How to Install a Slip on Flange

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. However, customers should bear in mind that this initial cost saving may be diminished by the additional cost of the two fillet welds required for proper installation. Moreover, weld-neck flanges have a higher life expentancy than slip-on flanges under duress.

slip on flange 2 - How to Install a Slip on Flange

Slip on Flange is essentially a ring that is placed over the pipe end, with the flange face extending from the end of the pipe by enough distance to apply a weld bead on the inside diameter.

The OD of slip on flange is also welded on the back side of the flange. Slip on flanges have a lower material cost than weld neck flange and are more easily aligned.

Slip on flange may also be used a lap joint flanges if Type B or Type C stub ends are used.

Materials of Construction

Slip on flanges can be made from a number of different materials depending on the piping material and the requirements of the application. Selection depends on factors such as environmental corrosion, operating temperature, flow pressure, and economy. Some of the most common materials include carbon steel, alloy steel, stainless steel, cast iron, copper, and PVC.

Carbon steel is steel alloyed primarily with carbon. It has a high hardness and strength which increases with carbon content, but lowers ductility and melting point. For more information on carbon and alloy steels.

Alloy steel is steel alloyed with one or more elements which enhance or change the steel’s properties. Common alloys include manganese, vanadium, nickel, molybdenum, and chromium. Alloy steels are differentiated based on standard grades. For specific information on individual types of alloying elements.

Stainless steel is steel alloyed with chromium in amounts above 10%. Chromium enables stainless steel to have a much higher corrosion resistance than carbon steel, which rusts readily from air and moisture exposure. This makes stainless steel better suited for corrosive applications that also require high strength. For more information on stainless steel alloys.

Cast iron is iron alloyed with carbon, silicon, and a number of other alloyants. Silicon forces carbon out of the iron, forming a black graphite layer on the exterior of the metal. Cast irons have good fluidity, castability, machinability, and wear resistance but tend to be somewhat brittle with low melting points.

Aluminum is a malleable, ductile, low density metal with medium strength. It has better corrosion resistance than typical carbon and alloy steels. It is most useful in constructing flanges requiring both strength and low weight. For more information on aluminum.

Brass is an alloy of copper and zinc, often with additional elements such as lead or tin. It is characterized by good strength, excellent high temperature ductility,reasonable cold ductility, good conductivity, excellent corrosion resistance, and good bearing properties. For more information on brass and other copper alloys.

PVC or polyvinyl chloride is a thermoplastic polymer that is inexpensive, durable, and easy to assemble. It is resistant to both chemical and biological corrosion. By adding plasticizers it can be made softer and more flexible.

Types of flanges:

Welding Neck Flanges (Weldneck Flanges) – are distinguished from other by their long tapered hub and gentle transition of thickness in the region of the butt weld joining them to the pipe. The long tapered hub provides an important reinforcement of the flange from the standpoint of strength and resistance to dishing. The smooth transition from flange thickness to pipe wall thickness is extremely beneficial under conditions of repeated bending, caused by line expansion/contraction or other forces, and produces a strength equivalent to that of a butt welded joint between pipes. This type of flange is preferred for every severe service condition, whether this results from high pressure, from sub-zero or high temperatures or from extreme loading conditions. This type of flange is recommended for handling explosive, flammable or costly liquids, where loss of tightness or local failure must be minimized.
Slip-On Flanges – continue to be the preferred flanges by most installing contractors because of their lower first cost, the reduced accuracy required in cutting the pipe to length, and the greater ease of alignment of the piping assembly. Unfortunately, their final installed cost is likely not much less (if any) than that of welding neck flanges. Their calculated strength under internal pressure is approximately 2/3 that of the weldneck flange and their life under fatigue is about 1/3 that of weldneck flanges. For these reasons, slip-on flanges are limited to line sizes 1/2″ to 2 ½” in the Class 1500 ANSI standard.
Lap Joint Flanges – are primarily installed with lap joint stubs, the combined initial cost of which is approximately 1/3 higher than that of comparable weldneck flanges. Their pressure holding ability is little, if any, better than that of a slip-on flange and their fatigue life is only 1/10 that of welding neck flanges. The primary use of lap joint flanges is in carbon or low alloy steel piping systems in services necessitating frequent dismantling for inspection and cleaning and where the ability to swivel flanges and to align bolt holes simplifies the erection of large diameter or unusually stiff piping. Their use at points where severe bending stress occurs should be avoided. 
Threaded Flanges – made of steel should be confined to special applications. Their chief merit lines in the fact that they can be assembled without welding. They are often used in extremely high pressure service applications, particularly at or near atmospheric temperature, where alloy steel is essential for strength and where the necessary post-weld heat treatment is impractical. Threaded flanges are unsuited for conditions involving temperature or bending stresses of any magnitude, particulary under cyclic conditions, where leakage through the threads may occur in relatively few cycles of expansion or contraction or stress. Seal welding is sometimes accomplished to overcome this weakness, but cannot be considered as entirely satisfactory.
Socket Weld Flanges – are available for use on small-size high pressure piping. Their initial cost is about 10% higher than that of slip-on flanges. When provided with an internal weld, their static strength is equal to that of double-welded slip-on flanges while their fatigue strength is 50% greater. Smooth bore conditions are possible by grinding the internal weld, unlike with slip-on flanges which require that the flange face be beveled and re-faced after welding. The internally welded socket type flange is becoming increasingly popular in the chemical process piping industry.

