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.
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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.
Threaded pipe saves fitters from labor- and skill-intensive pipe-joining techniques like flanging, butt-welding, brazing, and soldering. However, unlike these other pipe fittings, threaded pipe connections don’t form fluid-tight seals on their own. Especially under pressure, the threads leave space for fluids to leak out. That’s where pipe thread sealants come in.
Pipe dope, pipe joint compound, pipe thread seal—these are all different names for products used to create fluid-tight seals in threaded pipe joints. Pipe thread sealants:
In this article, we answer common questions about pipe thread sealants, including the difference between paste and tape sealants, whether thread sealant is the same as PVC glue (it isn’t!), and the difference between PTFE and Teflon™ (there isn’t one). Then, we show you how to apply pipe thread sealant to create water-tight joints in threaded pipe.
Before we get into how pipe thread sealant is used, let’s dig into some common questions about it. Namely, we’ll answer:
Simply put, thread sealant is a material that fills the gaps at threaded pipe connections, preventing fluids from leaking out. Besides creating fluid-tight seals, pipe thread sealants also lubricate the threads, making assembly quicker.
Pipe thread sealants come in two general varieties—paste and tape. Paste sealants are applied with a brush or finger directly to the male threads. They are sometimes referred to as pipe dope or pipe joint compound. Paste sealants can have different ingredients. Some use PTFE. Others consist of a mixture of minerals, rosin, and alcohols.
Some pipe thread sealants are anaerobic, meaning they dry only in the absence of air and in the presence of metal. Other sealants aren’t meant to dry at all. Read the product instructions to make sure you use the right sealant for the right application.
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Tape thread sealants are a mess-free alternative to paste sealants. Tape sealants have no solvent; instead, the sealant is made into a film and is sold in rolls of varying width, thickness, and density. Tape sealants are applied simply by wrapping the tape around and pressing it into the threads, and they are made of PTFE.
Lots of thread sealants are made of PTFE, but what is it? Polytetrafluoroethylene (PTFE for short) is a fluoropolymer—in other words, a plastic that contains fluorine atoms. Its useful properties include:
Besides the threads of pipes, PTFE also commonly coats the surface of nonstick pans. The same properties that make it great for sealing and lubricating pipe threads make it good for stick-free cooking.
No! Despite the word “tape” and the need for some sealants to dry or cure, pipe thread sealants are not adhesive. Remember, the advantage of threaded pipe is easy assembly and disassembly. Pipe thread sealant has a dual purpose. It seals the threads, but it also lubricates. It makes them easy to put together and take apart.
Pipe glue and pipe thread sealant are basically opposites. Pipe glue is used to make “slip” PVC joints permanent. PVC glue actually somewhat “melts” the male and female pipe so they fuse. If you put it on threaded PVC, you destroy the threads.
PTFE was first discovered by Roy J Punkett in 1938 while he was studying halocarbons in search of a new refrigerant for DuPont. Teflon™ is the trademarked named for PTFE. It and PTFE are the same chemical under different names. There is little more reason to buy sealant labeled “Teflon” instead of sealant labeled “PTFE” than there is to buy name-brand medication instead of the generic variety.
Applying thread sealant is easy, but there are still right and wrong (and better and worse) ways to do it. Whether you’re using paste or tape, there are two general guidelines:
Now, let’s get down to it. Here’s how to apply PTFE tape and pipe dope.
Apply PTFE tape to male threads only. Follow these steps:
Seeing is better than doing. This video explains how to apply PTFE tape without looking like a fool (or an apprentice):
In case you missed it, here’s a close-up of the right and wrong ways to wrap PTFE tape:
PTFE tape is sufficient on its own. But some people swear by using PTFE tape and pipe dope. The idea is that the small amount of time and money spent on double-sealing saves repair work later. Other tradesmen insist that you should never combine paste and tape. Either way, here’s how to apply pipe dope.
Applying pipe dope is easy. The important thing is to fill up the threads. A brush is good to have, but a finger works just as well. Follow these simple steps:
Again, it’s better to see it done. This video demonstrates the application of pipe dope:
As you can see, if you choose to use both pipe dope and PTFE tape, the tape goes on first. Apply the paste to the tape. If you’re putting dope over the tape, paint clockwise to avoid unraveling the tape. Remember, some pipe dope has to dry and requires contact with metal to do so. This means that it can’t be used with tape.
When using pipe dope or PTFE tape with plastic pipes, be careful not to overtighten. The lubrication provided by pipe thread sealant makes it easier to tighten too much and crack the fitting.
Whether you use pipe dope or PTFE tape, pipe thread sealant is essential for any threaded pipe application. Thread sealant serves two purposes: it creates a liquid-tight seal by filling gaps in threads and it makes assembly and disassembly easier by lubricating the threads. Pipe thread sealant is not glue or adhesive—in fact, it’s the opposite.
PTFE, the generic name for Teflon™, is a common material for pipe thread sealants because it is chemically unreactive, slippery, and stable at extreme temperatures. PTFE sealants can take the form of convenient tapes as well as pastes (sometimes called pipe dope). Pipe dope can be made of substances other than PTFE, too.
Outfit yourself for your next project. Shop PTFE thread-seal tape and PTFE pipe sealant compound on our website—perfect for fire sprinkler pipe installation. We also have commercial and residential fire sprinkler heads and sprinkler wrenches and other tools you need.
If you have questions or need help placing an order, call us at +1 (888) 361-6662 or email support@qrfs.com.
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