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.

Of course you can convert units, keeping in mind that "3", "1" and "927" are dimension coefficients chosen as the terms are dimensionless, so you can write down
[3*lbf^(-1)]*F+[1*lbf^(-1)*ft^(-1)]*M <= [927*in^(-1)]*D

Making the units conversion and factoring the inequality by 1.48274 (for cosmetics purpose only!), you get more precisely

[1*N^(-1)]*F+[1.093613*N^(-1)*m^(-1)]*M <= [54.11418*mm^(-1)]*D
or
SI unit
F+1.093613 M<=54.11418 D where F[N,] M[N*m], D[mm]

As you can see, in practice F+1.09M<=54.1D gives sufficient precision for equivalence with the formula using U.S. customary units.

Thanks Mariog,

But why you taken “3", "1" and "927" are dimension coefficients chosen as the terms are dimensionless

[3*lbf^(-1)]*F+[1*lbf^(-1)*ft^(-1)]*M <= [927*in^(-1)]*D

What I made: F is in lb. I made direct conversion of lb to N. then conversion US  SI not coming.

But you made that equation unit-less then converted. What is the reason behind that.

Please correct me if I am wrong.

You would think to the possibility to add "3F" to "M". In case the terms have dimensions, you cannot perform such sum.
In real world, would you sum your height with your weight? How you explain the result?

Thank you . Its clear now.