Rating of threaded fitting

Rating of threaded fitting is taken as Cl 300 & not as per ASME B16.11 viz. 3000# .

Spec : Cl 125 FF , galv CS , 0.125" CA

Pipe MOC : Galv CS A53 FBW / A53 / API 5LB

Large Fit MOC : CS A234 WPB

Small Fit MOC : Galv MI A 197 ( 2" & <)

Flange MOC : CS A105

T & Pr. lts : -20 to 366 F @ 150 to 150 psig viz 150 psig

Associated Pipe ( 3.4") : A53 Typ F , FBW , Sch 160 , T & C , Galv

Elbow – 90 ( ¾") : FNPT , A197 , Cl 300 , Galv

Coupling ( ¾") : FNPT , A197 , Cl 300 , Galv

Service : cooling water

Pipe ( ¾") is coming from header pipe( 10") .

Pl. explain the logic .

Thanks