各种管接头的标准

Standard Pipe Threads

There are many pipe thread specifications but they fall into one of two categories: The North American or US Standard and the Rest of the world.

The US has the National Standard Pipe Thread designated NPT and a variation of it, the dryseal thread designated NPTF. They are really the same thread. The dryseal thread differs in that the roots of the threads are slightly more truncated than the crests. The dryseal was designed to be sealed without thread sealant. If you are going to use thread sealant anyway, the fittings can be used interchangeably. Sometimes you will see NPTF used to designate National Pipe Thread Female and NPTM for National Pipe Thread Male. If someone tries to tell you this, insult them properly for being idiots.

The rest of the world is based off of the British Standard Pipe Threads. There are tapered and straight thread versions. Sometimes you will hear them called BSPT and BSPP respectively. The British don't call their threads that. They designate them as "R" and "R Tr" respectively. Germany, France, and Japan all have their own names for the threads. They are shown in the table.

The National Standard and the British Standard also differ in that their pitches or threads per inch are slightly different. If you pick up two fittings they will almost go together. Don't try to put them together though. At best, you'll just ruin both fittings. Only the nominal 1/2" and 3/4" have the same pitch. Still, don't try to put them together (it may work) because the thread forms are slightly different and you may have a difficult time putting them together.

Why the two thread systems are similar but not quite the same is a mystery to me. My theory (being an American) is that it was invented here and the Brits couldn't stand it so they changed it just slightly so they would never have to use an American made product. The rest of the Europeans (and Japan) couldn't stand using an English thread and further changed the name and called it metric. I'm sure the rest of the world would say that it was the Americans that changed it and made our own

Pipe is identified by it's nominal size. Originally, the OD of pipe was defined by it's ID. Given a wall thickness suitable to holding the required pressure (using the materials at the time) the OD was determined. With the advent of modern steels, the OD has remained constant and the ID has increased. Different wall thicknesses are now available to handle different pressures. They are defined by the ASME Boiler and Pressure Vessel Code and are designated as the Schedule number. Schedules 20, 40, 80, and 160 defined but the most common are Schedules 40 and 80.

Tubing is designated by its OD. A 1/2" tube ha a 1/2" outside diameter. It's inside diameter is something less. Hose on the other hand is designated by it's ID. A 1/2" hose has a 1/2" inside diameter. It is common practice when plumbing systems with hose, pipe, and tubing to use tubing the next nominal size up from the hose. Hose and pipe are typically plumbed at the same nominal size. Even though most Schedule 40 and 80 pipe has a larger ID, it is also rougher and the increased diameter