Threaded flanges
A threaded flange screws onto pipe with no welding at all. That makes it the flange of choice on galvanized and small utility piping, and in areas where hot work is restricted, and the wrong choice on lines with vibration or temperature swings.
A threaded flange, also called a screwed flange, has a tapered pipe thread cut in its bore so it screws onto threaded pipe without welding. ASME B16.5 sets the flange dimensions and calls for American National Standard taper pipe threads, NPT, to ASME B1.20.1. Threaded flanges suit small-bore and low pressure utility lines, galvanized pipe, and areas where hot work is restricted. We make them in A105, A350 LF2, 304L and 316L, most often in Class 150 and 300, with an EN 10204 3.1 mill test report for every heat.
How a threaded joint seals
NPT threads are tapered, so the joint tightens as the pipe is screwed in and the thread flanks wedge together. A thread sealant or tape fills the spiral leak path along the crest and root. The flange face then seals against its mate with a gasket like any other flange.
Where threaded flanges fit
- Compressed air, instrument air, nitrogen and water utility lines in small sizes.
- Galvanized pipe, where welding would burn off the zinc coating.
- Skids and packages that are assembled and taken apart in the field.
- Plants and platforms where hot work permits are hard to get, since no welding is needed.
Limits
A threaded joint can loosen and leak under vibration, bending and repeated temperature changes, and the thread removes part of the pipe wall. ASME B31.3 limits threaded joints in severe cyclic conditions, and many owners' specifications do not allow them on flammable or toxic lines, or require them to be seal welded. Where that applies, use socket weld or weld neck flanges. Seal welding needs a weldable flange grade; A105, A350 LF2 and the L grades of stainless all weld well.
For US projects we cut NPT threads to ASME B1.20.1, the thread ASME B16.5 specifies. If your drawing calls for a different thread, state it on the line list and we confirm it in the quotation.
Grades
| Grade | Specification | Typical service |
|---|---|---|
| A105 | ASTM A105 | Air, water, oil and gas utility lines |
| A350 LF2 | ASTM A350, Charpy tested at -50 °F | Outdoor and cold service |
| F304L, F316L | ASTM A182 | Corrosive utilities, instrument air, clean water |
How to order
A typical line reads: "Threaded flange RF, NPS 2, Class 150, NPT to ASME B1.20.1, ASTM A105, 25 pcs". Send the type, size, class, facing, bore or schedule, material and quantity for each line, and we quote every line as specified. Every order ships from our works in Gujarat, India, with an EN 10204 3.1 mill test report for every heat; ordering from India explains the documents, marking, packing and customs.
Frequently asked questions
Can threaded flanges be used on high pressure lines?
ASME B16.5 gives threaded flange dimensions in the higher classes, but vibration, temperature cycling and the reduced pipe wall at the thread make them a poor choice for high pressure or cyclic service. Most specifications use weld neck flanges there.
Should a threaded flange be seal welded?
Only if your piping specification calls for it. Seal welding stops leaks along the thread but makes the joint permanent; the flange grade must be weldable.
Do threaded flanges fit galvanized pipe?
Yes. Threading avoids welding the pipe, which would burn off the zinc. State the coating your project needs on the flange itself on the line list and we confirm it in the quotation.
Sources
- ASME B16.5, Pipe Flanges and Flanged Fittings, NPS 1/2 through NPS 24 Metric/Inch Standard
- ASME B1.20.1, Pipe Threads, General Purpose (Inch)
- ASME B31.3, Process Piping