Types of fittings used in flanges.

First, What Are Fittings in Flange Systems?

Think of a pipe system as a puzzle. Flanges are the big, flat pieces that bolt together to join sections of pipe, but fittings are the smaller, specialized parts that fill in the gaps. They're the elbows that turn corners, the tees that split flow, and the reducers that shrink pipe size. Without them, you'd just have straight pipes going nowhere. In flange systems, fittings do more than connect—they ensure the entire setup can handle pressure, resist corrosion, and stay sealed for years. Today, we're focusing on three workhorses: bw fittings , sw fittings , and threaded fittings . These three cover most industrial needs, from high-pressure oil rigs to everyday plumbing.

1. BW Fittings: The Heavy-Duty Welded Workhorses

Let's start with the tough guys: bw fittings , short for butt weld fittings. If you've ever seen a large industrial pipe system—like the ones at refineries or power plants—you've probably spotted these. They get their name from how they're installed: the ends of the pipe and fitting are cut flat (butted together) and welded around the circumference. It's like welding two pieces of metal pipe end-to-end, but with a pre-shaped fitting (elbow, tee, etc.) in between.

How BW Fittings Are Made

Most bw fittings are forged or extruded from solid metal, which gives them incredible strength. Manufacturers start with a billet of steel, stainless steel, or even nickel alloy (for corrosive environments), then heat and shape it into the desired fitting—elbow, tee, reducer, or cap. The key here is that the fitting's wall thickness matches the pipe's, so there's no weak spot where pressure could break through.

Why Choose BW Fittings? Strength and Durability

BW fittings shine in high-pressure, high-temperature settings. Since they're welded, there's no gap between the fitting and pipe—just a continuous metal bond. That makes them leak-resistant and able to handle pressures up to 10,000 psi (and beyond, with special alloys). They're also great for large-diameter pipes (think 2 inches and up) because welding creates a smooth interior, reducing flow resistance. If you're building a pipeline for crude oil or steam in a power plant, bw fittings are your go-to.

The Catch: They Need Skilled Welders

Nothing's perfect, right? BW fittings require precision welding. You can't just bolt them on—you need a certified welder with a steady hand. Welding also takes time: the ends of the pipe and fitting must be perfectly aligned, cleaned, and prepped before welding. And once they're welded, they're permanent. If you need to take the system apart for repairs, you'll have to cut the welds and ｒｅｐｌａｃｅ the fitting. That's why they're not ideal for small, easy-to-maintain systems.

Where You'll Find BW Fittings

You'll see bw fittings in places where failure isn't an option. Petrochemical facilities love them because they handle the high pressure of oil and gas. Power plants use them in steam lines, where temperatures can hit 1,000°F. Even shipyards rely on them for marine pipe systems—saltwater is brutal, but a well-welded bw fitting won't corrode easily. If your project involves petrochemical facilities or heavy industry, bw fittings are a safe bet.

2. SW Fittings: The Middle Ground for Smaller, High-Pressure Pipes

Next up: sw fittings , or socket weld fittings. These are like the little brothers of bw fittings—smaller, but still tough enough for high pressure. Instead of butting the pipe and fitting together, sw fittings have a "socket" (a recessed end) where the pipe slides in. Then, a single weld is applied around the outside, where the pipe sticks out of the socket. It's faster than butt welding and works better for smaller pipes (usually 2 inches or less in diameter).

The Socket Weld Design: Simple but Smart

Imagine a short pipe with one end hollowed out (the socket) and the other end open. You slide the end of your pipe into the socket until it hits a small stop inside (called a "shoulder"). Then you weld the joint where the pipe meets the socket's edge. That shoulder is key—it ensures the pipe is inserted the right distance, so the weld is strong and centered. The result is a joint that's almost as strong as a butt weld but takes less time to install.

Best For: Small Pipes, High Pressure, Quick Installs

SW fittings are perfect when you need high pressure but don't have room for bulky butt welds. They're common in hydraulic systems, where pipes are small but pressure is through the roof. They're also popular in instrument lines—like the small pipes that carry sensors and gauges in refineries. Since they don't require a full circumference weld (just a single pass around the edge), they're faster to install than bw fittings. That said, they're not great for very large pipes—anything over 2 inches starts to get tricky with the socket design.

