ERW Pipe is short for electric resistance welded pipe.

HFW Pipe is short for high frequency welded pipe.

According to API 5L Spec, the definition of HFW pipe is ERW pipe produced with a welding current frequency equal to or greater than 70 kHz.

So, HFW pipe is a kind of ERW pipe based on the welding current frequency. In general, when a customer require ERW pipe, he probably means HFW. That’s why steel pipe people usually regard them the same.

This specification covers nominal (average) wall steel pipe piles of cylindrical shape and applies to pipe piles in
which the steel cylinder acts as a permanent load-carrying member, or as a shell to form cast-in-place concrete piles.

ASTM A106 specification2 covers seamless carbon steel pipe for high-temperature service (Note 1) in NPS 1⁄8 to NPS 48 [DN 6 to DN 1200] (Note 2) inclusive, with nominal (average) wall thickness as given in ASME B 36.10M per ASTM A106.

This specification2 covers seamless and welded black and hot-dipped galvanized steel pipe in NPS 1⁄8 to NPS 26 [DN
6 to DN 650] (Note 1), inclusive, with nominal wall thickness (Note 2) as given in Table X2.2 and Table X2.3 in A53 spec.

In spec API 5L, the PSL means product specification level.