What is Cyclic Service in Pressure Vessel?

This would be a pressure vessel that cycles frequently from higher pressure to lower pressure. Local/regional Codes may have some definitions about how much the pressure difference is, and how often these cycles could occur. In extreme cases, metal fatigue could become a factor.

Have you tried doing any research on your own, like starting here? You need to understand your first question so you can figure out your second and third questions. These all sounds like Home Work ??, or your trying to design a tank that is beyond your abilities. Designing any pressurized tank should done by a Certified Engineer, particularly one's holding flammable gases.

Tornado has defined cyclic service for you. The other questions can only be answered by someone familar with the intended use of the vessel. There are standards relating allowable stress to number of cycles in design life.

←Look in here for your answers.