Equipment allowable nozzle load formula

[dasomjjang]

According to my document, there is below formula. Max. force and moment values of nozzles from piping stress analysis not covered in formula shall be met with the result calculated from the following stress formula.
Fr(max)=D*t*S/a
Mr(max)=D^2*t*S/b

Fr(max)= Max. resultant force in kg for nozzle in normal operating condition
Mr(max)= Max. resultant moment in kg-m of nozzle at reactive force of zero in normal operating condition
t = thickness of nozzle pipe
S = Allowable stress range due to the thermal expansion
D = Pipe size
a = Multiplication coefficient of reactive force
b = Multiplication coefficient of reactive moment

a = DN100 : 9.00 / DN150~DN200 : 7.06 / DN250 and larger : 5.88
b = DN100 : 25.0 / DN150~DN200 : 23.53 / DN250 and larger : 18.83
D = DN100 : NPS+1 / DN150~DN200 : NPS / DN250 and larger : (NPS+16)/3
I don't know where abve formula came from and what is the factor unit.
Please let me know the original formula and what does the mean the factor and the unit each factor.

 

[austsa]

If D is mm, t is mm and S is MPa, then Fr will be in Newtons and Mr in Newton-mm. Parameters a and b are dimensionless.