See Table 2. Introduce even one PVC threaded pipe or nipple, and the rating of the entire system must be reduced by 50 percent. This decrease in rating is due to a reduction of the fitting's wall thickness caused by threads.
In addition, most plastics, including PVC, are "notch sensitive. With these Do's and Don'ts in mind, many of the unnecessary headaches and costs of improperly installed systems can be avoided.
There are many different thread styles that are used in the PVC fittings industry. The following explains several of the commonly used thread styles and their sensitivity to bending loads.
Most plastics, including PVC, are notch sensitive. Glass, because it is a very notch sensitive material, is a very good example. To cut glass, a notch is scratched into the surface.
The notch produces a high stress concentration or stress riser, which is indicated by the red area in the diagram above. Applying a bending load will break the glass along the stress riser or notch. Threads can create the same stress concentrations, producing the related types of stress risers, which may result in fractures. The typical machine and pipe thread has a profile that is based on a "V" type notch. The stress that is actualized at the point of the "V" functionally reduces the strength of the thread substantially.
Some manufacturers produce Swing Joints with an alternate style of thread profile called "Buttress" threads. They promote the slanting notches of their threads as adding strength. The truth is these "Buttress" threads still have a "V" notch at the root of the thread profile that consequently make it sensitive to bending loads.
The strength of these fittings is still substantially reduced. It is a specialty thread that provides clearance with all diameter piping while contributing high strength. This element of design provides a high quality part that is less susceptible to failure. An added feature of the ACME style thread is that it provides, "free" and "easy" movement up to proper engagement. This feature prevents the "stick", "lock" or "gall" which is common with PVC threaded parts.
Recommendations for adding threaded plastic fittings to a system are discussed. While threaded thermoplastic systems are not recommended for high-pressure systems, piping layouts where leaks would be dangerous or for large pipe sizes more than 2 inches , they have two definite advantages.
They can quickly be dismantled for temporary takedown applications and they can be used to join plastic and non-plastic materials. The following recommendations for making threaded joints in a thermoplastic pipe and fitting should be followed and are adapted from Plastics Pipe Institute:. Transitions from plastic piping may be made with flanges, threaded fittings, or unions. To understand what happens when a threaded joint is tightened, we must understand the mechanics of tightening a joint.
Think of bolting two steel bars together. As more turning is applied to the nut, the resistance or torque increases. Extra turning of the nut and its travel along the threads applies a clamping force to the steel bars. The increase in torque is made up in part by the squeeze being applied to the steel bars.
At the same time the nut is trying to pull the bolt head through the hole in the bars. The pulling of the bolt or stretch is a key part of successful bolted joints. In many high tech applications the measure of clamping force is determined by bolt elongation or stretch as being more definitive than a torque reading.
The tensile strength of a shaft of steel, the bolt in this example, and its elongation are more consistent than the torque readings of bolts and nuts which may have with rust, lubrication, imperfect threads and tightening procedure. But, to the installer, tightness of the joint is commonly accepted as the resistance of the nut to turn or the torque necessary to rotate it further. This means that the feel of a tight joint is the result of applying loads, which deforms or stretches the joint fasteners.
As the two components are wedged together by more turns, the internal forces increase. With a 1 inch pipe thread, the taper angle means that each adjacent thread is.
This taper means that when the threads are finger tight, any additional wedging action of the two parts will cause strain in the female parts. Since virtually all materials are stronger in compression than they are when stretched. Even when both the male and female threaded parts are the same strength, or material, the female part will be stretched to failure before the male part has a compression load failure. Remember, the tightness of the joint is the result of the resistance to stretching of the materials.
Steel has a tensile strength, or resistance, to stretch roughly seven times more than PVC, which means a plastic joint will have a much lower torque, or feeling, than metal fittings. The greatest stress developed in a tapered pipe threaded joint is at the pitch diameter. The pitch diameter is a point that is midway between the root and the peak of the threads. Threaded pipe nipples are only available in sch Cutting threads into sch 40 PVC pipe results in a thin wall section, compromising strength.
Sch 80 pipe nipples must be used with both sch 40 and 80 threaded PVC pipe fittings. While the general rule of thumb is to use the same sch of fittings and pipe, this refers to systems using solvent weld fittings.
PVC fittings are very common and preform extremely well when assembled correctly. By keeping these do's and don'ts in mind, you can avoid unnecessary headaches and extra costs due to improperly installed systems. Tags: Fittings , PVC. Topics: Fittings , PVC. At BoshartU, our goal is to equip you and your team with the knowledge you need to be more successful.
We regularly publish new blogs and content that will help you become more comfortable and confident with our products. Subscribe to BoshartU Boshart. Related Articles. Subscribe to The Chronicles of Marketing Newsletter. The yellow tape is required by code for gas lines due to its ability to create a more consistent seal then the white tape. Find More Posts by psteeleb. Thanks for that info psteeleb. It often fails to help with sealing on the junk threads provided with PVC these days.
When I assembled my tank with tape I had about 4 aggravating drips with tape. I had to laboriously remove lots of piping. I reassembled it exactly the same way with Rectorseal and had zero leaks. Originally Posted by kcress. Used tape for years. Then tried the paste.
Originally Posted by LennyD4. It is called plumbers paste, PTFE paste, or a couple other names that escape me right now. It can be a bit messy at first, but a little goes a long way. David A. Crandall, M. Fatherree, M. Ken Feldman Jon E. Shimek, Ph. All times are GMT The time now is PM. TapaTalk Enabled. Use of this web site is subject to the terms and conditions described in the user agreement. Do you use Teflon tape for threaded PVC?
User Name. Remember Me?
0コメント