Simplest Slip On Flange Guide for Beginner

A slip on (SO) flange is the easiest flange for a beginner to understand and install: it slides over the pipe end and is held by two fillet welds - one outside, one inside - with no butt-weld bevel and no precision pipe cut required.

It costs less than a weld neck flange, aligns easily because the flange body rotates and slides on the pipe before welding, and is limited to low to medium pressure service because it has no tapered hub to transfer bending load into the pipe wall. That trade-off - cheap, fast and forgiving versus structurally weaker - is the whole decision in one sentence.

This guide keeps the beginner-friendly structure but adds what the page was missing: the standards behind the dimensions, a pressure-class table, a material selection table, the fillet-weld and gap rules your welder actually needs, an inspection and certification section, and the five mistakes that cause most new slip on flange leaks. Every dimension table that was already on this page is preserved unchanged further down.

Slip on flanges suit low to medium pressure - water, cooling, HVAC, firefighting, and moderate chemical duty. Use Class 150 or 300 in ASME B16.5 sizes up to NPS 24, two fillet welds (outside and inside), a gap of wall thickness plus ~3 mm between pipe end and flange face, and step up to a weld neck flange when the service is high pressure, high temperature, cyclic or requires radiographic inspection.

What Is a Slip On Flange?

A slip on flange is a ring-shaped flange whose bore is slightly larger than the pipe outside diameter, so the pipe slides into it and is secured with two fillet welds - one on the outside and one on the inside - with no butt-weld bevel required.

The internal diameter of the flange is a little bigger than the pipe OD, which lets it slide into position before welding. Because the bore is not part of the sealing surface, the same flange body can be used on a small range of wall thicknesses, and the flange can be rotated or slid along the pipe to line up bolt holes before the welder commits. While the basic ring shape stays the same, slip on flanges are mainly categorised by their face type (how they seal against the gasket) and by material (what they are forged from).

The two fillet welds do different jobs, and beginners often misunderstand this. The outer weld is the structural weld - it carries the pipe load. The inner weld is the sealing weld: it closes the crevice between the pipe OD and the flange bore, which is the exact geometry where crevice corrosion would otherwise begin in a chemical line. Removing the inner weld to save time is the single most common field shortcut, and on corrosive service it is the one that fails first. Our product page for the slip on (SO) flange lists the stock sizes and materials, and the flange range page covers the other flange types in the family.

Slip On Flange Types and Face Types

Face type is chosen by pressure and by what the flange bolts to - RF for general service, FF for cast iron and brittle equipment, RTJ for high pressure - and material is chosen by the fluid's corrosion demands.

Raised Face (RF)

The most common type. A small raised ring concentrates bolt load on the gasket for a stronger seal, and it suits most pressure classes and most gasket styles including spiral wound, sheet and ring gaskets. If nobody has told you otherwise, RF is the default you will be quoted.

Flat Face (FF)

A fully flush sealing surface used with flat-faced mating equipment - most often cast iron valves, pumps and equipment where a raised face would bend the brittle flange. The full contact area spreads bolt load, which protects the casting; the price is that FF joints need higher bolt load to seat the same gasket.

Ring Type Joint (RTJ)

A machined groove accepts a metal ring gasket to make a metal-to-metal seal. Smooth and RTJ faces rarely appear on slip on flanges and are more common on weld neck flanges, but RTJ slip on flanges do exist for high-pressure, high-temperature service where a mechanical, fire-safe seal is required. Where the duty is that severe, engineers usually specify a weld neck flange instead of an SO - our weld neck flange guide explains why.

Materials

Slip on flanges are forged in the same material families as the pipe they join: carbon steel for utility water and air, stainless steel for general process and hygienic duties, duplex and super duplex for chlorides and seawater, and nickel alloys where acids, high temperature or both rule out stainless. The table below maps material to forging standard, so you can write a compliant purchase order without guessing.

