Pipe threads are a lot like suction strainers in that they have no place in a modern, properly designed hydraulic system. HOWEVER, pipe threads are a legacy item, and as such, are still a feature of many (too many) hydraulic systems still in operation today. So every hydraulics user needs to know about them.
National Pipe Tapered (NPT – U.S.) and British Standard Pipe Tapered (BSPT – UK) were originally designed for water pipe plumbing (60 psi rating). Both NPT and BSPT thread forms engage on the sides (flanks) of the threads. This leaves a spiral leak path at the root and crest of the threads, which makes the use of a sealing tape or compound essential for any chance of a leak-free connection to be achieved. Trouble is, the use of such compounds introduces another potential contaminant that can find its way into the hydraulic system (and often does). And even when a sealing compound is used, a reliable, leak-free connection is not assured.
National Pipe Tapered Fuel (NPTF) is another pipe thread which differs from NPT in an important way. The threads of NPTF are designed to engage at the root and crest of the threads first, and then the flanks of the thread. This is meant to eliminate the spiral clearance associated with NPT threads, and therefore, not require the application of a sealing compound. Which is why NPTF is also referred to as Dry Seal.
Although NPTF is a superior pipe thread to NPT, it cannot be relied upon for a reliable, leak-free connection, should not be used on any new hydraulic system, and be engineered-out, if possible, on legacy systems.
For those not willing to take my word for it, the National Fluid Power Association (NFPA) recommends that neither NPT nor NPTF, and by extension, BSPT, are used in hydraulic applications. In other words, using pipe threads on a modern, high pressure hydraulic system is a mistake. And for 6 more costly mistakes you want to be sure to avoid, get “Six Costly Mistakes Most Hydraulics Users Make… And How You Can Avoid Them!” available for FREE download here.