How to decide between a slip on / weld neck flange? 8

[arksurat]

I have a 24" SA 106 -Sch 100 Seamless pipe(Max Working pressure=1440 psig @ 25 Deg C, petroleum crude) for vertical seperator with top head as 600# RTJ flange of SA 105 and bottom as formed end of SA516- GrB.
How to decide between a slip on flange or weldneck flange?

 

[PAN]

In corrosive service and high pressure-temperature, I always use welding neck flange. This is also easy to inspect the weld by RT. In your example, I decide to use welding neck flange.

For further consideration, slip-on flange is not suitable in shock and vibration

 

[Balukv]

General practice in the Refinery sector is to use Weld neck flanges for all corrosive and high pressure applications. Hydrogen carrying services do not recommend Slip-on flanges for any class of rating. Slip-on flanges are used only on utility services like Water etc.

For your service, a Slip-on flange is certainly not recommended or suitable.

 

[kstaylor]

Also see thread 794-16557 in the Boiler & Pressure Vessel Engineering forum.

 

[kumar081070]

Since You have a RTJ Flange that is Ring Type Groove . For 24 Inch Line you cannot consider Slip on Flange .You need to use a WNRTJ Flange i.e. Weld neck RTJ flange as the mating flange .
I suppose for the lower Flange with Formed ends u can go for a Weld neck Flange . i will be interested in knowing the size here .
Regards
Kumar

 

i thought that the neck of the flange has very little thing to do with the face of the flange.

the use of slipon is limited by B31. specifically, the criteria is whether the service is under severe cyclic load or not. if it is, a slipon is a no-no (pretty obvious, you dont get a full penetration with a slipon).

the flange face is another story. Either a rtj or raised face or full face will be acceptable provided that the pressure that tended to separate the jont falls within the range of the temp-pressure chart. i may add that the tyep of gasket use, the bolt amterial and the method of tightening the bolts has a lot to do of getting a leak tight joint.

 

[KernOily]

Most owners do not allow the use of slip-on flanges in any service other than utilities, steam not included. Why? (1) You can't get a full-penetration weld with a SO flange (2) Your component fatigue life is drastically reduced (see 31.3) (3) you can't radiograph the weld so you have to use other NDE methods to qualify the weld (4) the interface at the weld is a great place for crevice corrosion to start.

The flange face is a different issue than the connection type. I personally would not specify a SO flg in a service where a RTJ was needed and I don't know of any owners who would, either. Thanks!
Pete
P. J. (Pete) Chandler, PE
Principal Engineer
Mechanical, Piping, Thermal, Hydraulics
Processes Unlimited International, Inc.
Bakersfield, California USA
pjchandl@prou.com

 

[ralphsare]

308.2.1 says you can use a slipon flange (double welded) for service subject to severe corrosion and even under severe cyclic conditions. its a no-no where application will see a wide range of delta T.

however 308.2.4 says that its ok ro use under severe cyclic conditions provided it is "safeguarded" else use a WN.

let me add this. SO streght and fatigue life as pointed out is less than that of the WN. The safeguarding part will involve add'l burden that will probably cost more (many sleepless night) if you just utilized a WN.

 

[suraiya]

Need further clarification on the RTJ vs RF things. I have a piping system which require 2500# flange. Which type of flange face will be most preferable - raised face or RTJ, and for what reasons?

 

[PAN]

suraiya,

I always use ring-joint type of facing for such high pressure service. The internal pressure acts on the ring to increase sealing force on the joint.

If high cost is not your main concern, I recommend to use RTJ.

 

[welding1]

Slip on flanges are frequently used on pressure vessels. However the British Pressure Vessel Code (PD5500) restricts the maximum temperature for this type of flange to around 350°C because of creep. Fillet welds and fatigue don't go together and volumetric NDT of fillets is almost impossible. Therefore for critical componets that require high integrity, you need a weld neck, otherwise a slip on is perfectly OK.

Note with a slip on flanges you need a good weld on the under side as this forms the hub which is the highest stressed part of the flange.

If you want to know more about fatigue in welded joints go to

http://www.gowelding.com

Regards
John

 

[af246]

The pipe specs in our refinery does not allow to use slip on flanges.
One of the problems that occurs is the difficulty to UT and inspect the internal weld while the piping is on service.
We use SW flanges up to 2" anything over that is WN.

 

[JonnyWill]

SliP-ON flanges are as follows ASME B31.3 1996 Edition.
308,2,1 Slip On Flanges.(a) A Slip-on flange shall be double-welded as shown in Fig. 328,5,2B when the service is: (1) subject to severe erosion, crevice corrosion, or cyclic loading; (2) Flammable, toxic, or damaging to human tissue, (3) under severe cyclic conditions, (4) at temperatures below -101 DegC (-150 degF).
(b) The use of slip-on flanges should be be avoided where many large temperatures cycles are expected particularly if the flanges are not insulated.
The figure mentioned above shows a a typical detail for double-welded slip-on and socket welding flange attachment welds. Because of your process is high pressure the safest bet would be to go with the Weld Neck Flange
Jonny Will

 

[JohnBreen]

posted September 27, 2002 11:07 AM
--------------------------------------------------------------------------------
Hello,

Also......

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.

Regards, John.

 

[PAN]

JohnBreen,

Please clarify the method of internal welding at socket weld flange and small bore pipe.

 

[arto]

There was a very good & easy-to-use chart on Flange types[& other welds] in the Dutch Stoomwesen Code Par. W0301/72-12 App.2 {old copy -is it still in there?} Entitled "Suitability Grading," it was taken from a supplement to IIS/IIW-237-66 "Recommmended welded connections for pressure vessels" from the International Institute of Welding in London. It grades general weld configurations for fluctuating Pressure, creep, temp gradients, corrosion, easy assembly, crack resistance, lamellar tearing & inspection.

ps App.1 of W0301 has a 1-pager of weld preps - worth hanging on the wall