Chloe Zhang
Plastic piping with threaded fittings is known to provide reliable, leak-free service. However, some threaded plastic pipes may leak or break. The two types of threaded plastic pipes are Schedule 40 and Schedule 80. Schedule 40 PVC pipes are for socket fittings and are not recommended to be threaded. Schedule 80 PVC pipes, on the other hand, can be threaded and are considered stronger than Schedule 40 pipes.
| Characteristics | Values |
|---|---|
| Types | Schedule 40, Schedule 80, Schedule 120 |
| Use | To mechanically join two sections of plastic tubing |
| Pressure | Can contain pressure greater than 100 pounds per square inch (PSI) |
| Applications | Deep submersible well applications |
| Strength | Schedule 80 systems are stronger than Schedule 40 systems |
| Sealant | Use a non-hardening compound designed for plastic fittings |
| Taper | Male threads increase in diameter, female threads decrease in diameter |
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What You'll Learn
Schedule 80 vs Schedule 40
The terms "Schedule 40" and "Schedule 80" refer to the thickness of a pipe's walls. The higher the schedule number, the thicker the pipe walls. This means that Schedule 80 pipes have thicker walls than Schedule 40 pipes. The increased wall thickness of Schedule 80 pipes makes them more robust, able to handle higher pressure and heavier loads, and more durable.
Schedule 40 pipes are suitable for many general-purpose applications, but they may not handle extreme pressure or stress as effectively as Schedule 80 pipes. Schedule 40 pipes are best for applications involving relatively low water pressure, such as home repair or irrigation projects. Schedule 40 pipes also have a lower pressure rating than Schedule 80 pipes.
Schedule 80 pipes are commonly used in heavy-duty applications and are generally safer and more durable for pressurized liquid applications. They are ideal for industrial and chemical applications.
It is important to note that the Nominal Pipe Size (NPS) and Outer Diameter (OD) measurements for a given pipe are not affected by different pipe schedules. The increased wall thickness between schedules occurs as a reduction in the Inner Diameter (ID) measurement.
When assembling threaded plastic joints, it is important to finger-tighten and then add no more than one to two turns. It is also recommended to use a sealant that is non-hardening, compatible with plastic, and does not add slipperiness.
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Using sealants
Plastic piping with threaded fittings usually provides reliable, leak-free service. However, a small percentage of them may leak or break due to improper assembly of threaded joints and not using the correct thread sealant. The right sealant for threaded joints is non-hardening, compatible with plastic, and doesn't add slipperiness.
Teflon tape, also known as plumber's tape or thread seal tape, is a popular choice for sealing threaded connections. It is easy to apply, inexpensive, and effective for many types of fittings. However, it is not recommended for use with threaded PVC connections as it can create too much stress on the fittings and cause cracking.
Pipe dope, or pipe thread sealant, is a paste-like substance that binds to threads to create a watertight seal. It is useful for metal fittings and can be used with wicking for a more secure seal. Pipe dope is also suitable for connections that may be subject to vibration or temperature changes as it remains flexible after application.
When selecting a sealant, it is important to choose one that is certified by the manufacturer to be harmless to the fitting material and will not contaminate the fluid in the pipe. It should also lubricate the joint enough to allow for smooth tightening and the uniform meshing of the mating pipe threads.
To assemble a threaded plastic joint, finger-tighten it and then add no more than half to three-quarters of a turn. This will prevent the female part from stretching or undergoing strain, which can cause splitting.
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Taper
The taper on threaded plastic pipes refers to the slight increase in diameter of each successive male thread, while female threads get successively smaller. This is specified in the American National Standard B2.1, which all pipe manufacturers voluntarily follow to assure customers of quality materials. The taper allows male and female threads to compress and wedge together, forming a stronger, leak-resistant seal. This is especially important for plastic pipes as they are ""notch sensitive", meaning that any notches or threads on the smooth wall of the pipe can cause a significant loss of strength.
The National Pipe Taper (NPT) is the most common tapered pipe thread standard used in the United States and Canada. The NPT taper rate is defined as 1 inch of diameter in 16 inches of length or 0.75 inches per foot. The taper allows NPT fittings to form a seal when torqued, as the flanks of the threads compress against each other, creating a gas or liquid-tight connection.
To assemble a threaded plastic joint, it is recommended to finger-tighten and then add no more than half to three-quarters of a turn. This is because too many turns can cause the female part to stretch and split, and smaller diameter joints are more susceptible to this. Additionally, over-tightening can lead to leaks, especially without the use of a proper sealant.