Blind flanges – are pipe flanges used to seal the end of a piping system or pressure vessel openings to prevent flow. Blind pipe flanges are commonly used for pressure testing the flow of liquid or gas through a pipe or vessel. Blind pipe flanges also allow easy access to the pipe in the event that work must be done inside the line. Blind pipe flanges are often used for high pressure applications. Slip on Pipe flanges with a hub have published specifications that range from 1/2″ to 96″. Sunny Steel provides Blind pipe flanges in all material grades, specifications and sizes.


Pressure Capacity

Pipe Sizes

Applications / Advantages


Very high


Closing pipes, flow pressure testing

Lap joint



Systems requiring frequent disassembly




Low installation cost, simple assembly

Socket weld



Smooth bore for better fluid flow




Attachment without welding

Welding neck



High pressures and extreme temperatures

When to use slip-on vs. lap-joint flanges

Weld Neck Flanges:
Weld Neck Flanges are distinguished from other flange types by their long tapered hub and gentle transition of thickness in the region of the butt weld that joins them to pipe or a fitting. A weld-neck flange is attached to a pipe or a fitting with a single full penetration, “V” bevel weld. The long tapered hub provides an important reinforcement of the flange proper from the standpoint of strength and resistance to dishing. The smooth transition from the flange thickness to the pipe wall thickness by the taper is extremely beneficial under conditions of repeated bending caused by line expansion or other variable forces, and produces an endurance strength of welding neck flanged assemblies equivalent to that of a butt-welded joint. This type of flange is preferred for severe service conditions, whether loading conditions are substantially constant or fluctuate between wide limits.
The weld neck flange is used in each of the seven flange ratings and has the advantage of requiring only one weld to attach it to the adjacent pipe or fitting.
The key dimension for a weld neck flange is the length through the hub from the beveled end to the contact face of the flange. This “length” includes the bevel, the tapered hub, and the thickness of the plate part of the flange and the raised face. To obtain the correct dimension you must look at a correctly constructed flange dimension chart (see the “Tools” button on this website) or a flange manufacturers catalog. Electronic piping design software will normally already have the correct dimension built-in.
It is important to understand and remember that the (1/16″) raised face on the Class 150 raised face and on the Class 300 raised face flanges is normally included in the length dimension. However, the ¼” raised face is not included in the chart or catalog length dimension for the Class 400 and higher pressure rated flanges. The raised face dimension for Class 400 flanges (and up) normally must be added to the chart or catalog length to arrive at the true total length of these higher-pressure flanges.
Slip-on Flanges:
Slip-On (SO) 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.
Lap Joint Flanges:
A Lap Joint Flange is a two piece device that is much like a weld-neck flange but also like a loose slip-on flange. One piece is a sleeve called a ‘Stub-end” and is shaped like a short piece of pipe with a weld bevel on one end and a narrow shoulder on the other end called the hub. The hub is the same outside diameter as the raised face (gasket contact surface) of a weld neck flange. The thickness of the hub is normally about ¼” to 3/8″. The back face of the hub has a rounded transition (or inside fillet) that joins the hub to the sleeve.
The other piece of a Lap Joint Flange is the backing flange. This flange has all the same common dimensions (O.D., bolt circle, bolt hole size, etc.) as any other flange however it does not have a raised face. One side, the backside, has a slight shoulder that is square cut at the center or pipe hole. The front side has flat face and at the center hole an outside fillet to match the fillet of the “Stub-end” piece. The flange part of the Lap-joint flange assembly is slipped on to the stub-end prior to the sleeve being welded to the adjoining pipe or fitting. The flange itself is not welded or fixed in any way. It is free to spin for proper alignment with what ever it is joining to.
The “Stub-end” can normally be purchased in two lengths. There is a short version, about 3″ long and a long version of about 6″ long. It is prudent for the piping designer to know which version is in the piping specification.
