I want to know what is the deciding yardstick to go in for a slip on or socket welded flange ? In other words what are the relative merits and demerits of each ?

Hello,

Lots of issues involved in this one. Size limitations, cost, welding difficulty, pressure and importantly, fatigue.

Go here:

http://lightning.prohosting.com/~piping/flange/fngmain.htm

Regards, John.

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.

A major demerit of SW flanges over silp on:
Because of geometry, welding is possible only from one side on SW flanges which leaves cavity between the pipe outside wall and the inner diameter of flange, meaning there is always entrapped fliud in this space. The fluid being in static condition in the cavity causes so called crevice-corrosion.
That is why, on many occasions, SW flg. gets ruled out in the pipeclasses particularly for corrosive and dangerous fluids.

How acceptable is a slip-on flange directly welded onto a elbow?

Whilst i would not recommend this for stress critical lines, i am being asked if it is ok for normal service (amb@10bar)
I am sure it is ok and indeed, normal practice in some industries, yet i can not find any information to back up my belief.

While this would fall IMHO under the misuse of listed components it is indeed commonly done.

The flanges were developed for straight pipe just as the elbows were, so finding something in writing that says "gee this is swell thing to do" is very unlikely although its commonly done.

Kind of like motor speed laws... almost everybody speeds a little bit but according to the letter of the law over is over.