Connections of pipes, valves, fittings, and components can be very time consuming during the installation and maintenance of a system. Connections are often required to be leak proof and provide some mechanical support against gravity. For this purpose, fittings are used to adapt different sizes or shapes and often utilize a variety of connection technology. Among various connection methods, threads are one of the most common for joining pipes and fittings, including valves.
A thread is a ridge wrapped around a cylindrical or a conical shape, which by mating to another thread can provide a joint. Threads are designed in numerous sizes and shapes to serve a wide range of purposes. Ensuring the compatibility of the threads in a connection and the formation of a proper seal are important in achieving a reliable joint. Threads are identified by their design characteristics and are developed to provide certain degrees of joint reliability. When selecting a product with threads, it is important to understand these important thread specifications:
Threads can be internal (female) or external (male). All matching thread connections consist of a male and a female thread type. In a given connection, it is important for the threads to have the same design characteristics to ensure adequate leak tightness and/or mechanical performance. Generally, threaded port connections of valves are female, as seen in Figure 1 on the right.
Figure 1: A hexagon nipple fitting (left) and a solenoid valve (right)
The helix (or spiral) of a thread can be designed to be either right-handed or left-handed. As default, majority of threads are right-handed and follow the right-hand grip rule. The right-hand grip rule means that when a thread is engaged in a connection, it moves upward if rotated in the direction of the right hand. The letters “LH” in a thread spec stand for a left-handed thread and is rotated in the direction of the left hand to move the thread up. Figure 2 shows the differences along with the right-hand rule.
Figure 2: Handedness in Threads
The helix of a thread can be wrapped around a conical or cylindrical shape. A thread on a conical shape, as seen in Figure 3 on the top, creates a tapered thread. A thread on a cylindrical shape, as seen in Figure 3 on the bottom, creates a straight or parallel thread. The ‘taper angle’ is the angle between the conical shape and the centerline of the pipe.
Tapered threads are designed to create leak-tightness through compression and deformation of the male and female threads into each other in a connection. On the other hand, straight threads do not create a leak-tight joint on the threads and are often sealed with O-rings or gaskets. The design differences of these thread types are further discussed here.
Figure 3: Tapered (top) vs. Parallel (bottom) Thread
The major diameter of a thread is the larger of the two extreme dimeters of the thread profile. This is measured from the thread tips or crest for external (male) threads and from the groove or root of the thread for internal (female) threads. Minor diameter is consequently the smaller of the two diameters. The depth of a thread is the difference between the major and minor diameters. The pitch diameter (PD) is measured from a line that intersects the flanks of a thread and is half way between the major and minor diameters. Figure 4 shows these terms on a male thread.
Figure 4: Thread Dimensions
Thread angle or flank angle is the angle between thread flanks (Figure 4). Most commonly used thread angles are 60° and 55°.
Pitch is the distance between crests of two consecutive threads. Threads per inch or TPI of a thread is essentially the reciprocal of the pitch of that thread. For example, a thread with a TPI of 12 has a pitch size of of an inch. While TPI is used to describe imperial threads, pitch is often used for metric threads and is given in mm.
The number of starts of a thread determines the number of ridges that are wrapped around to create the thread. The number of starts determines the number of threads a screw moves when turned 360°. One-start threads are the most common pipe thread. Figure 5 is color coded to show the difference in ridge numbers per start.
Figure 5: Number of Starts in a Thread
The cross-section shape of a thread is called its form, different examples of which is shown in Figure 6. Threads can be triangular, square, trapezoidal or other shapes. The triangular shaped threads are also called V-threads. Although V-threads are represented as triangular, in practice, the tip and groove of the thread cannot have a perfectly sharp edge and is therefore truncated (cut short) at varying degrees. A Sharp V-thread is a thread where the tips of the triangles are not truncated.
Figure 6: Forms of Threads
When connecting two threads, it is important to take into consideration the hydraulic seal that they will create. Parallel and tapered threads can be combined to create two types of connections: Jointing threads and longscrew (fastening) threads.
Figure 7: Thread seal tape used for better sealing of jointing threads
Jointing thread connections create pressure-tightness through compressing the threads together. For that purpose, tapered male threads and parallel or tapered female threads are coupled together. In this type of joint, a positive seal is created by deforming the threads into each other through wrench tightening to proper torque. This means that repeated assembly and disassembly can distort the threads. During maintenance, it is important to check this type of joint for any leak paths or damage to the threads.
A sealing coating or a jointing compound on the threads such as a thread seal tape or a pipe dope is often used in this type of connection (Figure 7). When using seal tape, only two turns of sealant is required. It is important to note that an O-ring seal can not be used with a tapered male thread as it will not allow the threads to be tightened completely.
Figure 8: BSPT Connection
Some precise threads are also known as “dry fit” or “dry seal” and do not require any kind of seal material. In these joints, the pressure-tightness is solely created by compression of threads and are especially used to create a gas-tight seal or when the sealant could contaminate or react with the media (e.g. oxygen).
As mentioned, jointing threads may become susceptible to leakage through the course of tightening and untightening during repair or assembly. When a taper male thread is tightened to a parallel female thread, the seal is only made at the base of the female port with 1 or 2 threads due to difference in taper (Figure 8). That means that the area where the crest of male and the root of female thread meet, can form a spiral leak path. This is especially the case with BSPT connections. BSPT threads are commonly used for low-pressure applications and is not recommended for medium or high-pressure systems. This problem is diminished when both male and female threads are tapered (e.g. NPT connections).
Figure 9: O-ring on a male thread
When both male and female threads are parallel, pressure-tightness is not achieved on the threads. In this type of joint, a reliable seal is created by compression of a soft material (an o-ring seal or a washer) or a flat gasket between the shoulder of the male pipe and the interior surface of the female thread.
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