Plan 53M is same as Plan 53B. Please refer to API standards for definition of each Plan. A good example where you can use plan 53C is where you have varying high pressures. (Offshore) In this case Plan 53C follows the changing pressures and maintains a delta P. The delta P may is dependant on the piston ratio for example it may be 10% increase on it pulse pressure normally discharge. If you apply Plan 53B to this then the barrier pressure needs to be set to the max pressure and the operating pressure will be high even at lower chamber pressures.
Some people have a preference in using Plan 53C. My preference 53B, I like its simplicity.
What is your application?