3. Threaded Fittings: The No-Weld, Easy-Repair Option

Now, let's talk about the most user-friendly option: threaded fittings . These are the ones you'll find under your sink or in home plumbing, but they're also used in light industrial settings. Instead of welding, they have threads (like screws) on the inside or outside that twist onto matching threads on the pipe. It's the oldest pipe connection method, but it's still going strong for a reason: no special tools, no welding, and easy to take apart if something leaks.

Thread Types: NPT, BSP, and More

Not all threads are created equal. The most common type in the U.S. is NPT (National Pipe Taper), where the threads get tighter as you twist the fitting on. In Europe, you'll see BSP (British Standard Pipe), which can be parallel (BSPP) or tapered (BSPT). The taper is what creates the seal—when you twist the fitting onto the pipe, the threads compress, blocking leaks. For extra security, people often add Teflon tape or pipe dope to the threads before twisting.

Pros and Cons: Convenience vs. Pressure Limits

The biggest plus of threaded fittings is how easy they are to install. No welding, no heat—just a wrench and some tape. That makes them ideal for DIY projects or small systems where you might need to take things apart later (like home plumbing or small compressed air lines). But there's a catch: they're not great for high pressure. The threads create small gaps, even when tight, which can leak under extreme pressure. Most threaded fittings top out around 1,000 psi, which is fine for household use but not for a petrochemical plant.

Where Threaded Fittings Shine

You'll find threaded fittings in places where pressure is low and accessibility is key. Think of the pipes under your kitchen sink, the sprinkler system in your yard, or the small air lines in a workshop. They're also used in temporary setups—like construction sites where pipes might need to be moved later. In industrial settings, they're often paired with other fittings: for example, a threaded elbow might connect a small gauge to a larger bw-fitted pipe.

BW vs. SW vs. Threaded: Which Fitting Should You Choose?

Now that we've covered the basics, how do you pick the right one? It all comes down to three things: pressure, pipe size, and installation needs. Let's break it down with a quick comparison:

Real-World Examples: Fittings in Action

Let's make this concrete with a few examples of how these fittings work in the field.

Example 1: A Petrochemical Refinery

Refineries deal with high pressure, high temperatures, and corrosive chemicals—perfect for bw fittings. The main crude oil pipelines (6+ inches in diameter) use bw elbows and tees to navigate the refinery. These fittings are welded on-site by certified welders, ensuring no leaks even when oil is pumping at 5,000 psi. Nearby, smaller instrument lines (1 inch or less) use sw fittings to connect sensors to the main pipes. The sw design allows for tight bends in tight spaces, and the single weld is quick to inspect.

Example 2: A Marine Shipyard

Ships need pipe systems that can handle saltwater, vibration, and constant movement. Here, bw fittings are used for the main seawater intake pipes—large, thick-walled steel pipes that supply cooling water to the engine. The welded joints hold up against the constant pounding of waves. For smaller systems, like the freshwater lines to the crew quarters, threaded fittings are used. If a pipe springs a leak mid-voyage, the crew can quickly twist off the threaded elbow and ｒｅｐｌａｃｅ it—no welding required in the tight confines of a ship's hull.

Example 3: A Home Plumbing System

Your house is a threaded fitting playground. The pipes under your sink? Threaded elbows and tees connect the faucet to the main water line. The reason? If a pipe freezes and bursts, you can unscrew the fitting with a wrench and ｒｅｐｌａｃｅ it in 10 minutes. No need to call a welder—just a trip to the hardware store for a new threaded coupling.

Final Thoughts: Fittings Are the Backbone of Flange Systems

At the end of the day, bw fittings , sw fittings , and threaded fittings are the unsung heroes of any pipe system. They might not get as much attention as the big flanges, but without them, your pipes would be stuck in a straight line, leaking everywhere. Whether you need to handle 10,000 psi at a refinery or fix a leaky sink at home, there's a fitting for the job. The key is to match the fitting to your pressure, pipe size, and installation needs—and when in doubt, ask a pro. After all, a well-chosen fitting is the difference between a system that lasts 20 years and one that leaks next month.