Table 1. Slip on flange material selection and the standard that covers the forging

Material	Common grades	Forging standard	Typical slip on flange service
Carbon steel	ASTM A105	ASTM A105	Utility water, air, fire protection, low-pressure steam
Austenitic stainless	304/304L, 316/316L, 321, 347	ASTM A182 F304L / F316L	General chemical, food, pharmaceutical, moderate chlorides
Super austenitic	904L, 254SMO (F44)	ASTM A182 F904L / F44	Sulphuric and phosphoric acid, high-chloride cooling water
Duplex	S31803 / S32205 (2205, F51/F60)	ASTM A182 F51 / F60	Seawater, chloride process streams, higher strength lines
Super duplex	S32750 / S32760 (2507, F53/F55)	ASTM A182 F53 / F55	Aggressive seawater and offshore piping
Nickel alloys	Inconel 625, Hastelloy C276, Monel 400, Incoloy 825	ASTM B564	Hot acids, sour gas, caustic, FGD and pulp mill service

Two practical notes on that table. First, the forging standard is not the whole specification: the flange still has to meet ASME B16.5 (or B16.47) for dimensions, facing and pressure-temperature rating, and the material standard only tells you what the metal is. Second, nickel alloys are forged to ASTM B564 - not A182, which covers stainless and duplex only. Mixing those two up on a purchase order is a common beginner error and one that mills will quote around rather than correct. For alloy-specific detail see our guides to Hastelloy C276 flanges, 904L flanges and Inconel 600 flanges

Slip On Flange Specification and Standards

Slip on flanges are made to ASME B16.5 for NPS 1/2 to 24 in classes 150, 300, 600, 900, 1500 and 2500; above NPS 24 the dimensions come from ASME B16.47 Series A or B. Dimensions are set by NPS and pressure class, and the key measures are outside diameter, thickness, hub OD, flange length, raised face diameter, bolt circle, socket bore, bolt holes, bolt size and mass.

Four dimensions do the real work. The socket bore is the slip fit that gives the flange its name - it is slightly larger than the pipe OD, and it is why a slip on flange cannot be used to align a pipeline precisely. The flange thickness (C) is the pressure-bearing dimension. The bolt circle (PCD) and bolt hole count/size decide which mating flange it will actually bolt to. The raised face diameter and height decide which gasket lands on the sealing surface.

Above NPS 24 the two B16.47 series are not interchangeable: Series A follows the older MSS SP-44 dimensions and Series B follows API 605, so a Series A flange will not bolt to a Series B flange of the same nominal size. Check which series the specification calls for before ordering - the difference is in the bolt circle. Our dimension references cover the ASME B16.5 flange dimension chart, B16.47 Series A, B16.47 Series B and the large diameter flange dimensions in detail.

Table 2. ASME B16.5 pressure classes at 100 deg F (carbon steel forgings, Group 1.1) - how much pressure the class buys you

Class	Rating at 100 deg F	Approx. bar	Where a slip on flange of this class is still sensible
Class 150	285 psi	19.6	Water, HVAC, fire protection, low-pressure steam - the classic SO duty
Class 300	740 psi	51.1	Moderate chemical and process lines; the practical upper limit for most SO flanges
Class 600	1480 psi	102.1	Possible but unusual - weld neck is normally specified from here up
Class 900	2220 psi	153.1	Weld neck or RTJ flanges, not slip on
Class 1500	3705 psi	255.4	Weld neck or RTJ flanges, not slip on
Class 2500	6170 psi	425.5	Weld neck or RTJ flanges, not slip on

Read this before you use Table 2. These are the widely published ASME B16.5 Group 1.1 (carbon steel) ratings at 100 deg F, shown so a beginner can see the shape of the class system. Ratings fall as temperature rises, and they differ by material group - a 316L flange is not rated identically to a carbon steel one. Select the final class from the current ASME B16.5 pressure-temperature tables for your actual material and design temperature, not from this summary.

Class 150 Slip On Flange Dimensions

The table below is the original Class 150 slip on flange dimension record from this page, preserved exactly as published. Use the socket bore column for the slip fit, the flange thickness (C) column for the pressure-bearing check, and the PCD and bolt columns to confirm the mating flange.

