You might want to obtain a copy of API Standard 611 General Purpose Steam Turbines Petroleum, Chemical, and Gas Industry Services. This standard covers steam turbines for mechanical drive services similar to NEMA SM 23. The most recent issue is the Fifth Edition, March 2008. Unfortunately, this issue includes a few references to NEMA SM 23 since SM 23 was withdrawn after the Fifth Edition of API 611 was issued.
Best of luck!