Dear Richard & Dave

I found two allowable equations in US unit & SI unit _ API 617 for COMPRESSORS

US unit 3F+M<=927D where F_lb, M_lb-ft, D_inch

SI unit F+1.09M<=54.1D where F_N, M_N-m, D_mm

I tried to convert from US system to SI system but I didn’t get that above equation.

But I manually calculated with both the formulas, both the results (% of Allowable) are matching.

What is the technical background of those equations, why conversion is not possible?

please correct me, if I am wrong.