When to use slip on flanges?

Slip On Flanges are preferred by some contractors, over the Weld-neck, because of the lower initial cost. However, this may be offset by the added cost of the two fillet welds required for proper installation. The strength of the slip-on flange is ample for it's rating, but its life under fatigue conditions is considered to be only one-third that of the weld-neck flange.

The slip on flange may be attached to the end of a piece of pipe or to one or more ends of a pipe fitting. The slip-on flange is positioned so the inserted end of the pipe or fitting is set back or short of the flange face by the thickness of the pipe wall plus 1/8 of an inch. This allows for a fillet weld inside the SO flange equal to the thickness of the pipe wall without doing any damage to the flange face. The back or outside of the flange is also welded with a fillet weld.

A variation of the Slip On flange also exists. This is the Slip-On Reducing Flange. This is simply a larger (say a 14″) Slip-On flange blank that, instead of the Center (pipe) hole being cut out (or drilled out) for 14″ pipe it is cut out for a 6″ (or some other size) pipe. The SO Reducing flange is basically used for reducing the line size where space limitations will not allow the length of a weld neck flange and reducer combination. The use of the Slip-On Reducing Flange should only be used where the flow direction is from the smaller size into the larger size.

Slip On flanges or SO flanges are commonly lower in price than weld-neck flanges, and to this effect are a popular choice for our customers.

It is welded both inside and out to provide suffcient strength and prevent leakage.

Slip-on flanges are all bored slightly larger than the O.D. of the pipe. They are preferred over welding neck flanges many users due to their lower intial cost, but final intallation cost is probably not much less than that of the welding neck flange because of the additional welding involved.


Low cost installation
Less time needed to spent on ensuring the accuracy of the cut pipe
They are somewhat easier to align
The slip-on flanges have low hub because the pipe slips into the flange before welding
The flange is welded both inside and outside to provide sufficient strength
They prevent leakage

How does a WELDING NECK FLANGE work in the pipeline system?

As you know WELDING NECK FLANGES are one of the popular flange in the pipeline industries. As it have many advantages like:

1. Easy installing.

2. Not easy to change its shape.

3. High quality in sealing.

4. Wildly used.

The welding neck flanges are wildly used in Petroleum, Natural gas, Refineries, Petro-Chemical, Ship building & Paper making, Power project, Boiler fabrication etc.

Blind flanges have the face thickness of a flange, a matching face type, and similar bolting pattern. Blind flanges can also be used to seal a nozzle opening on a pressure vessel. Because it is bolted, the blind flange provides easy access to the interior of a vessel or pipe, unlike a cap that is welded. Figure 4.25 represents the drawing symbol for the blind flange.

What is plate flange?

Plate flange is also named plain flange, flat flange and slip on flange etc. A plate flange is a flat, circular disk that is welded onto the end of a pipe and allows it to be bolted to another pipe. Typically used in fuel and water pipelines, the two plate flanges will be bolted together with a gasket in between them. The plate flange will have bolt holes all around the perimeter and will be used to create junctions, tees and joints.

When building a pipeline, the length of the pipes used are not always known. By manufacturing the products separate from the plate flange, the welders can cut the pipe to length and weld a plate flange in place to join the pipes at any needed length. The plates can also be welded to the pipe on a slight bias, allowing two pipes to be joined that may not be precisely lined up.


A flange is a cast or forged steel product used to connect a pipe with another mechanical device. In the oil and gas industry, forged steel flanges are preferred due to their intrinsic strength and durability.


The key sizes for a pipe flange are:

the nominal size (i.e. the bore size of the flange, to match the bore size of the connecting pipe)

the flange rating (which designates the pressure/temperature performance of the flange, i.e. 150#, 300#, 400#, up to 2500#)

the schedule for welding neck flanges (that should match the pipe schedule)


Pipe flanges are available in multiple types, the standard ones are the welding neck, blind, socket weld, lap joint, threaded, etc.). There are also some special types of flanges, like the swivel flanges, the expander/reducer flanges, the "Nipoflange" / "Weldoflange", and the orifice flanges.

As a general rule, welding neck, slip-on, and socket weld flanges are used for high-pressure applications that require long-lasting flanged joints. Threaded flanges can be used with a lower pressure piping system, and if vibrations are not present.

Lap joint flanges are used in connection with stub ends either to facilitate the alignment of the bolts of the two mating flanges or to reduce the cost of noble materials in high-grade flanged joints (example, in an Inconel piping system, the stub end connected to the pipe may be in Inconel, whereas the lap joint flange can be of a lower grade, thus saving the overall weight of the expensive Inconel material).


The key material grades for pipe flanges, examined in a separate article, are:

ASTM A105 (carbon steel flanges for high-temperature service)

ASTM A350 (CS flange for low temperature)

ASTM A694 (high yield carbon steel flange for line pipes)

ASTM A182 F1 to F91 (alloy steel flange)

ASTM A182 F304, F316, F321 (stainless steel flange)

ASTM A182 F51, ASTM A182 F53/55 (duplex and super duplex flange), and higher grades (Inconel, Hastelloy, Monel flange).

The pipe and the flange material shall, of course, match (according to the ASTM piping materials selection chart).