Because of it’s two piece configuration, the Lap Joint Flange offers a way to cut cost or simplify work. The cost saving comes when the piping system requires a high cost alloy for all “wetted” parts to reduce corrosion. The sleeve or Stub-end can be the required higher cost alloy but the flange can be the lower cost forged carbon steel.
The work simplification comes into the picture where there are cases that require frequent and rapid disassemble and assembly during the operation of a plant. The ability to spin that backing flange compensates for misalignment of the bolt holes during reassembly.
Well I’ve seen Slip-on flanges used in a chemical processing plant – all low pressure steel lines. I imagine that the cost savings over weld necks would have been substantial. The same plant had (loose) lap joint flanges on the SS lines. Again, all mostly low pressure Sch10 stuff and weld necks would have considerably added to the cost. 
Not sure about the statement in rneill’s post “Lap joint flanges have very low fatique resistance and should not be used in cyclic or vibrating service.” The stub end is butt welded to the pipe end, so that’s Ok for cyclic service, unless the reference was to Slip-on type flange connections where fillet welds exist.
Certainly any oil and gas work I’ve been involved in has all been weld neck flanges.
Generally, if you consider the additional welding time for the two fillet welds on a slip on flange, while the purchase price is lower, there is usually no savings in installation cost. Consequently, my comment about the only real advantage being to make up for misaligned flanges.
Regarding the Sch10 lap joint flanges, weld necks are available in Sch10 as well but I do agree there can be cost savings on lap joints since the hub is usually only carbon steel.
The ring on the lap joint flange is going to impose mechanical loadings into the stub end on the pipe which are going to oscillate based upon the cyclic loads that are present. This will to an extent cause the stub end to act like a coat hanger being bent back and forth.  It’s not the weld that is subject to fatigue but the base material in the stub end of the lap joint flange.
Why Slip of Flange connection is not strong as Weld Neck and Socket Connection?

Slip-on Flanges. Slip-on flanges are preferred to weld-neck flanges by many users because of their initial low cost and ease of installation. Their calculated strength under internal pressure is about two-thirds of that of weld-neck flanges. They are typically used on low-pressure, low-hazard services such as fire water, cooling water, and other services. The pipe is ‘‘double-welded’’ to both the hub and the bore of the flange, but, again, radiography is not practical. MP, PT, or visual examination is used to check the integrity of the weld. When specified, the slip-on flanges are used on pipe sizes greater than NPS 2¹⁄₂ (DN 65).
Flange Types. Several different flange types are permitted by the standards listed in Table C7.5, as principally covered by ASME B16.5. These include different types of attachment to the pipe including threaded, lapped, and welded as covered in more detail by Chap. A2 of this handbook. Of the welded type, most flanges are the butt-welded, slip-on, or socket-welded types. The majority of flanges are butt- welded, which are more commonly referred to as the welding neck flange. Socket- welded flanges are typically limited to small-diameter connections less than NPS 2 (DN 50). Slip-on flanges fit over the outside diameter of the pipe and are attached with fillet welds at both the pipe end and off the hub end of the flange.
Typical restrictions on the use of slip-on flanges include:
● While available in most pressure classes, slip-on flanges are more typically limited to Class 300 or lower pressure rating in process plant piping. The available raised face gasket seating area can preclude commonly used gaskets in the case of slip- on flanges for higher class piping.
● They are limited to services with design temperatures below 750°F (400°C).
● They should not be used where the specified corrosion allowance exceeds 0.125 in (3 mm).
Many pipe designers are reluctant to use slip-ons for higher pressures, since (1) the joint between the flange and pipe is not as strong as in the welding neck type; and (2) the junction of the flange and pipe is more susceptible to corrosion.
Welding Slip on Flange to Long Radius Elbow