Table 3. Class 150 slip on flange dimensions (ASME B16.5) - original data, preserved unchanged

NPS	DN	OD (mm)	Flange Thick. (mm)	Hub OD (mm)	Flange Length (mm)	RF Dia. (mm)	RF Height (mm)	PCD (mm)	Socket Bore (mm)	Bolt Holes	Bolt Size UNC	Weight (kg)
1/2"	15	90	9.6	30	14	34.9	2	60.3	22.2	4	1/2"	0.8
3/4"	20	100	11.2	38	14	42.9	2	69.9	27.7	4	1/2"	0.9
1"	25	110	12.7	49	16	50.8	2	79.4	34.5	4	1/2"	0.9
1 1/4"	32	115	14.3	59	19	63.5	2	88.9	43.2	4	1/2"	1.4
1 1/2"	40	125	15.9	65	21	73.0	2	98.4	49.5	4	1/2"	1.4
2"	50	150	17.5	78	24	92.1	2	120.7	61.9	4	5/8"	2.3
2 1/2"	65	180	20.7	90	27	104.8	2	139.7	74.6	4	5/8"	3.2
3"	80	190	22.3	108	29	127.0	2	152.4	90.7	4	5/8"	3.7
3 1/2"	90	215	22.3	122	30	139.7	2	177.8	103.4	8	5/8"	5.0
4"	100	230	22.3	135	32	157.2	2	190.5	116.1	8	5/8"	5.9
5"	125	255	22.3	164	35	185.7	2	215.9	143.8	8	3/4"	6.8
6"	150	280	23.9	192	38	215.9	2	241.3	170.7	8	3/4"	8.6
8"	200	345	27.0	246	43	269.9	2	298.5	221.5	8	3/4"	13.7
10"	250	405	28.6	305	48	323.8	2	362.0	276.2	12	7/8"	19.5
12"	300	485	30.2	365	54	381.0	2	431.8	327.0	12	7/8"	29.0
14"	350	535	33.4	400	56	412.8	2	476.3	359.2	12	1"	41.0
16"	400	595	35.0	457	62	469.9	2	539.8	410.5	16	1"	54.0
18"	450	635	38.1	505	67	533.4	2	577.9	461.8	16	1 1/8"	59.0
20"	500	700	41.3	559	71	584.2	2	635.0	513.1	20	1 1/8"	75.0
24"	600	815	46.1	663	81	692.2	2	749.3	616.0	20	1 1/4"	100.0

Class 300 Slip On Flange Dimensions

The matching Class 300 record is preserved below in the same format, so the two classes can be compared column by column.

Table 4. Class 300 slip on flange dimensions (ASME B16.5) - original data, preserved unchanged

NPS	DN	OD (mm)	Flange Thick. (mm)	Hub OD (mm)	Flange Length (mm)	RF Dia. (mm)	RF Height (mm)	PCD (mm)	Socket Bore (mm)	Bolt Holes	Bolt Size UNC	Weight (kg)
1/2"	15	95	12.7	38	21	34.9	2	66.7	22.2	4	1/2"	1.2
3/4"	20	115	14.3	48	24	42.9	2	82.6	27.7	4	5/8"	1.4
1"	25	125	15.9	54	25	50.8	2	88.9	34.5	4	5/8"	1.4
1 1/4"	32	135	17.5	64	25	63.5	2	98.4	43.2	4	5/8"	1.8
1 1/2"	40	155	19.1	70	29	73.0	2	114.3	49.5	4	3/4"	2.7
2"	50	165	20.7	84	32	92.1	2	127.0	61.9	8	5/8"	3.2
2 1/2"	65	190	23.9	100	37	104.8	2	149.2	74.6	8	3/4"	4.6
3"	80	210	27.0	117	41	127.0	2	168.3	90.7	8	3/4"	5.9
3 1/2"	90	230	28.6	133	43	139.7	2	184.2	103.4	8	3/4"	7.7
4"	100	255	30.2	146	46	157.2	2	200.0	116.8	8	3/4"	10.0
5"	125	280	33.4	178	49	185.7	2	235.0	144.4	8	3/4"	12.7
6"	150	320	35.0	206	51	215.9	2	269.9	171.4	12	3/4"	17.7
8"	200	380	39.7	260	60	269.9	2	330.2	222.2	12	7/8"	26.0
10"	250	445	46.1	321	64	323.8	2	387.4	276.3	16	1"	36.0
12"	300	520	49.3	381	71	381.0	2	450.9	327.1	16	1 1/8"	52.0
14"	350	585	52.4	413	74	412.8	2	514.4	359.2	20	1 1/8"	68.0
16"	400	650	55.6	470	83	469.9	2	571.5	410.5	20	1 1/4"	91.0
18"	450	710	58.8	533	89	533.4	2	628.7	461.8	24	1 1/4"	111.0
20"	500	775	63.5	584	95	584.2	2	685.8	513.1	24	1 1/4"	141.0
24"	600	915	70.0	692	106	692.2	2	812.8	616.0	24	1 1/2"	211.0

