It is not clear what you are trying to do, and you show code, but not a compilable project where it would be clear how your objects are instantiated (if they are), and which methods, if any, are hooked to your spinedits.
However, you declare TPumpControl so that it inherits from TMainForm, and you redeclare the DutyCycleXFSEChange method, which is very strange, and wrong.
When you call DutyCycleXFSEChange you spiral into recursive calls.
Basically, your UnitB is a nonsense. What are you trying to achieve?