Butt Fusion vs Electro Fusion - Pipelines, Piping and Fluid Mechanics engineering

You don't say it but I assume you're talking about Poly Ethylene (PE)??

It is possible to use butt fusion for fittings, but normally this is only feasible in a factory setting as you need a lot of jigs and things to hold the bits in the right place whilst you heat them up and then hold them steady whilst they cool. Out in the field it is far easier and higher quality to use fusion couplings.

Have a look at this - download the technical guidelines document - it's excellent
The difference - butt fusion melts the PE directly (often a hot plate) and whilst still "molten" fuse it ot another also heated PE thing under force which extrudes seom PE and tis then allowed to cool with the materials fusing to each other in a homogeneous manner

electrofusion has a fitting which is in close contact with another piece of PE. within that fitting there are small electric wires which heat up, causes both surfaces to melt and then once the electricity is turned off, again it fuses. Because the metal wires are left in the fitting and to make it in the first place is a bit more, fusion is more expensive, but much easier to accomplish in the field.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it. It is interesting that while many, including up into the hallowed halls of Congress and some special interest industry, were quick to blame e.g. some gray cast iron pipes with ancient joining technology (that had incidentally provided at least some level of service since the time of Grover Cleveland) for the unfortunate, deadly and initially well-publicized East Harlem gas pipeline blast last March, some information that has dribbled out in the more than a year since have indicated maybe, "Not so fast...."
The article e.g. at e.g. says there may well have been some involvement with separation of a "T-shaped connector" in a relatively brand new (~3 years old) plastic pipe and service installation near the scene. While apparently some very small leaks were also found in the surrounding main area, it will be interesting to see (the final report may still not be out in this matter after more than a year!) what exactly is involved here, as electrofusion is in general attempted not only in socket type, but also for lateral/service connections.
In this regard, it appears from at least the preliminary "Operations Group Factual Report" this year, I also happened to see not long ago at , that the East Harlem "T-shaped connector" that separated from the main had in fact been "electrofused" in at least some extent to the plastic main. I had noticed this reported separation was described by the third-party gas pipeline safety consultant in the WSJ report as, "This is going to leak a lot of gas,"
For any not familiar, there are various requirements for different butt fusion and electrofusion joining procedures and fusers of plastic pipes in the Code of Federal Regulations (CFR).