Flange Materials, General Requirements

Flanges and flanged fittings shall be castings, forgings, or plates

Bolting materials shall conform to ASME B16.5, Table 1B.

The material for flanges in the pipeline service shall be suitable for welding. The carbon equivalents of the used flanges shall match with the pipe material

Cast Iron and Ductile Iron Flanges

Gray cast iron flanges shall not be used for process piping within the battery limits of any plant. The only exception shall be for fire systems applications. The material shall be ASTM A 126, Class B.

Ductile iron flanges may be used, in proprietary systems, for example, plastic-lined steel piping, as back-up for lapped joints flanges

ASME B16.1 Class 125 and class 250 cast iron flanges may be mated with ASME B 16.5 class 150 and 300 steel flanges respectively. However, care shall be exercised to ensure that a flat-faced cast iron flange shall mate only with a flat-faced steel flange, and vice versa.

Carbon Steel Flanges

Carbon steel flanges shall not be used in services above 425 °C

High-temperature service: Standard carbon steel material shall be ASTM A 105, a material that can be safely used for temperatures between minus 29 °C and 425 °C.

Low-temperature Service: Carbon steel flanges used for services below minus 29 °C, shall conform to the impact-testing requirements of ASME B 31.3. ASTM A 350-LF2 shall be the standard material for low temperature applications

High-Yield Service: High strength carbon steel flanges ASTM A694 should fit API Std. 5L pipe Grade X42 to X65.

Low-Alloy Flanges

Material for low-alloy steel flanges (11/4 Cr – 1/2 Mo) shall be ASTM A 182-F11. Material for intermediate alloy steel flanges (11/2 Cr – 5 Mo) shall be ASTM A 182-F5.

Stainless Steel and Non-ferrous Flanges

Usually, weld neck flanges shall match the metallurgy of the pipe in any material class. Austenitic stainless steels, however, may in certain cases be interchangeable. For example, type 347 and 321 stainless steels are compatible. Flanges that are double stamped, or double graded, and are so marked. For example, low carbon grades such as 304L, and 316L may be substituted, for the 'straight' grade, provided that the 'L' grade meets the physical requirements of the application.

When pipe material is forged, weld neck flanges shall be forged. When pipe material is not forged, material for weld neck flanges shall be subject to client approval.

Non-ferrous pipe flanges (copper, cupronickel, and aluminum) are used in marine and aeronautical applications.

Pipeline Service Flanges

Flanges for pipeline service shall match SMYS, and carbon equivalency specified in ASME B31.4 and B31.8.

NACE Service Flanges

When an in-plant service has water and H2S concentrations above the limits specified in NACE MR0175, that service shall be considered as NACE service. Flanges for use in NACE service shall be in accordance with NACE MR0175 special requirements. The purchase description shall specify 'NACE service'.

Coated, Painted and Lined Flanges

For specific applications, flanges may be coated, painted or internally lined (with Teflon, for example) to enhance the resistance of the metal to the aggression of corrosive or erosive fluids.


The ASME B16.5 specification covers flanges for piping applications up to 24 inches in diameter, whereas the ASME B16.47 specification covers pipe flanges above 24 inches (series A and B are available).

More in detail, the following ASME standards apply to main and companion flanges in pipe works:

ASME B16.1: this standard applies to integral cast iron flanges and blind flanges

ASME B16.5, Classes 150, 300, 600, 900, 1500 up to NPS 24 and class 2500 up to NPS 12. Class 400 carbon steel flanges shall not be used

Flanges larger than NPS 24 to be specified in accordance with ASME B16.47. ASME B16.47 series A for NPS 26 to NPS 60 in class 150 to 900 replaces these flange sizes in MSS SP-44

Series B replaces API 605 in sizes NPS 26 to 60 (ASME B16.47 Series B is used for pipelines and is restricted to flanges used for joints)

MSS SP-44 shall be used for steel pipeline flanges for sizes smaller than ASME B16.47 where the material grade is not listed in ASME B16.5

Flanges of unlisted materials and flanges not covered by the above standards shall be designed in accordance with ASME Section VIII Div 1, Appendix 2, and for blind flanges, in accordance with ASME Section VIII Div 1, Section UG-34

Tolerances for flanges shall be in accordance with ASME B16.5, section 7 for flanges up to NPS 24, and ASME B16.47 for flanges over NPS.

Pipe flanges for European piping systems are covered by the EN 1092-1 specification (weld neck flange type 11, plate flange type 01, blind flange type 05, threaded flange type 13).


Flanges and flanged fittings shall be marked in accordance with MSS SP-25. The following shall be included in the marking:

Pressure rating class

ASME B16 designation

Nominal pipe size

The letter 'R' and the corresponding ring groove number for ring joint flanges

The letters 'PL' shall precede the grade symbol followed by the material grade of the pipe

Type of flange facing

Schedule or wall thickness for weld-neck flanges

plate flange designs are uniform in any given size regardless of materials used to create them. This allows a 6-inch (15 cm) black pipe flange to mate perfectly with a 6-inch stainless steel flange. The plate flanges will have a serrated finish on the inside mating surface, which allows the plate to seat into the gasket material. This ensures a perfect seal between two joining pipes.
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