The ASME Boiler and Pressure Vessel Code (2004 Edition), Section VIII, Div Introduction lists pressure vessels that are exempt from the scope of VIII, Div 1.

The following notes could apply to your application;

U-1c, 2c - pressure containers on reciprocating parts or are integral components on rotating pumps, compressors, etc..

U-1c, 2i - vessels having an inside diameter, width, height or cross section diagonal of 6" or less, with no limitation on length or pressure.

Of course, the above exemptions only apply if the Jurisdiction (which is the authority that regulates boilers and pressure vessels where the object will be installed) does not have more stringent requirements regulating pressure vessels. You need to check on this to be sure.