and failure to comply is a criminal offence. As you would imagine, they are both very similar but also do have some distinct differences. Because the codes are law, they must be adhered to when equipment is installed in a specific region. Most manufacturers therefore have both approvals. When installing a safety valve however, to be compliant with these codes you not only need to use an approved supplier, but you also need to select settings, and install them in accordance with the codes.
The governing standards and recommended practises are API 520 (USA) and EN4126 (Europe). API 520 incorporates more detail on correction factors (more conservative) for back pressure and viscosity, hence is the most widely used for valve sizing.
Europe
PED 97/23/EC - Pressure Equipment Directive - Classifies pressure equipment into four categories (I, II, III, IV)
EN4126 - Safety devices for protection against excessive pressure
USA
ASME I - Direct fired pressure vessels
ASME III - Nuclear power plants
ASME IV - Heating Boilers
ASME VIII - Unfired pressure vessels
ANSI/ASME PTC 25.3 - Performance test codes
API 520 - Sizing selection and installation of pressure relieving devices in refineries, Part 1 Design, Part 2 Installation
API 521 - Guide for pressure relieving and depressurising systems
API 526 - Flanged steel pressure relief valves
API 527 - Seat tightness of pressure relief valves
Others
Korea - KS B 6216 - Spring loaded safety valves for steam boilers and pressure vessels
Japan - JIS B 8210 - Steam boilers and pressure vessels - spring loaded safety valves
Australia - SAA AS 1271 - Safety valves, other valves, liquid level gauges and other fittings for boilers and unfired pressure vessels
International - ISO 4126 - Safety valves - general requirements
The European PED, from a legal perspective replaces the very many local codes in all European member states such as BS (UK), TUV (Germany), AFNOR NFE (France), Stoomwezen (The Netherlands), ISPESL (Italy).
Some European users may also stipulate their local code however in addition to the PED. Local codes have different ways of presenting things but often the results are the same.
Compliance with the PED allows a manufacturer to CE mark their product.
Further information on ASME and API
ASME B16.34
Applies to valves with flanged, threaded, and welding ends. This standard covers pressure/temperature ratings, dimensions, tolerances, materials, non-destructive examination requirements, testing and marking. This standard is not specifically applicable to Safety Relief Valves but is often used by manufacturers as good engineering practice.
ASME B16.5
Applies to pipe flanges and flange fittings. Provides allowable materials, pressure / temperature limits and flange dimensions for standard ANSI flanges.
ASME Stamps:
ASME itself does not certify the valves. This is done by a National Board (NB) and they certify the valves capacity and the valves compliance with the ASME code. It is then published in the "Red Book" - NB18, along with the true flow co-efficients as measured and approved by them. A copy can be found at www.nationalboard.org