Why Slip of Flange connection is not strong as Weld Neck and Socket Connection? 5

[ramprakash11]

Flanges have many types like - Welding Neck, Slip On, Socket Weld, Lap Joint, Threaded, and Blind flange. Here are few description about flanges.

Welding Neck Flange - Welding Neck Flanges are easy to recognize at the long tapered hub, that goes gradually over to the wall thickness of a pipe or fitting.These flanges are bored to match the inside diameter of the mating pipe or fitting so there will be no restriction of product flow.

Slip on Flange - The calculated strength from a Slip On flange under internal pressure is of the order of two-thirds that of Welding Neck flanges, and their life under fatigue is about one-third that of the latter.

Socket Weld Flange - Socket Weld flanges were initially developed for use on small-size high-pressure piping. Their static strength is equal to Slip On flanges, but their fatigue strength 50% greater than double-welded Slip On flanges.

Slip on flange has equal static strength as Socket weld strength. Still, Socket connection is more powerful than slip on flange. Above information, I have taken after reviewing may companies like CHW Forge, Bharat, Maharashtra Forgings and many more.

 

[moltenmetal]

Slip on Flange - The calculated strength from a Slip On flange under internal pressure is of the order of two-thirds that of Welding Neck flanges

Rubbish.

Slip-on flanges have the same B16.5 pressure ratings as the other types- so unless you're designing outside a piping code, any "calculated strength under internal pressure" is pretty much meaningless.

The stress intensification factors used in flexibility and fatigue analysis are of course different. Anything attached with fillet welds rather than full penetration welds is going to suffer from this problem. The benefit of fillet welds though is substantially higher welding productivity. Radiography of a socket-welded joint doesn't tell you anything useful so doing it is a waste of time- you can see that as a benefit or a disadvantage depending on your perspective.

Socket welding flanges, like most socket welding components, are used to 2" NPS and no higher. Socket welding flanges rely on one fillet weld so their SIFs are quite unfavourable. They also have a huge crevice for the initiation of corrosion and to store material to ensure that you can never really clean the piping. But they find lots of successful use in small lines.

Slip-ons have a smaller raised face (with an ID greater than the OD of the pipe), which means that if you use spiral wound gaskets, you need to use special dimensions. They require two welds, but don't have the crevice. They also allow easy adjustment for alignment during fitting.

 

[LittleInch]

ramprakash.

You really need to back up your statement and look closely at whatever reference you got that from. There is a lot of difference between guidelines and "rules of Thumb" and what is officially permitted by the code.

I agree with everything molten metal says.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bimr]

From Piping Handbook by Mohinder L. Nayyar:

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.

http://www.wmhuittco.com/images/Article_2_Piping_Design_Part_2_Flanges.pdf

 

[LittleInch]

yup - a guideline. A quote from a piping manual is not a calculation referenced back to anything.

I don't much like slip on flanges, but you need something better than that before you consign them to only have 2/3 the "calculated strength under internal pressure".



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[KevinNZ]

"Their calculated strength under internal pressure is about two-thirds of that of weld-neck flanges"

Where does this come from?

Use the pressure rating in the flange code which are the same for all flange types.

You can design custom flanges to the pressure vessel codes but you will be making your life hard if you start checking standard flange dimensions to code calculations.

As posted above, The only difference the pipe code looks at is SIFs for the flange type for pipe stress. There are also some size application limits, Pressure rating is not effected.

 

[racookpe1978]

Many times faster Field fit, more accurate field alignment (or correction of mis-alignment) from old pipes or old equiment, and the need for accomodating installation uncertainities are very big advantages. Sometimes.