Frequently Asked Questions About Compression Fittings
Q. "Should I use PTFE thread sealing tape or pipe joint compound on my compression threads?"
A. No, you definitely should not. PTFE tape and pipe joint compound (also known as "pipe dope") should only be used on "IPS" ("iron pipe size") threads, which are normal pipe threads. IPS threads make their seal along the threads themselves; when a piece of IPS-threaded pipe is screwed into a fitting, PTFE tape and/or pipe dope is used to assist that seal on the threads. Compression fittings, on the other hand, seal between the beveled end of the ferrule/sleeve and the beveled female part of the fitting, and so using pipe dope or tape on compression threads could actually prevent the fittings from making an adequate seal.
Q. "I have clear vinyl tubing, can I use compression fittings with it?"
A. Brass compression fittings will work on some sizes of clear vinyl tubing and most sizes of low density polyethylene tubing. However, it really depends on the outside diameter and the inside diameter of the plastic tubing. When using plastic tubing, the "plastic sleeve" must be used and the "brass insert" must be used. The plastic sleeve is used to seal against the outside of the plastic tubing and the tapered end will seal into the compression fitting. Plastic sleeves are used because the brass compression sleeves can more easily cut into the plastic tubing, potentially causing a leak. The brass insert is used to keep the inside of the plastic tubing from collapsing as the compression fitting is tightened. The size of the plastic sleeve and the brass insert must be chosen according to the outside diameter dimension of the plastic tubing to be used. The brass insert must be able to fit snugly inside the plastic tubing yet still allow the plastic sleeve and compression nut to slide over the outside of the plastic tubing.
Q. "How does a compression fitting work to seal water?"
A. The compression nut is tightened clockwise to cause the compression sleeve to be squeezed between the fitting and the nut. As more pressure is applied to the nut, the ends of both sides of the sleeve dig slightly into the pipe on both sides of the sleeve, causing the sleeve to be compressed on to the tubing. The water seal takes place as the outside of the sleeve comes into contact with the inside of the nut, and at the same time, on the inside of the fitting on the other side, The sleeve is effectively sealed on both sides when it is properly squeezed between the nut and the fitting and compressed onto the tubing. Please note that over tightening the nut will cause a leak.