Note: For NPS 26-60, use ASME B16.47 (Series A/B). Weights are theoretical, based on stainless steel density; actual weights vary with material and tolerances.

Slip On Flange Advantages

Slip on flanges win on cost, alignment ease, installation speed and space - the trade-off is lower strength and poorer inspectability than a weld neck flange, so they belong in low to medium pressure service.

Lower initial cost

An SO flange has no long tapered hub, so it uses less raw material and less machining than a weld neck flange of the same size and class. In commodity carbon steel the saving is modest; in a nickel alloy such as Hastelloy C276 or Inconel 625 it is significant, because you are paying for a much smaller mass of expensive metal. That is why alloy slip on flanges still appear in chemical plants where the pressure class is low.

Ease of alignment

Because the flange slides over the pipe, bolt holes can be rotated into line with the mating flange before welding, and the pipe does not have to be cut to extreme length precision - the flange position is adjustable along the pipe axis. Field fitters value this most on large, awkward assemblies where a weld neck flange has to be tacked, checked and re-cut.

Simplified installation

Two fillet welds replace one full-penetration butt weld. Fillet welding needs no edge bevel and no root gap control, which makes it faster and easier to execute in the field - especially in positions where a welder has limited access to the joint root.

Space efficiency

The low profile matters on compact skids and inside equipment where axial clearance is tight: a slip on flange connects without the long neck a weld neck flange needs behind the flange face.

The limitation you must design around

The fillet welds create a crevice between the flange bore and the pipe OD. In chloride or acid service that crevice is a corrosion site, and the joint is less tolerant of bending and cyclic load than a full-penetration butt weld. Neither of these is a reason to avoid slip on flanges - they are reasons to keep them out of high-pressure, high-vibration and severe-corrosion duty. Where those conditions exist, step up to a weld neck flange, or use a lap joint flange with a stub end when you need frequent dismantling in alloy pipe.

Slip On Flange Installation Steps

Installing an SO flange takes five steps - clean and square the pipe, slide the flange on with a gap, weld outside, weld inside, then inspect - and the two fillet welds are what make the joint both strong and leak-tight.

1. Step 1 - Preparation. Cut the pipe end square and remove burrs, rust and debris. Clean the inside of the flange bore and the outside of the pipe so both weld surfaces are bare metal.

2. Step 2 - Position the flange. Slide the flange onto the pipe and leave a gap between the pipe end and the flange face: wall thickness plus about 3 mm is standard practice, so the pipe end cannot touch the gasket or distort the face as it expands.

3. Step 3 - External fillet weld. Run a full fillet weld around the outside where the pipe meets the flange back. This weld carries the primary structural load.

4. Step 4 - Internal fillet weld. Run the second fillet weld inside the bore where the pipe end meets the flange. It seals the crevice between pipe and flange, which is where corrosion attack would otherwise start.

5. Step 5 - Inspection. After cooling, inspect both welds - dye penetrant for austenitic stainless and nickel alloys, magnetic particle only on ferromagnetic parts - and check the flange face for weld spatter before bolting up.

Two welding details separate a joint that passes hydrotest from one that weeps in service. The gap: leaving the pipe end short of the flange face stops the pipe end touching the gasket as the line heats up, and it keeps weld heat away from the sealing face. The weld sequence: stagger the start and stop points of the two welds, and on austenitic stainless or nickel alloys use low heat input with matching filler - ERNiCrMo-4 for C276, ER80S-B2 type or matching stainless filler for stainless - because these alloys distort and segregate if they are overheated. Flange faces should be protected during welding, and any spatter must be removed before bolting: a scuffed raised face is a leak path.

