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Malleable Iron Fittings for Instrument air service

Malleable Iron Fittings for Instrument air service

Malleable Iron Fittings for Instrument air service

This subject had a reference under thread404-7823: Malleable Iron Fitting Rating,

Why is that Instrument air service of 7 bar (g) pressure cannot use MI fittings. Why is it required to use class 3000 threaded forged fittings. In some cases it is required to go for class 6000. I do not see it used although allowed by B31.3. ASTM A197 is a listed material and the PT and service conditions does not exclude use.

RE: Malleable Iron Fittings for Instrument air service

I'm not aware of any code prohibition against the use of A197 cupola malleable iron 150# B16.3 threaded fittings in instrument air service at 7 bar(g). If such a restriction exists, it is likely a customer/user rule rather than a code rule, and based on bias rather than on experience. We use galvanized steel pipe and 150# MI threaded fittings from 1/2" through 2" for instrument air, nitrogen and cooling water on nearly every plant we build.

We do restrict their use to Category D services and do not use them for either steam or condensate unless it is of very modest pressure. We wouldn't use them with 150 psi saturated steam. Maximum service temperature is 366F (177 C) which happens to match perfectly with the upper limit of the anaerobic pipethread sealant we use in addition to heavy 1.3-1.5 s.g. teflon tape we use with these components.

We don't use 300# MI fittings though. In fact we keep them out of our facility entirely. They are too easily confused with 3000# A105 carbon steel components- they have very similar appearance and it is easy to mistake the markings too, and that can cause a serious problem.

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