When you begins the process of threading pipe, you must understand how the tapered threads will create an seal within the threaded joint. The threads work due to the fact that the male threads and the female threads wedge together as the threads advance into each other. Due to the fact that the male and female threads wedge together, there is an increases in the contact between the thread flanks as the threads deepen into each other.
This increase in the contact between the thread flanks creates an interference fit between the two pipe threads, and this interference fit creates the seal between the two pipes. Thus, tapered pipe threads doesnt require the use of a gasket or O-ring to seal the joint between two pipes of the same size; the threads create the seal between the two pipes. In order to create this seal, a person must ensure that the threads engage to the correct depth.
How tapered pipe threads seal pipes and how to fit them
If the depth is not achieved, there will be a leaks between the two threaded pipes. If the depth is too great, the fitting may crack or the threads may become stripped. In order to ensure that the proper depth is achieved, a reference chart for different tap sizes will allow a person to determine the drill diameter and the engagement depth for those sizes.
The reference chart will provide a depth for the engagement of the threads, and there are two measurement for that depth. The first measurement is the hand tight depth, which is the depth to which the person can assemble the fitting using only hand pressure. The second measurement is the wrench tight depth, which is the depth at which the person must use a wrench to achieve the seal.
The wrench tight depth will indicate the number of turns that is required for the fitting to be tightened past the hand tight depth to create the interference fit between the two threads. However, it is important for the depth that the fitting is tightened to not go past the wrench tight depth; otherwise, thin walled pipes may split, and the threads may become damaged in the process. There are different type of tapered threads.
One example of a type of tapered pipe thread is an NPT thread. The NPT thread is a standard tapered thread that leaves a small gap between the two male and female threads; a sealant must be used at the gap to prevent fluid from escaping the joint created by the two threaded ends. An alternative type of thread is an NPTF thread.
The threads for an NPTF fitting are cut to a tighter tolerance than NPT threads, and the two NPTF threads will crush against each other to create a seal without the use of a sealant. Since the NPTF threads do not require a sealant to create a dry seal between the two threads, NPTF threads are often used in systems that use hydraulic and fuel fluids, which contain chemicals that would otherwise destroy a sealant. Another distinction between different types of threads is that NPT threads are different than BSPT threads.
The NPT threads have a sixty degree angle to the threads, while BSPT threads have a fifty-five degree (Whitworth) angle to the threads. Due to the different angles to the threads, there will be a lack of compatibility between the two types of threads; thus, the two types of threads will not create a seal between them when connected. The incorrect use of these two different types of threads will result in a leak in the fluid system.
Another decision that a worker must make is which sealant to use in the joint between two pipes. For instance, people often use PTFE tape along with threads for water and air lines; the tape is chemically inert and will not react with the water or air flowing through the lines. However, anaerobic compounds will cure in the absence of air, and are used in hydraulic fluid ports that experience vibration.
For steam lines, which can reach high temperatures, thread paste is used; the paste remains pliable despite high heat. Another skill in creating good, even tapered threads is in the tapping process. The tap must be started in a square manner to the workpiece to ensure that the threads are not created at an angle.
Cutting fluid can be used to allow for the tap to be rotated with less torque and to allow for the threads to be formed more easy. Periodically during the tapping process, the tap should of been backed off the workpiece every half turn to remove the metal chip created during tapping; otherwise the chips will become lodged into the tap. Finally, the softness of the metal of the workpiece must be considered; metals like aluminum are softer metals than metals like cast iron.
