Metric pipe threads are a system of measurement that allow for metric pipe fitting to be connected to metric pipes. If the person uses the correct form of metric pipe threads, the threads will form a seal. However, if the individual use the incorrect form of metric pipe threads, the connection will leak.
The dimensions of metric pipe threads are standardize so that any fitting manufactured to those standards will have the ability to connect to any pipe manufactured to those standards. In order to create successful metric pipe threads, the person must understand the relationship between nominal sizes, thread pitch, and thread angle. In order to understand metric pipe threads, the individual must realize that the nominal size of a metric pipe thread is not the same than the outer diameter of that metric pipe thread.
How Metric Pipe Threads Work
For instance, a DN15 size metric pipe thread is often referred to as a half-inch pipe, but the actual outer diameter of that pipe is approximately 21 millimeter. Instead, the nominal size refers to the internal bore of the metric pipe, but the metric pipe thread is formed along the outside diameter of the pipe. Additionally, the thread pitch of the metric pipe thread will change according to the size of the metric pipe; larger pipes has deeper thread pitches to allow for the even maintenance of hydraulic pressure along the pipe, while smaller sizes of pipes utilize finer thread pitches to perform the same function.
Metric pipe threads often utilize a specific angle in their design, as do many of the threads along the length of the pipe. Metric pipe threads use a 55 degree angle, known as the Whitworth angle. This angle is different from the 60 degree angle use in the North American standard for metric threads.
Additionally, metric pipe threads may be of either the tapered or the parallel form. Tapered metric pipe threads are those that narrow along the length of the thread, while those that have the same diameter from one end of the thread to the other are referred to as parallel metric pipe threads. Because of the narrowing diameter of the tapered metric pipe threads, a wedge is formed that allow for the threads to seal when tightened.
In contrast, because the diameter of the metric pipe threads does not decrease along the length of the parallel threads, those types of threads do not naturaly form a wedge, and sealant are required to prevent leaks from those connections. Depending upon the form of the metric pipe threads, certain methods of sealing will have to be used. For instance, tapered male metric pipe threads will naturaly form a seal within the tapered female metric pipe threads due to the wedge shape of the threads.
In contrast, because the metric pipe threads of the parallel threads do not form a wedge when they are tightened, one must use external sealants to create a connection that does not leak. Many of those that use metric pipe threads utilize PTFE tape to seal the connection between two metric pipe threads of the same size. In instances in which the machinery containing those metric pipes may experience vibration, anaerobic sealants may be used to prevent leaks caused by the vibrations of the machinery.
Additionally, in hydraulic systems, O-rings are often used to seal the metric pipe threads; these O-ring allow the machinery to be disassembled and reassembled numerous times without the leaks of the metric threads. In all instances, however, the type of sealant should be according to the movements and pressures of the machinery in which the metric pipes are installed. The material of the metric pipe and the size of that metric pipe determine the pressure that the metric pipe threads can hold.
Pipes made of materials like steel can hold high amounts of pressure due to the strength of the metal and its ability to form deep metric pipe threads. Pipes made of materials like brass will hold medium amount of pressure, and pipes made of plastic will hold the least amount of pressure due to the relatively weak strength of that plastic material. Additionally, the diameter of the metric pipe also plays a major role in the amount of pressure that can be held by that pipe; the smaller the diameter of the metric pipe, the higher amount of pressure that it can hold.
This is due to the fact that the total amount of force that is applied to a small diameter metric pipe joint is less than the total force that is applied to a large diameter joint of metric pipes. In addition to understanding how the metric pipe threads should be formed and what types of seals should be applied to those threads, individuals must also avoid making mistake in the installation of those metric pipes. For instance, many individuals attempt to force the metric pipe threads to become seated by utilizing brute force to those pipes.
However, if too much force is applied to those metric pipes, the pipes may crack along their length. Instead, the number of turns of the two metric pipes after they are initially hand-tightened should be to the number that is recommended for the specific size of metric pipe threads. Additionally, two wrenches should be used during installation of the metric pipe threads so as to avoid potentially twisting the metric pipe during installation of the metric fitting.
In instances in which the individual must determine the type of metric pipe threads that are present on the joint of two metric pipes, certain steps can be used to determine the form of the metric threads. First, the individual must measure the outer diameter of the metric threads. In addition, a thread pitch gauge can determine the pitch of the threads.
If the diameter of the metric threads decreases along the length of the joint, then the joint has a tapered metric pipe thread. If the diameter of the joint is the same along the length of the joint, the joint has a parallel metric pipe thread. If the angle of the thread is measured to be 55 degrees, then the joint contains a metric pipe thread.
In no instances, however, should an individual attempt to thread pipes of different angle; regardless of the amount of sealant applied to the joint, the joint will not form a seal.
