If I had a dollar for every time I found PTFE tape wrapped around a compression fitting nut, I could retire early. Thread sealant and anti-seize both come in a tube or a roll, and they both go on threads, so people treat them as the same thing. They aren't. One seals a leak path. The other keeps metal from locking up. Here's how to tell which job you're doing before you reach for anything.
First, Figure Out How the Connection Seals
Every threaded connection seals one of two ways, and that decides everything else.
Tapered threads (NPT) seal on the threads themselves. As the male and female threads wedge together, the metal deforms, but there's always a small spiral leak path left at the crest and root. A thread sealant fills that path and lubricates the threads so you can make up the joint without galling.
Straight threads (SAE/O-ring boss, UNF, BSPP and similar) do not seal on the threads. The threads only hold the parts together. The seal comes from an O-ring, a gasket, or a metal-to-metal face. Sealant on these threads doesn't help, and excess can work its way past the seal and into your system.
Compression tube fittings seal at the ferrules, not the nut threads. Never put tape or sealant on the nut or body threads of a tube fitting. It can throw off your make-up, keep the ferrules from seating properly, and end up downstream in a regulator or instrument. Assemble them clean and follow the manufacturer's turns from finger-tight.
When to Use Thread Sealant
Use a thread sealant on NPT and other tapered pipe threads. A few habits that save rework:
- Clean the threads first. Old compound, cutting oil and chips all get in the way.
- Leave the first thread or two bare. Whether it's paste or tape, that keeps material from getting pushed into the flow stream.
- If you use PTFE tape, wrap in the direction of the threads so tightening doesn't unravel it, and don't overdo the wraps. We recommend 3 wraps for thick mil tape, thinner tape might require 5 wraps.
When to Use Anti-Seize
Anti-seize is not a sealant. Its job is to prevent galling and seizing so a joint can be tightened correctly now and taken apart later. Stainless steel on stainless steel is notorious for galling, where the surfaces tear and cold-weld under load. Once that happens, the part is often scrap.
Reach for anti-seize on stainless bolting, studs, flange hardware, and threaded parts that see high heat or get taken apart regularly. It also helps you get more consistent torque, because less of your effort is lost to thread friction. Just remember that lubricated threads reach tension with less torque, so follow the torque values for lubricated threads when they're specified.
Special Service Needs Special Attention
Oxygen service is the big one. Ordinary compounds that contain hydrocarbons can be a fire hazard in oxygen systems. Use only products specifically rated for oxygen service, and keep everything cleaned to the spec your system calls for. The same careful approach applies to hydrogen, potable water, food and pharma, and high-purity lines: check the product's approvals before it goes on.
Also think about the base metal and the temperature. Different anti-seize formulations (nickel, copper, metal-free and others) suit different materials and environments. When in doubt, read the data sheet or ask.
Quick Rule of Thumb
Tapered thread that seals on the threads? Sealant. Straight thread or tube fitting? No sealant, seal on the O-ring, gasket or ferrule. Fastener or threaded part that needs to come apart again, especially stainless? Anti-seize.
A-Best Industrial Inc. Fittings, Valves, and Tubing is an authorized distributor of Anti-Seize Technology compounds and sealants, and we carry the compression tube fittings and pipe fittings they go with. We're happy to help you match the right product to your service.
Need a Hand Picking the Right Compound?
Call us at 310-833-3353 or email sales@abesttube.com. We're open Monday–Friday, 8:00 AM–5:00 PM Pacific, with after-hours emergency service when you need it.