Table 5. Gasket choice and bolt-up guidance for beginner slip on flange joints

Gasket type	Best suited to	Bolt-up notes for a beginner
Sheet / compressed fibre	Water, air, low-pressure utility, FF and RF faces	Single-use only; do not reuse a compressed gasket, and do not over-torque it into the raised face
Spiral wound (SS + graphite or PTFE filler)	Process lines, steam, moderate temperature and pressure	The workhorse for RF flanges; tighten in a cross pattern in at least two passes
PTFE envelope / virgin PTFE	Chemical and hygienic service where contamination matters	Low bolt load; creep is real, so re-torque after the first thermal cycle
Ring joint metal gasket	High pressure, high temperature, RTJ grooved faces	Groove must be clean and undamaged; gasket is not reusable and requires controlled bolt load
Full-face gasket	Flat face flanges on cast iron equipment	Covers the whole face and protects brittle equipment from bending load

Torque figures are deliberately absent from that table: the correct value depends on bolt size, bolt material, thread lubricant, gasket factor and target bolt stress, and it comes from your project's bolted-joint procedure (typically derived from ASME PCC-1). What a beginner should take away is the sequence - tighten in a star or cross pattern, in at least two passes, ending at the specified torque - and the discipline of re-torquing after the first thermal cycle on PTFE gaskets. For the pipe side of the assembly, our pipe and tube range and forged fittings range cover the mating components.

What Mistakes Do Beginners Make With Slip On Flanges?

Five mistakes cause most new slip on flange problems: skipping the inner weld, leaving no gap to the flange face, welding with the flange face unprotected, reusing a compressed gasket or over-torquing one side of the joint, and specifying a slip on flange for high-pressure or cyclic duty where a weld neck belongs.

The inner weld is the one that is invisible after assembly, which is exactly why it gets skipped - and it is also the weld that prevents crevice corrosion between pipe and flange bore. The gap is the second silent error: without wall thickness plus roughly 3 mm of clearance, the pipe end can bear on the gasket or distort the raised face. The third is heat damage - leaving spatter, arc strikes or a warped face on a sealing surface, which no gasket can recover from.

On the specification side, the classic beginner errors are dimensional and documentary rather than mechanical: ordering a Series A large-diameter flange against a Series B drawing, assuming Class 300 is safe at its 100 deg F rating without checking the design temperature, or ordering a nickel-alloy flange to ASTM A182 when the correct forging standard is ASTM B564. Each of these is cheap to prevent on the purchase order and expensive to discover at site. Our nickel alloy flanges for subsea and deepwater article shows how much specification detail a severe-service job actually carries.

Where Are Slip On Flanges Used?

Slip on flanges are best for low to medium pressure systems where cost and ease of installation matter more than ultimate strength: water and firefighting mains, cooling water circuits, HVAC piping, low-pressure steam, and moderate-pressure chemical lines.

Water and firefighting systems. Their ease of installation makes them ideal for municipal water lines and fire suppression systems, where pressures are moderate and the sheer number of joints makes welding time the dominant cost.

Cooling water circuits. Power plants and refineries use SO flanges extensively in secondary cooling loops - a duty that is low pressure but often chloride-bearing, which is where the material choice (316L, duplex or super duplex) matters more than the flange type.

Chemical processing. In 316L, 904L or a nickel alloy, slip on flanges serve non-hazardous chemicals and general process streams at moderate pressure. Where the fluid is a hot acid or contains chlorides, the alloy and the inner weld become the critical decisions - see our C276 flange guide and the ASTM B564 N10276 flange dimensions reference.

HVAC and building services. Large-bore heating and cooling piping in commercial buildings runs comfortably in Class 150 carbon steel or 304/316 stainless.

Low-pressure steam. Suitable where pressure and temperature do not fluctuate violently - steady steam is kind to a fillet-welded joint, while cycling steam is not, because fatigue cracks initiate at the weld toe.

Packaged equipment and skids. Because the flange slides on, it tolerates the small dimensional variations in prefabricated assemblies; our F53 weld neck flange shipment case study shows how flange specification detail is handled on prefabricated orders.

