What Is a Threaded Flange? NPT Connection

A threaded flange has a tapered internal thread (NPT per ASME B1.20.1) that screws directly onto a pipe with matching external threads. No welding is required. This makes threaded flanges useful where hot work is restricted, where the pipe material is difficult to weld, or where the connection must be easily disassembled.

Key Specifications

Property	Detail
Standard	ASME B16.5
Size range	NPS 1/2 to NPS 4 (occasionally to NPS 12)
Pressure classes	150 through 2500
Thread type	NPT (ASME B1.20.1) tapered thread
Face types	RF, FF
Common materials	A105 (CS), A182 F304/F316 (SS)
Seal method	Thread engagement + thread sealant (PTFE tape or compound)
Weld	None required (optional seal weld sometimes added)

When to Use Threaded Flanges

Threaded flanges are specified in specific scenarios:

• No hot work zones: Explosive atmospheres, live plant environments where welding permits are restricted
• Low-pressure utility piping: Air, water, low-pressure steam (Class 150)
• Galvanized piping: Welding destroys the zinc coating; threaded connections preserve it
• Temporary or field connections: Easy to install and remove with pipe wrenches
• Small-bore instrument lines: Quick installation for gauge connections and sample points

Common Mistake

Using threaded flanges in cyclic or vibration-prone service. The thread path creates a spiral leak path, and vibration can loosen the connection over time. For these conditions, socket weld flanges provide a far more reliable seal.

Threaded vs Socket Weld Flanges

Both serve small-bore piping. The choice depends on service conditions and project requirements.

Feature	Threaded	Socket Weld
Welding	None	Single fillet weld
ASME B31.3 SRF	0.60	0.75
Leak path	Spiral thread path	None (welded)
Disassembly	Easy (unscrew)	Requires cutting
Vibration resistance	Poor	Good
Temperature limit	Moderate (thread sealant limits)	High
Cost	Lower	Slightly higher

The SRF values tell the story: threaded connections are rated at only 60% of butt-weld strength, while socket welds reach 75%.

Pro Tip

Some project specs allow a “seal weld” on threaded flanges—a light fillet weld over the threads after assembly. This eliminates the spiral leak path and improves fatigue resistance. However, once seal-welded, the connection is no longer easily removable.

Thread Engagement

Proper thread engagement depth matters. For NPT connections, minimum thread engagement varies by size:

NPS	Min. Thread Engagement (mm)	Number of Threads
1/2	13.5	8
1	16.2	8
2	19.1	8
4	25.4	8

Per ASME B1.20.1, NPT threads have 11.5 TPI for NPS 1/2 and 3/4; 8 TPI for NPS 1 and above.

Under-engagement leads to joint failure under pressure. Over-engagement can crack the flange body. Use a thread gauge to verify fit.

Read the full guide to flanges