Socket weld flanges
Small-bore process, utility, hydraulic and refrigeration lines are often joined with socket weld flanges: the pipe slips into a socket and one fillet weld makes the joint. Here is where they fit, where they do not, and how to order them.
A socket weld flange has a counterbored socket that the pipe end slips into, joined by a single fillet weld on the outside. ASME B16.5 covers socket weld flanges in small sizes only: NPS 1/2 to 3 in Classes 150, 300 and 600, and NPS 1/2 to 2 1/2 in Class 1500, whose dimensions Class 900 shares in those sizes. We make them in A105, A350 LF2, 304L and 316L, with the bore matched to Sch 40, Sch 80 or the schedule you give, and an EN 10204 3.1 mill test report for every heat.
How a socket weld joint is made
The pipe is pushed into the socket until it bottoms, then pulled back about 1/16 in (1.5 mm) before welding. ASME B31.1 and B31.3 both show this gap, which lets the pipe grow with the heat of welding instead of cracking the root of the fillet weld. Below the socket, the flange bore matches the inside diameter of the pipe schedule, so the flow passes a step only at the gap. That is why the schedule matters when you order: a flange bored for Sch 80 on Sch 40 pipe leaves a ledge in the line.
| Class | Sizes | Rating of A105 at 100 °F |
|---|---|---|
| 150 | NPS 1/2 to 3 | 285 psig |
| 300 | NPS 1/2 to 3 | 740 psig |
| 600 | NPS 1/2 to 3 | 1,480 psig |
| 900 and 1500 | NPS 1/2 to 2 1/2 (same dimensions) | 2,220 and 3,705 psig |
There is no Class 2500 socket weld flange in ASME B16.5; at that class small lines use weld neck flanges.
Where socket weld flanges fit
- Small-bore process, drain, vent and instrument connections where a butt weld on thin small pipe is hard to make well.
- Ammonia refrigeration lines in food and cold storage plants, in A105 or impact-tested A350 LF2. See flanges for food and beverage plants.
- Hydraulic, lube oil and fuel lines on skids and ships.
- Steam and condensate branches at traps and stations.
Where to use something else
The gap at the bottom of the socket is a crevice, and ASME B31.3 says socket welded joints should be avoided where crevice corrosion or severe erosion can occur. For the same reason they are kept off clean coolant loops such as the technology cooling loops of liquid-cooled data centers, and off lines that will be chemically cleaned. A fillet weld also cannot be radiographed the way a butt weld can, so specifications that call for full volumetric examination use weld neck flanges instead. Above NPS 3, use weld neck or slip-on flanges.
Grades
| Grade | Specification | Typical service |
|---|---|---|
| A105 | ASTM A105 | General carbon steel service down to -20 °F |
| A350 LF2 | ASTM A350, Charpy tested at -50 °F | Refrigeration and cold service |
| F304L, F316L | ASTM A182 | Corrosive, instrument and hygienic utility lines |
| Duplex S32205, super duplex S32750 | ASTM A182 F60 and F53 | Seawater and chloride service |
How to order
A typical line reads: "Socket weld flange RF, NPS 1, Class 300, bored for Sch 80, ASTM A105, 40 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
What is the difference between a socket weld and a slip-on flange?
Both slide over the pipe. A socket weld flange has a shoulder at the bottom of the socket and takes one fillet weld outside; a slip-on flange has a through bore and takes two fillet welds, one at the hub and one at the face. Socket weld flanges are made in small sizes only, NPS 3 and below.
How big should the gap be before welding?
About 1/16 in (1.5 mm) between the pipe end and the bottom of the socket, as shown in ASME B31.1 and B31.3. Follow your welding procedure and piping specification.
Can you bore socket weld flanges for Sch 160 or XXS pipe?
Give the schedule or the pipe inside diameter on each line and we confirm the bore in the quotation.
Sources
- ASME B16.5, Pipe Flanges and Flanged Fittings, NPS 1/2 through NPS 24 Metric/Inch Standard
- ASME B31.3, Process Piping
- ASME B31.1, Power Piping
- ASTM A105/A105M; ASTM A350/A350M; ASTM A182/A182M