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Threads

The "Do's and Don'ts" of Assembling Threaded Plastic Fittings

Today, there are millions of miles of plastic piping with threaded fittings, providing reliable, leak-free service. However, a tiny percentage of those threaded plastic fittings may leak or break. The reason for this is improper assembly of threaded joints.

Here are some of the Do's and Don'ts of PVC joint assembly:

  • Don't over-tighten joints by giving them "one more turn to be sure."Do finger tighten plus one or two turns - No More.
  • Don't wrap Teflon tape or Teflon paste or pipe dope to add bulk to or to lubricate the joint. Do use a sealant for threaded joints.
  • Don't use "stronger" Schedule 80 threaded fittings on the assumption that they may solve the problem of splitting through over-tightening.
  • Do use only Schedule 40 threaded fittings with Schedule 40 pipe and fittings.
  • Don't over-tighten.
  • Do finger tighten plus one or two turns.

On threaded male PVC fittings each successive thread is slightly larger in diameter than the one before it. Female threads get successively smaller. This is called taper and the amount of taper is specified (1¾ degrees) in the American National Standard B2.1. All pipe manufacturers voluntarily follow these standards to assure their customers they are receiving quality materials.

Because the threads are tapered, additional turns cause the female part to stretch or undergo "strain." This will split the female fitting just as a wedge, driven by a sledgehammer, will split a tree stump.

The amount of strain increases as the size of the pipe decreases. Therefore it is easier to split smaller diameter threaded joints than larger ones. It is also easier to over-torque smaller diameter fittings because their resistance to torque is less. Table 1 gives Strain and Tensile Stress levels according to pipe diameter.

"Stress" (tensile stress) is the force exerted by the strain of the male thread multiplied by the resistance of the PVC. The resistance of PVC is 400,000 pounds per square inch (psi). The strain per turn past finger tight for one-inch PVC pipe is .00447, so the stress per turn is 1,788 psi. Thus, a one-inch threaded PVC joint that is tightened four turns past finger tight will develop a tensile stress of 7,152 psi. The joint is bound to fail since the stress exceeds the 7,000 psi tensile strength of PVC, without even adding the tensile stress caused by the pressure inside the irrigation system (up to a maximum of 2,000 psi).

Table 1

Strain and Tensile Stress levels of PVC Threaded joints

(Schedule 40 and 80)

 


 

 

 

 Finger-tight+2 turns

 Size

Strain/Turn

Stress/Turn

Max Allowable

Hydrostatic Stress

 (IPS)

 (in/in)

 (psi) 

 (psi) 

 ½

 0.00588

 2352

 6704

 ¾

 0.00461

 1844

 5688

 1

 0.00447

 1788

 5576

 1 ¼

 0.00349

 1396

 4792

 1 ½

 0.00302

 1208

 4416

 2

 0.00239

 956

 3912

 2 ½

 0.00287

 1148

 4296

 3

 0.00234

 936

 3872

 4

 0.0018

 720

 3440

The right way to assemble a threaded PVC joint-Schedule 40 or 80 is finger tight plus one to two turns-no more. Two turns past finger tight plus the stress of the system pressure is within the tensile strength of one-inch PVC. ([1,788 psi x 2] + 2,000 psi = 5,576 psi).

Don't use Teflon tape, Teflon paste or pipe dope. Do use a sealant.

Teflon tape, Teflon paste and pipe dope is intended for metal pipe and fittings. Metal to metal fitting joints are more difficult to tighten than plastic; the surfaces tend to gall without the aid of such lubricants as Teflon or pipe dope. Plastic fittings do not need this lubrication.

When Teflon tape is wrapped around plastic male threads it adds to the strain and tensile stress. The tendency of most installers is to incorrectly wrap several thickness of tape around the male threads, increasing stain and stress further.

Teflon paste and pipe dope, just like Teflon tape, make threaded joints slippery. Their use on PVC fittings can be an invitation to over-torque.

When working with threaded plastic fittings do use a proper sealant. The right sealant for threaded joints is non-hardening, compatible with plastic and doesn't add slipperiness.

A non-hardening compound is forced by water pressure into potential points of leakage, thereby performing a true sealing function. Tapes and hardening pastes permit a leak path to develop when a joint is backed off, mechanically flexed, or expands with rising temperatures.

A sealing compound must be compatible to plastics. Many brands of pipe sealant contain oils, solvents or carriers that can damage plastic. A proper sealant must be certified by the manufacturer to be harmless to the fitting material and to not contaminate fluid in the pipe.