As far as SIF or k values goes, most of the ones you see in the B31 codebooks.
B31.3 only gives you 30% (I think) of the fatigue strength for a SO flange vs a WN.  This is because  a SO flange is welded using a fillet weld and not a full pen groove weld (“buttweld”).  Also, there is less NDE available for this type of joint vs a buttweld, so you can’t verify the integrity of the joint, and correspondingly the codebook kicks you in the butt for using a SO flange.  If you have any loads transferred to this joint in the form of reactive moments or forces due to pipe supports, equipment nozzles, seismic loads, wind, etc., I would be VERY careful when using this setup.  Make sure you have identified all your load cases and anticipated operating scenarios, e.g. startup, hydrotest, seismic event, steam-out, operator standing on the pipe, etc.  Any one of those may overload this connection.  If I were doing this myself, when setting the allowable for this connection, I would probably derate the allowable by a goodly factor relative to the allowable stress for a standard WN to BW elbow.
Most of the owners of chemical plants/refineries/oil fields that I’ve worked with will not allow the use of SO flanges, period, in any service, even one such as this, without a rigorous analysis showing that the use of a SO in any proposed application is OK, and that usually means an FEA.
So in this case, you have a SO flange welded directly on a LR elbow.  During fit-up, due to the curvature of the ell you will have a larger gaposis between the SO flange and the ell on the ID of the ell vs the OD of the bend, this being caused by the need to square the bevel on the ell with the groove on the bore of the SO flange, i.e. align the CLs of the two fittings.  They have to be squared (1) so the CL doesn’t get screwed up and (2) so the end of the ell doesn’t interfere with the gasket on the companion flange.  This means more weld metal has to be deposited by the welder on the OD side than on the ID side, which provides more opportunity for weld defects since the welder is filling a larger gap on the OD.  The additional weld metal may also distort the elbow but I don’t think that will add up to be a problem in this case.  It might look funky though.
Lastly, you can buy LR ells with straight tangents on them and that will get you out of this problem.  Or you can have them formed (bent) from straight pipe.
How to Install a Slip on Flange

Slip on flanges usually “slip” over a pipe and are welded in place. This allows the flanges to swivel freely to easily align bolt holes prior to installing and welding in place. 

Slip on Flange - How to Install a Slip on Flange

Slip On Flange Dwg - How to Install a Slip on Flange

  1. Slip On Flange
  2. Filled Weld Outside
  3. Filled Weld Inside
  4. Pipe

Flanges are most often used to connect pipes that have diameters of more than 2 inches. The flange’s joint consists of two matching disks of metal, separated by a gasket, that are bolted together to achieve a secure seal with the gasket material. The flange is attached to the pipe by screwed fittings. The flange uses force provided by the bolts to pre-load the gasket. When internal pressure is applied, there is enough contact stress between the flanges and gasket to maintain a seal.
Check the flange and pipe to make sure there is no damage that may prevent a proper seal from being formed.
Slide the slip-on flange onto the pipe using your hand. Determine where you will be welding the flange to the pipe. In most cases, you will be welding the flange to the end of the pipe.
Use a welder to weld the inside and outside of the flange to the pipe. This will create a strong seal between the pipe and flange.
Perform a pressure test to ensure the welded seal does not leak. Turn on the water leading to the pipe and observe the pipe for any leaks.

