Pipe flanges join pipes, valves and equipment with a bolted connection, and the four main types are slip-on, weld neck, threaded and blind flanges. Hotline Steel Trading supplies all four types from Dubai to projects across the UAE, in carbon steel and stainless steel from 1″ to 24″, to ANSI, DIN, BS and JIS standards. This guide explains how each flange type works and how to choose the right one before you request a flange quote.
At a glance
- Slip-on flanges slide over the pipe and are welded inside and outside.
- Weld neck flanges have a long tapered hub for high pressure and temperature.
- Threaded flanges screw onto threaded pipe with no welding.
- Blind flanges close off the end of a pipe or nozzle.
- The flange standard and pressure class must match on both sides of a joint.
In this guide
What is a pipe flange?
A pipe flange is a ring or plate fixed to the end of a pipe so it can be bolted to another flange, valve or piece of equipment. Two flanges are bolted together with a gasket between them to make a sealed joint that can be taken apart later for inspection, cleaning or replacement.
That ability to separate the joint is the main reason flanges are used instead of welding everything in place. Pumps, valves, strainers and meters are usually flanged so they can be removed without cutting the pipe. The way the flange attaches to the pipe is what separates the four main types, and each type suits different pressures, sizes and installation conditions.
Slip-on flanges
A slip-on flange has a low hub and a bore slightly larger than the outside diameter of the pipe, so the pipe slides into the flange before welding. The flange is then welded on both the inside and the outside for strength and to prevent leakage.
Slip-on flanges are easy to line up on site because the pipe can be positioned before the welds are made, and they cost less than weld neck flanges. They are widely used on water, cooling and general service lines at lower pressures. Hotline Steel Trading also supplies JIS 5K and 10K slip-on flanges from 1″ to 12″ for projects specified to Japanese standards.
Weld neck flanges
A weld neck flange has a long, tapered high hub that is butt welded to the pipe, which transfers stress from the flange into the pipe wall. This reduces stress concentration at the base of the flange, so weld neck flanges are the common choice for high pressure, high temperature and cyclic service.
Because the joint is a full penetration butt weld, it can be inspected by radiography, which many oil and gas and process specifications require. Weld neck flanges are welded directly to pipe or to butt weld fittings such as elbows, tees and MS concentric reducers, so the bore of the flange should match the schedule of the pipe or fitting it joins.
Threaded flanges
A threaded flange has a tapered internal thread that screws onto a threaded pipe end, so no welding is needed. This makes it useful where hot work is restricted or where a quick, low cost connection is needed on small diameter pipe.
Threaded joints are generally used on smaller sizes and lower pressures, because cutting a thread reduces the pipe wall and the joint can loosen under heavy vibration or temperature changes. They pair naturally with threaded pipework such as GI pipe and GI threaded fittings in building services.
Blind flanges
A blind flange is a solid plate with no bore that closes off the end of a pipe, valve or vessel nozzle. It is bolted to a matching flange in the same way as the other types.
Blind flanges are used to end a line that will be extended later, to isolate a section for maintenance and to close a line during pressure testing. Because the whole face carries the internal pressure, a blind flange must be ordered in the same pressure class as the flange it is bolted to, and larger sizes in higher classes become very heavy.
Flange types compared
The right flange type depends mainly on pressure, how the pipe is joined and whether the line needs to be welded on site. The table compares the four types.
| Type | How it attaches | Typical service | Main advantage |
|---|---|---|---|
| Slip-on | Pipe slides in, welded inside and outside | Lower pressure water and general lines | Easy alignment, lower cost |
| Weld neck | Butt welded to pipe or fitting | High pressure, high temperature, cyclic | Strongest joint, can be radiographed |
| Threaded | Screws onto threaded pipe | Small bore, low pressure, no hot work | No welding needed |
| Blind | Bolted to another flange | Line ends, isolation, pressure testing | Closes the line and allows later extension |
Slip-on, weld neck, threaded and blind flanges from 1″ to 24″, supplied from Dubai.
Flange standards, materials and sizes
Flanges are made to a dimensional standard and a material specification, and both have to match the project drawings. The table below lists the flange standards and sizes Hotline Steel Trading supplies.
| Flange group | Standard and material | Ratings and sizes |
|---|---|---|
| Carbon steel | ANSI B16.5, ASTM A105 | Class 150 and Class 300, 1″ to 24″ |
| Stainless steel | ASTM A182 F304, F304L, F316, F316L, 316Ti and F321 | 1″ to 24″ |
| DIN and BS | DIN 2632 and 2633, BS 4504 | PN10, PN16 and PN25 |
| JIS | JIS slip-on | 5K and 10K, 1″ to 12″ |
Stainless steel grades such as F316 and F316L are chosen for better corrosion resistance, while ASTM A105 carbon steel covers most general industrial and building services work, matching carbon steel ERW pipe and seamless pipe. For full details see our flange suppliers in Dubai page.
How to choose a flange
Choosing a flange means matching it to the line it will sit on, starting with the design pressure and ending with the size. Working through these points in order gives a supplier everything needed to quote.
- Check the standard on the project drawings, for example ANSI B16.5, DIN, BS 4504 or JIS.
- Select the pressure class or PN rating that covers the design pressure and temperature.
- Choose the flange type: weld neck for demanding service, slip-on for general lines, threaded where welding is not possible, blind to close a line.
- Match the material to the pipe, for example ASTM A105 for carbon steel or ASTM A182 F316L for stainless steel.
- Confirm the nominal size and, for weld neck flanges, the bore to suit the pipe schedule.
- List quantities, including blind flanges needed for testing.
Frequently asked questions
What does Class 150 mean on a flange?
Class 150 is a pressure class under ASME B16.5. A higher class, such as Class 300, has thicker dimensions and a higher pressure rating at the same temperature, so the class must match the design pressure of the line.
Can an ANSI flange be bolted to a DIN flange?
In most cases no. ANSI, DIN, BS and JIS flanges use different bolt circles, bolt sizes and dimensions, so both mating flanges should be ordered to the same standard and rating.
Which flange standards does Hotline Steel Trading supply?
Hotline Steel Trading supplies flanges to ANSI B16.5 with ASTM A105 and ASTM A182 materials, DIN 2632 and 2633, BS 4504 PN10, PN16 and PN25, and JIS 5K and 10K slip-on flanges.
Request a quote for flanges
Send the standard, class, type, material, sizes and quantities and Hotline Steel Trading will reply with price and availability.
