What's a Maximum Allowable Operation Pressure for ball valves since I haven't find this parameter in API 6D and ASME B16.34.
Maximum allowable operating pressure can be stated or are normally given or described for:
a) single components as: vessels, instruments, valves and pipelines and pipeline components etc.
b) complete constructions assembeled as: vessels with equipment, process constructions with pipelines, instruments vesssels and valves etc.
Maximum allowable working pressure is the maximum pressure that regularily during the process can occur as a normal part of the process, or continously permanently be present during the process.  It can thus never (of course) be above the normal allowed operating pressure for the given designed pressure class (allowed pressure related to the temperature) for the valve or component.
For robust components as ball-valves, the allowed maximum working pressure, certified and allowed from the factory, is normally equal to the pressure given by the pressure class.
That seat thightness and body normally are tested above pressures for the pressure class, do not indicate that mawp is allowed to be set above the pressure class limit.
For the total design with other components the mawp can be set a bit lower than the pressure class as a safety limit.
Some norms (EN (European - equal for different European countries) for vessels among others) describe how different pressure limits are to be understood, and allowed variations. 

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