How to Use Slip-On Flanges on a Steam Pipe

Pipe is assembled with other pipe, fittings, and flanges either by welding or threading. There are specific codes describing the permissibility of threading or welding. With welding you also need to be aware of several options. Flanges and fittings are either slip-on or weld-neck. Slip-on fittings slip onto the end of the pipe. The flange is then welded around the contact points on the inside and outside of the pipe and the flange. Slip-on flanges are not considered as strong a joint as weld-neck or butt-welded connections. With butt-welded or weld-neck flanges, the two pieces—flange and pipe—are prepped and then welded together, with full penetration (a welder carefully lays a bead and builds up layers around the entire surface of the gap between the two pieces).
Socket welding describes when a slip-on fitting, usually used for small diameters, is inserted into the fitting until it bottoms out. The pipe is then pulled back from the bottom and welded to the fitting. Failure to pull the pipe back can cause weld failure due to stress.

Select the flange size that is appropriate to the steel pipe. The fit needs to be snug and not loose. The slip-on flange consists of a bored hole with a diameter just slightly larger than the steel pipe.

  1. Slip the flange over the end of the pipe base.
  2. Thread the flange pins to tighten the flange’s grasp on the pipe. The pins will keep the pipe securely in place inside the flange. There are usually six to eight pins in place. Use a screwdriver to tighten the pins all the way.
  3. Try to pull the pipe out of the flange to make sure that the pins are efficiently tightened. If they aren’t, re-tighten them with the screwdriver. Inspect the pins to make sure they are down all the way and that the pipe cannot move inside the flange.
  4. Use a hammer to slightly tap the flange in place. The flange face should be flush with the end of the steel pipe.
  5. Use a 12-inch wrench with 85 pounds force to tighten the pins. Apply 80 to 85 foot-per-pounds torque on the pins, or until the pin heads break off.
  6. Lubricate the gasket of the pipe and stretch it over the pipe end, with the beveled edge positioned in the field flange.

Two fillets ARE easier and quicker to do than one butt weld completed 100% from the outside and which must pass RT.  Both welds are available for inspection visually and by MP/LP etc. when the piping is disassembled- just not RT.  And contrary to another poster’s assertion, you have 1/2 the risk of a single weld leading to a failure since you have two welds EACH of which should be strong enough for the job.
Some folks drill a small hole through the flange to vent the cavity between the two welds:  it helps vent trapped air during completion of the 2nd weld and helps detect cracks in the face weld earlier.
At 2″ and smaller we use socket weld flanges:  one fillet weld is quicker to do than two.
There’s a lot less material in a SO than in a WN- a benefit if you’re using something other than carbon steel and LJs are off the table.
All of that said, I just plain STOP using either SW or SO beyond 600# class on piping.  As body flanges on small vessels made of piping, where the flange never sees moments or forces other than hydrostatic and bolt loading, I’ll permit their use beyond 600 class.
In piping, the stress concentrations on the fillet welds and the thermal stress/cycling fatigue risk, and the residual stress on these welds due to the heat input during completion of the 2nd weld and subsequent shrinkage make a SO flange riskier than a single butt welded WN, and make a SW flange very risky.  Hence the reduced fatigue life stated previously.
Don’t forget that they also need gaskets with a bore equal to the OD of the pipe, NOT its ID.  Use the wrong spiral-wound gasket and you could have hte gasket unravelling inside the piping.

Source: China Slip On Flanges Manufacturer – Yaang Pipe Industry Co., Limited (

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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Please notice that you might be interested in the other technical articles we’ve published:






  • Difference Between Pipe Elbow And Pipe Bend

  • Where to get high quality alloy steel pipes

  • Distinguish Inferior Steel Pipes


  • The Heat Treatment Process of Steel Pipes



  • Where to get high quality blind flanges

  • Where to get high quality orifice flanges

  • Where to get high quality lap joint flanges

  • Where to get high quality plate flanges

  • Where to get high quality slip on flanges

  • Where to get high quality ring type joint flanges



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