Plumbing Supply Lines

They are in every bathroom, kitchen, and laundry room. These are the rigid, semi-rigid, and flexible lines that connect shutoff valves to faucets, toilets, dishwashers, refrigerators, and washing machines. In the days when copper was the prime plumbing material, supply lines were of no exception. However, innovations and economics have driven the plumbing market away from copper and toward plastics. While some products have been on the market for a long time, the widespread use and application has increased to the common plumber and do-it-yourselfer. Our experience has shown that the traditional installation practices used on copper components are carried over into the installation of plastic components. Unfortunately, the material's properties such as strength, chemical resistance, and rigidity differ greatly from those of copper. The installers are not the only culprit in recent failures. Most readers will recall the advent of polybutylene (PB) piping some years ago, which was touted as "the product of the future." Over time, this material has been found to become brittle and fail without warning. Donan's experience finds that most of the failures encountered occur at the fittings. These failures may be a result of installation errors, poor design, or material failures. As newer products become more attractive to the customer, so does the desire for their production.

The most common type of connection in use for supply lines is compression fittings. To have a secure, failsafe installation two aspects have to be achieved-water tightness and mechanical attachment. For this reason, compression fittings typically have straight pipe threads opposed to tapered pipe threads, which tighten and seal as the two components interlock. Straight pipe threads provide only the mechanical attachment and compressive force on the fitting. The watertight aspect of the fitting is achieved via a rubber gasket or compression sleeve, which is a brass or Delrin® material. Thus, sealing materials such as Teflon® tape, pipe thread sealant, or plumber's putty is not required.

Figure 1

In compression fittings with a compression sleeve or "ferrule", as it is sometimes called, the mechanical attachment is provided when a compression sleeve is tightened sufficiently. This causes the sleeve to permanently seat around the tubing and deform against the opposing male fitting. As a result of this action, an impression is created in the compression sleeve and the tubing. Once the compression sleeve is installed, it cannot be removed from the tubing even when the supply line is disconnected. If the compression nut was not tightened sufficiently, then the compression sleeve will not grip the tubing, thus making it susceptible to pullout. Note that some specialized compression fittings, namely those with rubber compression sleeves or gripper rings, have components that are able to be slid off the tubing when disconnected.
In comparison, compression fittings with rubber gaskets have a flanged component with the compression nut fastened to the tubing. Here the rubber gasket is merely a sealing mechanism, while the flange component and compression nut provide the mechanical integrity of the fitting. The problem with this assembly of components occurs when the old standards of copper installation are applied to crosslink polyethylene (PEX) tubing. PEX tubing or braided hoses are now found in almost all supply line locations. Years ago the common toilet supply line was rigid, chrome-plated copper tubing nicely routed from the shutoff valve to the fill valve. A coupling nut and rubber cone washer created the connection at the toilet tank. The rubber washer provided the watertight seal and rigidity of the copper tubing which resisted pullout forces. A problem now occurs when the copper tubing is replaced with PEX tubing. In most cases the PEX tubing remains connected to flush valve, especially if exceptionally long lengths of tubing are fed into the valve inlet. However, inadvertent impacts to the semi-rigid PEX tubing during cleaning or flooring installations can pull the end of the tubing to the edge of the rubber washer. As a result, fluctuations in the water pressure due to water hammer or thermal expansion can cause the tubing to come loose and result in flooding.

As previously mentioned, Teflon® tape and sealing compounds are not required in compression fittings. However, many plumbers and trade magazines recommend their use. While Donan has found that the correct type of compound can sometimes aid in the assembly of the fitting, the probability of its application causing adverse conditions is too great.

The theory behind using pipe thread compound in compression fittings is that it seals and lubricates the outer surface of the brass compression sleeve, making it easier to tighten and thus, it is more secure. The problem originates when the compound works its way between the inner surface of the compression sleeve and the tubing. Now the very mechanism that is intended to grip the tubing and resist pullout forces has just been lubricated. A second problem stems from compound on the threads of the compression nut. Because the compression nut is easier to tighten, it becomes susceptible to over-tightening, which results in a circumferential fracture of the nut as shown in this image.

Figure 2

(Note plumbing compounds are not to be used in Delrin® compression sleeves.) In addition, no petroleum-based compounds are to be used with plastic components because they are chemically harmful and can result in a failure of the connection.

The installation of compression fittings is relatively easy, though to be confident in a lasting connection, a feel for how tight to make a compression nut is a learned task. With experience, most plumbers gain the skill to find the range between insufficiently and overtightened fittings. In crucial, prone to overtightening situations, the manufacturers will provide warnings such as "hand tighten only," "hand tighten plus ¼ turn," or "hand tighten plus ¼ to ½ turn." One important aspect in the assembly of these types of connections is to ensure that there is an adequate length of the supply line installed in the fitting. A short supply line leaves less material to be "grabbed" by the compression fitting, and the parts can separate suddenly like the previously described PEX tubing. Furthermore, the use of a brass insert, as a tubing stiffener, is recommended in all compression fittings installed on tubing that can be deformed by the associated compressive forces. Without the insert, the tubing can potentially deform, thus weakening the mechanical grip of the fitting. Again, the potential result is the tubing coming loose and causing a flood.

Since the installation of standard compression fittings has become common, problems arise when new products require a slightly different method that is not intuitively obvious. For example, Watts® makes compression fittings with an integral brass compression sleeve called an Ander-Lign universal fitting. These are compression nuts you would typically find in a Lowe's® or Home Depot® home center. Even though it contains a brass compression sleeve, its unique design allows for installation on all types of tubing. Typically, brass compression sleeves are reserved for rigid to semi-rigid piping (i.e. copper, chlorinated polyvinyl chloride (CPVC), PEX, etc.); while plastic (Delrin®) compression sleeves are used for different types of flexible plastic tubing (i.e. polyethylene (PE), polypropylene (PP), PEX, PB, etc.). The problem with the Ander-Lign fitting is that it generally requires more tightening torque than conventional compression fittings. Consequently, it is often insufficiently tightened, and the tubing can withdraw due to water pressure. In situations where the fitting is attached to a coupling that is not rigidly secured to a wall, the difficulty in achieving the required torque is amplified. In addition, personal experience has found that some of these fittings will never seat on the tubing no matter how much torque is applied to the fitting.

Figure 3

While on the topic of plastic tubing replacing copper tubing, it is also worthy to note the increasing trend of failures associated with brass PEX fittings. The characteristics of failure are a circumferential fracture of the barb fitting where the tubing connects, as shown in this image. Donan Engineering has had metallographic studies completed that illuminate the root cause of failure but cannot be publically addressed at this time due to ongoing cases.

Plumbing failures have increased as the materials and installation techniques have changed. Damage resulting from the failures is limited only by the duration of the leak or discharge. Study of these failures is crucial to identify the responsible parties, as well as to highlight and change improper installation techniques and improve faulty designs.