A335 is an ASTM standard for seamless ferritic alloy-steel pipe for high temperature service and is also found in section II of the ASME Boiler and Pressure Vessel Code. As the standard states, "Selection will depend on design, service conditions, mechanical properties, and high-temperature characteristics." The grades you mentioned, P11, P21, and P22 are all chromium/molybdenum steels that vary slightly in the amounts of chromium and molybdenum they contain. This variation is enough to cause slight differences in physical properties that might be important for you application. An example of this is the limiting design metal temperature. According to ISO 13704, the limiting design metal temperature for P11 is 1100° F but for P21 and P22 is 1200° F. Other physical property differences could come into play when determining design details and pipe/tubing thickness. I suggest you consult the applicable design standards for you application and also seamless ferritic alloy-steel pipe manufacturers for aid in selecting a specific grade.