1. Connections where the screw thread provides both the mechanical join and the hydraulic seal.
2. Where the thread provides the mechanical connection but there is a separate hydraulic seal.
The best known and most widely used connection in the first category is the American National Pipe Tapered Thread (NPT). It is sometimes just referred to as IPT or EPT, the I or E indicating Internal or External. This thread is based upon the old Briggs design and has a 60° angle (Figure 1) and the diameter tapers ¾" per foot (1 in 16). The thread is usually designated in fractional inch dimensions. This dimension refers to the nominal bore of the tube or pipe and the actual outside diameter of the thread is approximately 5/16" to 3/8" larger than the bore diameter. The chart below gives some brief details of the thread and the diagram (Figure 2) gives an indication of the pipe sizes. Over 1 inch bore up to 2 inches, the threads per inch are 11½, and over 2 inches, 8 threads per inch.
Although this thread is still very popular, it has a number of practical disadvantages. Firstly, when two parts are screwed together, there is still a leakage path between the crest of one thread and the root of the other so the connection requires some form of liquid or plastic sealant to fill up the gap and prevent spiral leakage along the thread. In the last decade or so, it has become very popular to use this thread with PTFE tape acting as the sealant. PFTE has a very low frictional resistance with the result that the two parts can easily be screwed together. The tapered thread acts as a very effective mechanical jack and if the internal port thread is machined into a casting, the casting can often be easily split.
A number of variations of the NPT thread has been introduced to overcome the problem of spiral leakage and they are generally known as Dryseal threads. (Covered by the SAE standard J476). The best known is the NPTF (the F standing for fuel). In this thread design, the root diameter has been increased so that when the internal and external threads are screwed together, there is an interference fit between the crest of one thread and the root of the other so as to seal against spiral leakage. (Figure 3).
Another variation of the Dryseal thread is the NPSF (National pipe straight fuel). It is used for internal threads and an NPTF external thread can be screwed into it to provide both a satisfactory mechanical connection and hydraulic seal. A combination of a parallel and tapered thread would not appear to be ideal but it is widely used. Many quick disconnect couplers for example have the NPSF thread and are recommended by the manufacturer for use with NPT (!) and NPTF.
There are a number of other variations for special circumstances. The PTF is identical to the NPTF thread except that one thread turn has been eliminated on the small end of the external thread. It can be used with the NPTF internal thread and also the NPSI internal thread.
The NPSI is very similar to the NPSF but is designed for use in less resilient materials. The 'I' at the end stands for intermediate. There is also the NPSM (National Pipe Straight Mechanical) thread, but this thread specification does not come into this category as a separate hydraulic seal is required when it is used.
Sometimes, one finds reference to an API line thread. In this case, API stands for American Petroleum Institute, and for all practical purposes can be considered the same as the NPTF Dryseal thread.
Another tapered thread is the British Standard Pipe taper (BSP) covered by British Standard 21. This uses the Whitworth thread form where the valley angle is 55° with a 1 in 16 taper. It is not interchangeable with the American NPT thread, although at the ½" and ¾" size, they both have the same number of threads per inch - 14. The BSP tapered thread is still widely used for low pressure plumbing, but it is no longer common on medium and high pressure hydraulic circuits.
In the second category, where the thread provides the mechanical connection, and a separate seal is required, the most widely used thread connection outside the American Continent is the British Standard Pipe Parallel Thread, (covered by British Standard 2779) sometimes known as BSPP or BSPF. This thread has been converted to a metric standard and is commonly known by a G or R designation. (The G stands for gas as the thread was originally designed for use on gas pipes. The R is an abbreviation for Rohr, the German word for Pipe). ½ G (or sometimes G½) is the same as ½" BSPP or ½" BSPF. The Japanese pipe connection is again almost identical with the metric equivalent of the BSP parallel thread.
With BSP Parallel, the hydraulic seal is usually formed at the mouth of the internal thread by using a suitable gasket. This can be a copper or aluminium ring or a lipped seal such as the Dowty seal which is usually rubber bonded to steel. But there are similar designs made by other manufacturers of all metal construction. The illustration in Figure 4 shows a section through a port connection using one of these seals. A variation of this type of connection is where the BSPF thread has been replaced with an appropriate metric module thread but it is not very popular.
Another sealing method that comes into this category is the SAE O-ring, Boss. This pipe connection was originally developed for the aircraft industry and uses a UNF bolt thread for mechanical connection with an O-ring for the seal. An illustration is shown below in Figure 5.
The designation of hydraulic ports using the SAE O-ring Boss can at times be a little confusing. For example, the dash numbers refer to the size of tubing recommended for the size of port. The dash number can be converted to inches by dividing it by 16; for example a -8 is 8/16 - ½ inch, a -24 is 24/16 - 1½ inch. In the case of thin wall steel tubing, it refers to the O.D. of the tube with hydraulic hose to the bore. The -8 size uses a ¾" - 16 UNF thread, the -24 a 1.7/8 - 12 UNF thread. The chart below shows different sizes.
The use of a bolt thread gives a very good mechanical connection and the O-ring an excellent hydraulic seal. Parts screwed hand-tight together will not leak at high pressure. This is not a practice, however, to be recommended as if they are only hand-tight, they can easily vibrate loose and even with a small gap, the pressure will extrude the O-ring, together with a lot of oil. A metric version of the 'O'-ring Boss has been designed and is covered by the ISO standard 6149. It is not yet widely used but is likely to be extensively adopted in Continental Europe.
The unified thread is also used with the flared type connection where the hydraulic seal is formed by two tapered faces, either at 37° or 45°, (Figure 6). The 37° flare is often referred to as JIC (Joint Engineering Council). The JIC organisation seems to have disappeared some years ago, but the standard continues. This sealing method is usually adopted at the end of a flexible hose where a loose nut is tightened on a suitable adaptor.
The thread size follows the same arrangement as for the O-ring Boss, e.g. the -8 size being ¾" -16 thread. The 45° flare or taper is an alternative to the 37°. It is covered by an SAE standard but is generally not so popular as the 37°.
A 60° cone (inclusive angle), Figure7, is normally used with the BSP Parallel thread and a high proportion of the flexible hoses used in Europe usually have this end fitting. The 37° flare is used in Europe, but is more popular in the USA.
Another connection which is commonly used for higher flow and higher pressures is the SAE 4-bolt split flange type, an illustration is shown below, Figure 8. Here, two flanges are clamped together by a split clamp, the hydraulic seal being made by a face located O-ring. A variation of this design is also available as a solid flange.
When rigid tubing is used for piping up the hydraulic circuit, compression ring fittings are widely used in Europe. In America, this type of fitting is sometimes known as a flareless fitting. Figure 9 shows a typical example, but there are very many small variations for which different advantages are claimed. Of this type of fitting, probably the Ermeto is the best known. All the compression ring fittings have a common principle of a tapered olive being pushed onto the outside diameter of the pipe by a nut squeezing the olive into a tapered recess.
The various types of pipe fittings and pipe connections mentioned are very widely used, but they do not comprise a complete list. There is nothing to stop manufacturers making non-standard versions and there are a lot of non-standard fittings available. For example, there is no British or International Standard for an O-ring seal of the type shown in Figure 5 using BSP parallel thread. Nevertheless, it is manufactured. There are similar variations for almost every pipe fitting that has been manufactured. Fortunately, some 90% of pipe fittings comply to a standard, but watch out for the exceptions!