When choosing a sealant, it is important to select one that is non-hardening, compatible with plastic, and does not add slipperiness to the joint. A proper sealant fills any voids between the threads, preventing leaks, and is forced by water pressure into potential points of leakage. It is also important to note that Teflon tape, Teflon paste, and pipe dope are intended for metal pipes and can add strain and stress to plastic joints, leading to potential leaks.
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PVC pipe threading
Threaded plastic pipe fittings provide reliable, leak-free service. However, some may leak or break due to the decrease in the fitting's wall thickness caused by threads. This decrease in thickness makes the plastic notch sensitive, causing it to lose a significant portion of its original strength.
Only Schedule 80 and Schedule 120 PVC pipes can be threaded as they have thicker walls. Schedule 40 PVC pipes have walls that are too thin for threading. When assembling a threaded PVC joint, it is recommended to finger tighten and then add no more than one to two turns. This is because too many turns can cause the female part to stretch or undergo "Strain", leading to splits in the fitting.
There are various tools available for threading PVC pipes, including electric drill pipe threader kits and PVC thread maker tools. These tools often come with multiple dies of different sizes to accommodate different pipe diameters. While threading can be done, some recommend gluing on threaded fittings instead to avoid weakening the PVC pipe.
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Installation guidelines
Threaded plastic pipes are used for mechanically joining two sections of plastic tubing. They are typically used to contain higher pressures, greater than 100 pounds per square inch (PSI).
When installing threaded plastic pipes, it is important to follow the correct procedures to ensure a smooth and stress-free assembly process. Here are some detailed guidelines for installing threaded plastic pipes:
- Selecting the Right Pipe and Fittings: Choose the appropriate pipe schedule and type for your specific application. For threading, only Schedule 80 and Schedule 120 PVC pipes are suitable due to their thicker walls. Schedule 40 pipes have thinner walls and are not recommended for threading as it compromises their strength.
- Preparing the Pipe: Place the pipe in a vise with the end to be threaded 6 to 8 inches from the clamping mechanism. Wrap a rag around the pipe at the mating point with the clamp to protect the plastic surface. Secure the pipe in the vise, being careful not to over-tighten the clamp as this can crack the pipe.
- Threading the Pipe: Insert the tapered arbor into the plastic pipe. Use a long handle to rotate the die and thread the pipe. Continue threading until the backside of the die is in line with the end of the pipe. Remember that male threaded fittings have threads that increase in diameter, while female threaded fittings have threads that decrease in diameter (taper).
- Removing the Die: To remove the die, turn it in a counter-clockwise direction. Clean the newly cut threads by wiping them with a rag. Do not use any cutting fluids on the PVC pipe while cutting threads as this can clog the die cutter and affect the thread quality.
- Assembling the Fittings: When assembling threaded joints, finger-tighten the fittings first, and then add no more than one to two turns, depending on the pipe schedule. For Schedule 40, one to two turns past finger-tight are sufficient. For Schedule 80, half to three-quarters of a turn is recommended.
- Using Sealants: Avoid using Teflon tape, Teflon paste, or pipe dope, as these are intended for metal pipes and can add strain and stress to plastic fittings. Instead, use a sealant specifically designed for plastic fittings. Choose a non-hardening compound that is compatible with plastic and does not overly lubricate the joint, as this can lead to over-tightening.
- Avoiding Over-Tightening: Over-tightening can cause the female part to stretch and split. This is more common with smaller diameter fittings, as they are more prone to stress cracking due to increased strain. Ensure you follow the recommended number of turns for your pipe schedule to avoid over-tightening.
By following these guidelines, you can ensure a proper installation of threaded plastic pipes, minimizing the risk of leaks and breaks.
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Frequently asked questions
Threaded plastic pipes are plastic pipes that use threaded fittings to mechanically join two sections of plastic tubing. This is used for containing higher pressures, greater than 100 pounds per square inch (PSI).
There are two main types of threaded plastic pipe: Schedule 40 and Schedule 80. Schedule 40 PVC pipe is for socket fittings and has a thinner wall thickness, while Schedule 80 has a thicker wall thickness and can be threaded.
Schedule 80 systems are considered stronger because they have higher pressure ratings than Schedule 40 systems. However, this is only true when the components are joined with solvent cement or welding. The outer diameter of both types is the same, but Schedule 40 is not recommended for threading due to its thin walls.
When assembling threaded plastic pipes, it is important to use the right sealant. A non-hardening compound is recommended as it seals potential leak points by being forced into them by water pressure. It is also crucial to ensure the sealant is compatible with plastics and does not over-lubricate the joint, as this can lead to over-tightening.