Slip On Flange vs Weld Neck Flange

Choose a slip on flange when budget and installation ease matter in low to medium pressure service; choose a weld neck flange for critical, high-pressure, high-temperature or high-vibration duty where strength and radiographic inspectability justify the higher cost.

The comparison below is the original SO versus WN table from this page, with two adjustments so the claims match what can actually be defended. The strength difference is retained but labelled as the rule of thumb it is, and the earlier 'service life about 3x' claim has been replaced with a statement about fatigue that does not depend on an unsourced multiplier.

Table 6. Slip on flange versus weld neck flange

Factor	Slip on (SO)	Weld neck (WN)
Connection	Slides over the pipe; two fillet welds	Long tapered hub; one full-penetration butt weld
Installation	Faster, less skill, no bevel, position adjustable	Slower, needs bevelled pipe end and alignment
Cost	Lower - less material and machining	Higher - long hub, more alloy mass
Strength	Moderate; commonly cited as about two-thirds of a WN joint (rule of thumb, not a code value)	High; the hub transfers load into the pipe wall and suits bending and fatigue duty
Pressure and temperature	Low to medium pressure; steady temperature	High pressure and temperature; cyclic and vibrating service
Inspectability	Fillet welds are difficult to radiograph or ultrasonically test	Full-penetration butt weld is RT/UT inspectable
Service life under severe duty	Good in steady conditions; fillet weld toe is the fatigue-sensitive point	Longer in cyclic service because there is no crevice and the weld is fully inspectable
Best use	Water, firefighting, cooling, HVAC, low-pressure steam, moderate process lines	Oil and gas, power, high pressure, hazardous or critical systems

Notice that none of these rows is really about the flange body - they are about the weld. A slip on joint is limited because a fillet weld cannot be volumetrically inspected and because it leaves a crevice, not because the forging is weaker. That is why the same forged material can be perfectly acceptable in one flange form and unacceptable in another, and why the material certificate alone never proves the joint is right for the duty. If your line is alloy and severe, compare lap joint and weld neck options before defaulting to slip on.

How Does a Slip On Flange Compare With Other Flange Types?

Within the forged flange family, the slip on flange is the cheapest and most forgiving but the least inspectable; lap joint suits frequent dismantling in alloy pipe, socket weld suits small-bore high-pressure lines, threaded suits no-weld tie-ins, and blind flanges close a line end.

Beginners usually meet five flange types in the same project, and each has a job the others do badly. The table puts them side by side so you can see why an engineer does not simply specify the cheapest one everywhere.

Table 7. Slip on flange compared with the other common forged flange types

Flange type	How it attaches	Strength / inspectability	Where it belongs
Slip on (SO)	Two fillet welds, outside and inside	Moderate / fillet welds hard to RT	Low to medium pressure utility and process lines
Weld neck (WN)	One full-penetration butt weld	High / fully RT and UT inspectable	High pressure, high temperature, cyclic, hazardous duty
Lap joint (LJ)	Loose flange on a stub end that is butt welded	Moderate to high / bolt holes can be rotated on assembly	Alloy and exotic piping that will be opened frequently
Socket weld (SW)	Pipe inserted into a socket, fillet welded	Moderate / crevice at socket bottom	Small bore, high pressure utility lines (typically NPS 1-1/2 and below)
Threaded	Screw thread, sealant required	Lower / thread roots are stress raisers	Low pressure, small bore, no-weld tie-ins and instrumentation
Blind	Bolted over a nozzle or line end	Matches the mating class	Isolating or closing a line end, hydrotest blinds, future tie-in points

Two of those alternatives deserve a beginner's attention. A lap joint flange lets you rotate the bolt holes freely on assembly - genuinely useful in expensive alloy pipe, because the flange itself contacts no fluid and can be a cheaper material than the stub end. A socket weld flange handles small-bore high-pressure utility lines where a slip on flange would be poor practice. Threaded flanges in lower-pressure service are covered in our threaded flange deep guide, and for closing line ends see the blind flange guide.

How Do You Select the Right Slip On Flange?

Selection is four decisions in order: confirm the flange form is acceptable for the duty, pick the pressure class and design temperature, pick the face type against the mating equipment, then pick the material against the fluid - and only then choose the size and dimension table.

