NPT stands for National Pipe Thread, a U.S. standard for tapered threads used to join pipes and fittings. Unlike standard straight machine threads, NPT threads are engineered to taper inward at a specific angle (1/16 inch per inch of length), allowing the male and female threads to compress against each other to create a tight, leak-resistant seal.
NPT has a tapered male and female thread that seals with Teflon tape or jointing compound. Pipe threads used in hydraulic circuits can be divided into two types: Jointing threads are pipe threads for joints made pressure tight by sealing on the threads and are taper external and parallel or taper internal threads.
What Are the Disadvantages To Using NPT Fittings?
Thread sealant or tape is typically required to complete the seal. Straight Pipe Threads need a gasket or O-ring to create a seal. While NPT and NPS threads will engage, they do not seal properly with each other.
Although relatively inexpensive, taper threads come with significant drawbacks. No specific torque value is required during installation, meaning it's easy to over-tighten and crack the female port, or under-tighten, causing spiral leak paths from poorly engaged or mated threads.
NPT (National Pipe Taper) threads are incompatible with most other thread standards (such as BSP or Metric) due to differing thread angles and pitches. Mixing them causes leaks and damages fittings. Always use the correct mating standards or an adapter for a secure, leak-free connection.
Apply 3 to 5 turns (wraps) of standard Teflon (PTFE) tape for most threaded pipe connections. However, the exact number depends on the tape's thickness and the type of material you are connecting.
Thread seal tape (also known as PTFE tape, Teflon tape, or plumber's tape) is a polytetrafluoroethylene (PTFE) film tape commonly used in plumbing for sealing pipe threads.
In daily life, if PVC drain pipes are not glued without pressure, they may not leak in a short period of time, but they will definitely leak after long-term use. This is because the contact parts of the PVC water pipe and the joint cannot fit perfectly, and there must be certain gaps.
However, recent failures and the lawsuits that have followed those failures lead to the idea that PEX plumbing systems aren't as good as once assumed. PEX systems can fail in either the pipe or in the fitting. The most notable failure occurs when the fittings fail and water starts leaking and causing visible damage.
When done incorrectly with poor sterilization protocols, or just bad luck, the skin and surrounding tissues can become badly infected, inflamed and sometimes even had tissue death.
ASME B1. 1 (Unified Inch Screw Threads) states that the last effective thread of an externally threaded fastener occurs about three threads from the end of the bolt or stud. So, a stud or bolt is not effectively joined with the nut until the stud or bolt extends at least three or more threads beyond the end of the nut.
PVC Pipes are Prone to Chemical Leaching
PVC pipes were long known to leach toxic chemicals into drinking water.
The "purple stuff" plumbers use is purple primer. It is a chemical solvent used to clean and soften the surface of PVC and CPVC pipes before applying pipe glue (solvent cement). The bright purple dye provides visible proof to building inspectors that the required primer was used.
Plumbers use a mix of forensic deduction and specialized technology to locate hidden leaks without tearing apart your home. They generally start by assessing the property for water damage and then utilize advanced tools to pinpoint the exact source.
3 Plumbing Pipe Materials You Don't Want In Your Home
Oatey® Purple Primer is an NSF-listed, aggressive primer for use on all schedules and classes of PVC and CPVC pipe and fittings. It removes contaminants such as surface dirt, grease and grime as well as softens the pipe surface to allow for a fast, secure solvent weld.
Apply 3 to 5 turns (wraps) of standard Teflon (PTFE) tape for most threaded pipe connections. However, the exact number depends on the tape's thickness and the type of material you are connecting.
If you are working on indoor fixtures like sink drains or faucet bases, plumber's putty is often the better choice because it is easy to apply and remove. On the other hand, for areas exposed to constant water, pressure, or outdoor conditions, silicone provides a stronger, long-lasting seal.
Always wrap plumber's tape (PTFE/Teflon tape) clockwise. By matching the clockwise tightening motion of the fitting, the tape wraps tightly into the threads instead of bunching up or unraveling when you screw the parts together.
Improper Application of Teflon Tape Wrong direction: If the tape is wrapped counter to the thread direction, it unravels when tightened. Too little tape: Less than 2–3 wraps may not seal properly. Too much tape: Overwrapping can prevent proper thread engagement or even crack fittings.
Liquid thread sealant, often referred to as pipe dope, is a paste or gel applied to threaded connections to create a tight seal. It typically comes in a tube or a can with an applicator brush and is used for various plumbing, gas, and industrial applications.