fittingplumbing
When is it appropriate to use tape, dope, or nothing at all when connecting pipe fittings? Several advised not to use any product when I posed this question. What is the purpose of each product, and what problems would they solve or could they cause?
To preempt duplicate question flags, this question is focused on tape vs. dope, and doesn't ask about case where neither is recommended.
If you're using both, the tape would go on first. Which is immediately evident, if you've ever tried to apply them the other way round. There's open debate over whether or not there are any advantages to using both products together, or if each product by itself is adequate.
Pipe dope; also known as pipe joint compound, is used to seal and lubricate the threads of a threaded pipe joint. It's available for both water and gas lines. It's applied by simply brushing it on the male threads of the pipe joint, before the joint is fit together. Pipe dope never hardens, and will not crumble and fall out of the joint.
Teflon Tape dope; also known as polytetrafluoroethylene (PTFE) Tape, or thread seal tape, is also used to seal and lubricate the threads of a threaded pipe joint. It's available in high-density and low-density flavors, for water and gas pipe. PTFE tape is applied by wrapping it in a clockwise direction, around the male threads of the threaded joint. PTFE tape is available in color coded versions (pink for water, yellow for gas), as well as the common white variety.
Whether dope is required at all, is dependent on the type of threads. The type of thread is determined by the tap or die used to create the threads, and should be labeled on the pipe or fitting.
This type of thread when mated, may contain slight gaps between the major and minor diameter of the threads. Because of this, a thread seal agent may be required. However, this type of thread is tapered at a rate of 1/16 (3/4"/foot), which often allows a seal to be made without a sealing agent.
May be listed as MNPT or FNPT, for male and female threads respectively.
NPTF threads are designed so that when the threads are mated, they actually deform to create a mechanical seal. Since the seal is created by the threads themselves, a thread seal agent is not required (though may be used as a lubricant).
May be listed as MNPTF or FNPTF, for male and female threads respectively.
This type of thread is similar to NPT, except that the thread does not have a taper. A thread seal agent is required, but should be selected differently due to the lack of taper.
May be listed as MNPS or FNPS, for male and female threads respectively.
NPSF like NPTF creates a mechanical seal due to the deformation of the threads, however, unlike NPTF NPSF threads are not tapered.
May be listed as MNPSF or FNPSF, for male and female threads respectively.
Which form of dope you choose is often based on who you learned from, personal preference, what's on hand, what type of pipe you're working with, etc. The following are my personal guidelines.
In this situation I'll reach for PTFE tape, since it's not likely I'll require long term corrosion protection. I'm just looking to lubricate the joint, so I can tighten it to create a leak free joint.
Because I don't want the threads to rust or react negatively, in this situation I'll use paste dope. The paste dope will give me the lube I need to get the pipes properly joined, while at the same time providing corrosion protection. The paste dope will never harden or flake off, so I know the threads will be protected for a long time.
If I'm working with water pipe of any material larger than ~1", I'll always use paste dope. I don't really have any particular reason for this, it's just the way I was taught.
When working with fuel gas pipes of any kind, I always use a paste dope labeled for this use. This is especially true when working with "black" pipe, since I want some corrosion protection in these joints.
For pipes and fittings with tapered threads, I'll simply follow the guidelines mentioned above.
When working with fittings with straight threads, paste dope should always be used. Tape dope is too thick, and can actually prevent a good seal in straight thread joints. Paste dope will spread and be pushed out of the way, and will not prevent the threads from properly engaging.
Best Answer
The only place tape is to be used is on threads, to be exact, tapered threads, the kind that are on pipe nipples, or the male threaded ends of valves for example. These are the kind that get tighter as you twist the parts together. If you look at the threaded end you can see the pipe is smaller at the end than farther down the length. Pipe dope can be used on these threads too. Pipe dope, otherwise can be used on the various types of compression fittings, the type that use a ferrule. When using metal pipe the type of pipe dope is not that critical, but if using plastic, be careful, there is pipe dope NOT for plastic.
In a nut shell, tape is for tapered threads only, dope can be used too. and either must be used or a leak will occur. Dope is to be used only on mating surfaces, flat, smooth or rounded. It is mainly insurance in my opinion, and I use it too, but the mating surfaces should make a tight fit on their own. I have had a many a leak before using dope, now the leaks are non-existent. Then again, maybe I just got better at it finally....