Malleable Iron vs Forged Carbon Steel Fittings

[NCENG78]

Well I'm trying to find some information on why I would use malleable vs forged CS fittings. Our current piping specificaion for general water service uses ASTM 53 grade B pipe and malleable fittings. Can someone please tell me when I should and should use malleable vs forged CS fittings. Thanks

 

[moltenmetal]

There are two classes of malleable iron SCREWED fittings: 150# and 300# under ANSI B16.3. 150# are common as dirt in both plain and galvanized form. Typically they're used in low hazard services like water, air, low pressure nitrogen, low pressure natural gas distribution etc.

The pressure/temperature rating for 150# fittings is 300 psig from ambient to 150 F, falling to 150 psig to the maximum temperature of 366 F (i.e. saturated steam at 150 psig). The 300# MI fittings have ratings which depend on their size and which are much higher but still MUCH lower than 3000# forged steel fittings.

We stop threading at 2", favouring Victaulic for these services in the larger sizes (EXCEPT LP steam and condensate- Victaulic is no good for those services!).

300# MI fittings are MUCH heavier, and in my experience much harder to come by. We don't have much use for them, preferring 3000# A105 forged steel fittings instead. Note that they're sometimes quite similar in exterior dimensions and that 300 marking looks an awful lot like 3000 sometimes. The risk of confusion between the two is pretty significant. In our shop that risk is significant enough for us to avoid 300# MI fittings entirely.

 

[MJCronin]

bdear......

You would use MI fittings where you wanted to save money..

molten....

ASME B31.1 (and other piping codes) have service restrictions on where MI fittings can be used.

Its always a good idea to check the code of record (and develop piping line specifications) before beginning a large and expensive project...

-MJC