Compressor mishap

[NC Rick]

Parallel threads are a lot harder to seal, they are intended to be used with a gasket or some other sealing device. That is why things you don't want to leak use tapered threads.

Parallel threads are fantastic assuming there is enough flange on the spigot to properly support the sealing element (oring or crush washer). To me it looks like you may be stuck with a tapered fitting and goop which will also work.

 

[Lo-Fi]

For a point of reference regarding parallel Vs taper threads in a pressure vessel: I have yet to see parallel threads on any of the heritage steam boilers I work on, all of which have at least a dozen or so tapered "washout" plugs. Larger fittings are always flanges with bolts.
Washout plugs are sealed using nothing more than heavy graphite grease, are removed regularly and very rarely give trouble despite suffering ~200psi pressure and a couple of hundred degrees thermal cycling.

The only caveat I'd add is that tapered threads do require more care when tapping and seating, but you simply won't beat the sealing ability a taper gives, all else being equal.

 

[NC Rick]

I don't know about steam. How about high pressure gas or hydraulic systems? Taper threads are rough, have price advantages and simple alignment control. Reliance of thread wedging as opposed to engineered sealing joins.