To understand when its appropriate to use a thread sealing tape, it's important to understand a little about the various types of threads on water treatment equipment and components. The two fundamental thread types are:

• Tapered threads
• Parallel threads

Tapered threads are very common on many connections that carry fluids. The tighter male tapered threads are screwed into female tapered threads, the greater the contact between mating threads, and the better the seal. The seal is created by contact between the male and female threads. The two most common types of tapered threads are:

• NPT National Pipe Thread Taper
• BSPT British Standard Pipe Taper

Most plumbing connections in the USA have NPT threads. Walk down the isle of plumbing fittings at your local hardware store and almost all of the threaded fittings have NPT threads. Use thread sealing tape with NPT threads.

BSPT threads can be found on some items imported from Europe or Asia. Although it's possible to screw a male NPT into a female BSPT fitting, you'll note the threads don't match, and the joint will leak - with or without Teflon tape. Use thread sealing tape when joining BSPT fittings.

Easy enough! Make sure the threads on joined parts match (use the same thread system - either NPT or BSPT), and use Teflon tape. But where is thread sealing tape not used?

Some threads are not tapered - no matter how far the male threads are screwed into the female threads, the joint doesn't tighten up. Some examples: a nut screwed onto a bolt, the threads on a filter housing, or threads on most fittings that utilize an oring. In this case, the seal occurs at the oring, not at the threads. BSPP (British Standard Parallel Pipe) threads can be found on fittings common in Europe and Asia - but are rare in the USA.

The take-home message: If you have mating tapered threads, use sealing tape.