Finally, a sealing compound must not lubricate the joint to the point that over-tightening is encouraged. Several sealants on the market meet all these requirements.

Don't use Schedule 80 threaded fittings in a Schedule 40 System. Do use the same Schedule threaded fittings with the same Schedule pipe and fittings.

Many plastic piping system installers who encounter problems with splitting assume Schedule 40 fittings are weak. They conclude that the problem can be solved by switching to "stronger" Schedule 80 fittings. There are several fallacies in this reasoning.

First, all the problems inherent in over-tightening apply as much to Schedule 80 systems as they do Schedule 40. While the walls of female Schedule 80 threaded fittings are thicker, wall thickness does not change stress and strain levels. See Table 1.

Second, installers believe Schedule 80 systems are stronger because they have higher pressure ratings than Schedule 40 systems. This is true only when comparing systems with components that have been cemented together with solvent. See Table 2. Introduce even one PVC threaded pipe or nipple, and the rating of the entire system must be reduced by 50 percent.

Table 2

Maximum Static Pressure Rating* of Type 1120 PVC at 73° F

 Size

Schedule 40

Schedule 80 

 

 (IPS)

 Solvent Weld

 Solvent Weld

 Threaded Joint

 ½

 600

 850

425

 ¾

 480

 690

 345

 1

 450

 630

 315

 1¼

 370

 520

 260

 1½

 330

 470

 235

 2

 280

 400

 200

 2½

 300

 420

 210

 3

 260

 270

 185

 4

 220

320

 160

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." When the smooth wall of a plastic part is notched, the part loses a significant portion of its original strength, just as a thick sheet of glass will break along a scribed line on its surface. This is why the presence of even one threaded fitting in a system requires a 50% reduction.

With these Do's and Don'ts in mind, many of the unnecessary headaches and costs of improperly installed systems can be avoided.

 

PVC Fitting Thread Style

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. The styles covered are the standard "V" thread, buttress thread, and the ACME thread.

Standard "V" Thread

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. This is why the working pressure is actually reduced 50% in systems that use threaded plastic fittings compared to those using only non-threaded fittings.

Buttress Thread

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.

ACME Thread

The ACME thread has a configuration that eliminates the "V" notch. It is a specialty thread that provides clearance with all diameter piping while contributing high strength. The ACME thread is less sensitive to bending loads because there is no "V" notch.

LASCO Swing Joints and Unions incorporate the ACME thread design. 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.

 

Threaded Plastic in Systems

LASCO Fittings Inc. has included this article, from the Plastics Pipe Institute, on threaded plastic in systems. 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:

  1. Thread only pipes that have wall thickness equal to or greater than those of Schedule 80 pipe.
  2. For pressure rated pipe of PVC and CPVC, reduce the pressure rating of the threaded pipe to one-half that of unthreaded pipe.
  3. To cut the threads, use only pipe dies designed for plastic. Keep the dies clean and sharp. Do not cut other materials with them.
  4. Vices for holding pipe during thread cutting and pipe wrench should be designed and used in such a manner that the pipe is not damaged. Strap wrenches are recommended. Wooden plugs can be inserted in to the end of the pipe if needed to prevent distortion of the pipe wall.
  5. The following general procedure for cutting threads may be used: - Use a diestock with proper guides so that the die will start and go on square to the pipe axis. Any burrs or sharp edges on the guide that can scratch the pipe must be removed. - Do not use cutting oil. However, a drop of oil may be rubbed onto the chaser occasionally. This prevents chatter and helps promote clean, smooth threads.
  6. Before assembly, the threads should be lubricated and sealed with a non-hardening pipe dope.
  7. In making up threaded joints, care must be taken not to over tighten the joint. Generally, one to two turns past finger-tight is adequate. Further tightening may split female threaded plastic parts.

Transitions from plastic piping may be made with flanges, threaded fittings, or unions. Flange connections are limited to 150 psi and threaded connections are limited to 50% of the rated pressure of pipe.

PLASTICS PIPE INSTITUTE (PPI)A Division of The Society of The Plastics Industry, Inc.
250 Park Avenue, New York, New York 10017  (212) 687-2675

 

Why Threaded Connections Fail

To understand what happens when a threaded joint is tightened, we must understand the mechanics of tightening a joint. First, let’s go over what takes place when a standard bolt and nut joint is tightened to clamp two objects together. Think of bolting two steel bars together. When the nut is started on the bolt the nut is “free-running” and the nut spins easily down the length of the threads. As the steel bars are clamped together the nut is no longer “free-running” but offers resistance to turning or torque.