Working in that order prevents the expensive mistakes. Deciding material first is how people end up with a beautifully corrosion-resistant flange welded to equipment it cannot seal against. The table below is the quick-selection version of the original comparison table on this page, extended with the standard or reference that settles each row.

Table 8. Beginner's slip on flange selection table

Selection question	Choose	Why	Standard or reference
General low-pressure process or utility service?	RF slip on flange, Class 150	Most common, lowest installed cost, seals well with a spiral wound gasket	ASME B16.5
Bolting to cast iron or brittle equipment?	FF slip on flange	Spreads bolt load and avoids bending the casting	ASME B16.5 flat face
High-pressure gas or oilfield service?	Weld neck or RTJ flange	A slip on fillet weld is not the right joint here	ASME B16.5 / B16.47
Corrosive fluid or chlorides?	316L, 904L, duplex or a nickel alloy	Match corrosion resistance to the fluid, and remember the crevice at the bore	ASTM A182 / ASTM B564
Frequent dismantling in alloy pipe?	Lap joint flange with stub end	Rotating bolt holes, cheaper flange material, butt-welded joint	ASME B16.5 lap joint
Small bore, high pressure?	Socket weld flange	Purpose-built for that size and duty range	ASME B16.5 socket weld
Tight budget, low or medium pressure?	Slip on flange	Lowest installed cost of the forged flange family	ASME B16.5
High pressure, vibration or required RT?	Weld neck flange	Stronger joint, fully inspectable, no crevice	ASME B16.5 / B16.47

What Inspection and Certification Should a Slip On Flange Come With?

A compliant flange delivery includes a heat-specific EN 10204 3.1 mill test certificate against the correct forging standard and the ASME B16.5 dimension and marking requirements - and where the project demands it, a 3.2 certificate countersigned by an independent inspector.

Check four things on the paperwork before the flange goes into the rack. First, that the certificate cites the forging standard you ordered (ASTM A105, A182 F316L, B564 N10276 and so on) and shows heat chemistry against that specification - not 'typical' values. Second, that the marking on the flange itself carries the manufacturer, material grade, size, pressure class and standard, which is what a third-party inspector will verify against the certificate. Third, that any supplementary test you ordered - ultrasonic examination, hardness, dye penetrant on the machined face - actually appears on the document. Fourth, that the heat number on the certificate matches the heat number stamped or tagged on the flange.

On the weld side, inspection follows the material: dye penetrant testing applies to austenitic stainless, duplex and nickel alloys, while magnetic particle testing only works on ferromagnetic materials such as carbon and low-alloy steel. Using 'PT or MT' as a generic phrase in a specification is a beginner giveaway - on a 316L or C276 joint only PT is physically meaningful. Volumetric inspection of the fillet welds themselves is limited, which is precisely why slip on flanges are not specified for duty that demands radiographic acceptance. Material test documentation for alloy flanges is covered in more detail on our ASTM B564 Monel 400 flange and 904L flange references.

What Drives the Cost of a Slip On Flange?

Four variables set the price of a slip on flange: material, size, pressure class and quantity documentation - and material dominates by a wide margin, which is why the same NPS 4 flange can cost ten times more in one alloy than another.

Material is the largest factor: carbon steel is the baseline, 304/316 stainless typically sits a few multiples above it, duplex and super duplex above that, and nickel alloys such as Inconel 625 or Hastelloy C276 at the top of the range. Size and class then multiply the mass you are paying for - a Class 300 flange is heavier than a Class 150 of the same bore, and a slip on flange is lighter than the equivalent weld neck, which is exactly where the SO saves money. Quantity matters more than beginners expect because setup costs are amortised over the order. Documentation and testing - 3.2 certification, ultrasonic examination, special facing finishes - are small percentages that can nonetheless complicate a small order.

Because price moves with alloy and class, the honest answer to 'what does a slip on flange cost' is a range, not a number: get a quotation against a complete specification. That is also the practical way to avoid the classic procurement error of comparing two quotes that are not for the same thing - different material standard, different pressure class or different certification. If you need indicative alloy pricing to build a budget, our 254SMO flange and duplex S32205 flange pages show how the same flange form is specified across alloys.

Frequently Asked Questions