Page 74 - Applied Process Design for Chemical and Petrochemical Plants Volume I

Fluid Flow 61

Welding neck flanges are distinguished from other types by their long tapered hub and gen-
tle transition of thickness in the region of the butt weld joining them to the pipe. Thus this type
of flange is prefemd for every severe service condition, whether this results from high pres-
sure or from sub-zero or elevated temperature, and whether loading conditions are substan-
tially constant or fluctuate between wide limits.

Slip-on. flanges continue to be preferred to welding neck flanges by many users on account
of their initially lower cost, the reduced accuracy required in cutting the pipe to length, and the
somewhat greater ease of alignment of the assembly; however, their final installed cost is
probably not much, if any, less than that of welding neck flanges. Their calculated strength
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.

Lap joint flanges are primarily employed with lap joint stubs, the combined initial cost of the
two items being approximately one-third higher than that of comparable welding neck flanges.
Their pressure-holding ability is little, if any, better than that of slip-on flanges and the fatigue
life of the assembly is only one-tenth that of welding neck flanges. The chief use of lap joint
flanges in catbon or low alloy steel piping systems is in services necessitating frequent dis-
mantling for inspection and cleaning and where the ability to swivel flanges and to align bolt
holes materially 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, are confined to special applications. Their chief merit lies in
the fact that they can be assembled without welding; this explains their use in extremely high
pressure services, particularly at or near atmospheric temperature, where alloy steel is essen-
tial 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 magni-
tude, particularly under cyclic conditions, where leakage through the threads may occur in rel-
atively few cycles of heating or stress; seal welding is sometimes employed to overcome this,
but cannot be considered as entirely satisfactory.

Socket welding flanges were initially developed for use on small-size high pressure piping.
Their initial cost is about 10% greater than that of slip-on flanges; when provided with an inter-
nal weld as illustrated, their static strength is equal to, but their fatigue strength 50% greater
than double-welded slip-on flanges. Smooth, pocketless bore conditions can readily be
attained (by grinding the internal weld) without having to bevel the flange face and, after weld-
ing, to reface the flange as would be required with slip-on flanges.

Figure 2-6. (continued on next page)