Stainless steel ASME pressure vessels are very expensive and long-lived pieces of process equipment. Many tanks in use today still have years of good service left in them, but processes keep getting cleaner year by year.A tank can easily become unusable when a new aseptic process comes along. The surplus of these once-useful tanks, and the prices and long lead times of new tanks (now as much as 16 weeks at some plants), accounts for the increasing number of tank retrofits being carried out across the country.

The first approach is to cut the old valve out of the tank and weld in a new one. If the tank is a pressure vessel, it must be hydrotested in the presence of an ASME pressure vessel inspector. This process brings it into compliance with the pressure vessel code. It has been re-coded.

The second approach avoids cutting anything out of the tank. Instead you would leave the old coded flange welded to the tank, unbolt the original diaphragm valve, and bolt on a new aseptic tank valve whose inlet is specially designed to fit the flange already on the tank. This is called a custom retrofit and avoids the need to recode the pressure vessel.