 
 
 
45lb torque on a 3/8" bolt yields 7,000lbs stretch force

 

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.

Now using the information we just went over, let’s explain what happens when a tapered pipe thread joint is tightened. Just like the bolt and nut, until clamping forces are present, tapered threads are “free-running” until clearance between male and female threads disappears. As the two components are wedged together by more turns, the internal forces increase.

A National Pipe Thread has a taper of 1¾°, which means that each male thread is slightly, larger in diameter than the one before it and the female threads get successively smaller. With a 1 inch pipe thread, the taper angle means that each adjacent thread is .0055 inches, or about the thickness of this page, different in diameter. As the male and female threads are turned past “free-running” the parts are wedged together causing the female piece to stretch while the male compresses slightly. 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.

Pitch Diameter

This means that for every turn past finger tight, or “free-running”, the female part is stretched more than the male is compressed. 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. It’s at the pitch diameter within a threaded connection that any crack or failure starts, and then propagates outward through the fitting wall. Because the crack originates at the pitch diameter any extra wall thickness of the female threaded component provides little protection from an over tightening failure.

To see why the highest loads are on the pitch diameter, we must see how the wedging action loads are distributed. Let’s use a 1 inch pipe thread for this example! Strain is the change in diameter for every revolution of the threaded joint, in this example the pitch diameter increases .0055 inches for each full turn. Since the pitch diameter at the end of the internal thread is 1.230 and the diameter increase of .0055 inch for each turn this yields a strain of .00447 inch/inch. Whereas, the change in pitch diameter on the outer wall of the fitting that measures 1.673 would be .00329 inch/inch

Notice that the stretch on the outside diameter of the female part is lower than that at the pitch diameter, showing where the most strain is located. Stress or tensile stress is the force created by the strain developed, multiplied by the resistance of the material, to enlarge, here PVC. Since the resistance to stretch, or tensile modulus, of PVC is 400,000 psi. This means the stress on this 1” threaded part at the pitch diameter is; .00447 x 400,000 or 1,788 psi/turn. Therefore with PVC having a tensile strength of 7,000 psi it is easy to see that just a few turns past finger-tight or “free-running” can cause PVC fittings to fail. If we tighten the joint 3.9 turns past finger tight we exceed the strength of PVC, and cause it to crack.

The right way to assemble a threaded PVC joint-Schedule 40 or 80 is finger tight plus one to two turns-no more. Two turns past finger tight plus the stress of the system pressure is within the tensile strength of one-inch PVC. The working pressure of PVC pipe is based on a 2000 psi stress level. What this means that a 1 inch female threaded connection is exposed to 7,364 psi hoop stress when tightened just three turns past finger tight and under at the rated working pressure of the pipe. As you can see, in this case the connection is on the verge of failure.

(1,788 psi x 3) + 2,000 psi = 7,364 psi

The table below shows the stress per turn, turns to failure and strain that is generated in the other size of pipe thread joints. It is important to notice that the most common threaded connections, those under 1 inch, can crack a female PVC fitting with just a few turns past finger-tighten.

 

How, then you ask, should a plastic fitting joint be made correctly? First we must recognize that the female threaded part needs to be the strongest. If the joint is made of different materials such as metal and PVC then the male threaded part must be plastic to provide the least chance of joint failure. If the joint is all plastic and a thread sealant is used, its chemical make up must be compatible with the materials involved. Since sealant or tapes that contain Teflon® reduce the friction, they will mask the loads and stress being applied during the tightening sequence. Because of the clearance between the root or valley and the peaks of the mating threads, there is a small spiral leak path that extends the length of the threaded connection. This leak path must be sealed, and this is the reason for thread sealant. Notice that I did not say lubricant. The lubricating qualities of thread sealants can cloak the resistance the installer expects when tightening a joint. This leads to over tightening to get the “feel” of being leak free, while exerting the excessive stress of wedging the male and female components together.

The procedure to make leak free joints that will not cause split fittings is simple! Make-up the joint to finger-tight, not hand tight, then tighten 1 to 2 more turns. This method will provide a joint that is leak tight without causing excessive stress within the connection. It is important to realize that pipe thread sealant; especially those made with Teflon®, lubricate the threads and mislead the installer to believe the joint is not tight.