The NEMA has published old (very old!) nozzle force and moment limitations for turbines and rotating machinery. But all observers challenge those limits - which are based on a nominal diameter of the nozzle and total "area" removed from the casing of the turbine or pump by all nozzles as impractical, overly restrictive, and not based on actual turbine casing strength.
The original limits were drawn up in the 50's based on rules of thumb used at the time, and those equations and their limits have not been updated based on FEA or any real-world analysis since then even though the NEMA document itself have been updated. Use the turbine manufacturer's limits, if those are published.
The API has published different limits based on vessel pressures usually much higher than the steam turbine nozzles and thin steam turbine casings assumed by the NEMA, but I am aware of no such "Use twice the API limit (for pumps and compressors)." Yes, for steam nozzles, the NEMA limits on forces and moments have been tested on actual steam turbine casings for nozzles less than 10 inch diameter. Those steam nozzles tested do resist up to twice the NEMA limit, but you cannot extrapolate that series of tests to "all nozzles made of all materials at all pressures at all temperatures".
