Piping Component: Types Of Flange And Pressure Rating

Flanges are used in piping for making joints which can be opened at a later date. They are usually used for connecting piping to equipment, valves & control valves etc.

A flanged joint is composed of three separate and independent, although interrelated components: the flanges, the gasket, and the bolting, which are assembled by yet another influence, the assembler. Proper controls must be exercised in the selection and application for all these elements to attain a joint, which has acceptable leak tightness.

CLASSIFICATION OF FLANGES

Flanged joints may best be classified by dividing them into two general parts.

Methods of attachment of Flange to Pipe.

Type of Facing on contact surface of flange proper.

Methods Of Attachment Of Flange To Pipe

Screwed flanges:

This flange can be rapidly connected to threaded pipe without welding.

Screwed flange joints have the following inherent weaknesses and disadvantages which restrict their use to moderate operating conditions.

a.Necessity of careful alignment of bolt holes.

b.Flange faces may become distorted when attaching flanges to  pipe.

c.Pipe frequently extends through flange beyond the face.

d.Tests indicate that in tension a threaded joint has less

   than 60%of the strength of the pipe.

e. Material removed in threading results in a thinner wall section

       readily affected by erosion and corrosion.

Threaded connection is susceptible to leakage under any type of cylic operation. This flange is not recommended for use on services which involve bending or thermal cycles.

Usually screwed flanges are used for Galvanized piping.

Sometimes, steel screwed flanges are seal welded to guard against leakage through the threads.

Slip on Flanges:

Slip on type of flange is widely used because of its low initial cost.

Slip on flange is attached to pipe by fillet weld. The welding is either at the back or at back and face. Usually this flange is double welded and vented with the use of 1/8” pre drilled holes through hub. During welding at the face, care should be taken so as to avoid distortion and refacing.

The strength of this flange as calculated from internal pressure consideration is approximately two-thirds that of a corresponding welding neck type of flange.

The use of this type of flange is usually limited to moderate services where pressure fluctuations, temperature fluctuations, vibrations and shocks are not expected to be severe. The fatigue life of this flange is approximately one third that of a welding neck flange.

Slip on flange is not recommended for corrosive and/or critical services.

Use of slip on flange is usually limited to class 300 (refer para on pressure temp. rating) and design temperature not exceeding 500° F.

It is essential to ensure proper alignment of bolt holes before welding this type of flange.

Welding - neck Flanges

Welding-neck flanges have a long, tapered hub between flange ring & weld joint. This hub provides a more gradual transition from the flange ring thickness to the pipe wall thickness, thereby decreasing the discontinuity stresses and consequently increasing the strength of the flange. Also welding area is sufficiently away from the face to avoid undue distortion.

This type of flange is attached to pipe by having butt weld which can be radiographed if required.

This type of flange is preferred for extreme service conditions such as repeated bending from line expansion or other forces, wide fluctuations in pressure or temperature, high pressure, high temperature and sub zero temperature.

This type of flange is recommended for the handling of costly, inflammable or explosive fluids where failure or leakage of a flange joint might bring disastrous consequences.

While ordering this type of flange it is necessary to indicate schedule number thickness or I.D and O.D. of connected pipe so that bore of the flange as well as butt welding ends can be machined to suit pipe.

The welding neck flange also requires accurate alignment of bolt holes before welding.

Socket-welded flanges

These flanges are usually used upto 2” size for lines where socket welded joints are permitted.

In this type of flange, hub part has a socket to suit pipe and flange is attached to pipe by having a fillet weld at the back. There is no provision for face welding in this type of flange.

Their use and limitations are same as those applicable to slip-on flanges.

Blind Flanges:

These are used to block-off the ends of piping and valves. These flanges are also used extensively to blank off pressure vessel openings such as handholds and inspection ports.

Blind flanges absorb high bending stresses but do not have to absorb stresses caused by thermal expansion or by weight of the piping system.

Flange Facings

Raised Face

Steel flanges with a raised face are extensively used because of the simplicity of the design.

Moderate unit gasket pressure is maintained with conventional machining and assembly methods and facing is suitable for average service conditions. However for severe service involving high pressure, high temperature, thermal shock or cyclic operation, this type of flange facing may not be satisfactory.

Flanges with rating of ASME 150 and 300lbs have 0.06 inch high raised face which is included in the minimum flange thickness. However, flanges having higher ratings have 0.25 inch high raised face which is additional to the minimum flange thickness.

The raised face is machined with spiral or concentric grooves. Depth and spacing of these grooves vary depending upon finish required to suit the gasket used for making joint. The edges of these grooves serve to deform and hold the gasket.

As per ASME B16.5, for raised face, either a serrated concentric or serrated spiral finish having from 24 grooves/inch to 40 grooves/inch shall be used. The cutting tool employed shall have an approx.0.06 inch or larger radius. The resultant surface finish shall have a 125 micron to 500 micron roughness. The finish of contact face shall be judged by visual comparison with Roughness Average ‘Ra’ standards and not by instruments having stylus tracers and electronic amplification (Refer ASME B46.1)

The facing should not have any radial tool mark since the same would provide shortest path for fluid to escape.

The facing should be properly protected during transportation and handling at site.

Flat ring composition gaskets are normally used having a width equal to the width of the raised face where as flat metal gaskets may be used having a width equal to that used with large tongue-and-groove type face.

Flat face:

Flanges  with  flat  faces  using  full-face  gaskets  are  usually  used  for   making connections to ASME 25lb and 125 lbs. Cast iron flanges and flanged components. This decreases the leverage in bolting, thereby reducing the tendency for the flange to crack.

Male & Female Facings:

They have the disadvantage that the two mating flanges are not identical. For this reason, these flanges are not as widely used as raised face flanges.

Male & female facings have the advantage of confining the gaskets thereby minimizing the possibility of blow-out of the gaskets. The outer diameter of the female face serves to locate and retain the gasket.

Male & female facings offer no protection against forcing the gasket into the line or vessel.

Tongue-and-Groove Facings:

Tongue-and-Groove facings have the advantage of confining the gasket. The presence of retaining metal on either side of the gasket gives protection against possibility of blow out of the gasket as well as protection against deforming soft gaskets into the interior of the line or vessel. This is an advantage over the male-and- female type of face.

The Gasket is less subject to erosive or corrosive contact with the fluid.

In service, the tongue is more likely to get damaged than the groove, therefore the tongue should be placed on the part that can be easily replaced.

Usually for services, where tongue-and-groove facings are used, all  equipment nozzle flanges, valve flanges, and instrument flanges etc. are provided with groove face and mating piping flanges with tongue face.

Pressure-Temperature Ratings

Pressure-Temperature ratings are determined as per Annexure D of ASME B 16.5 For easy reference, ASME B16.5 Table 1A and Table 2 - 1.1 (rating for Group 1.1 material) are attached in section 7.0.

For any temperature below -20°F, the rating shall be no greater than the rating shown for –20 O F.

Socket welding and screwed flanges are not recommended for service above 500 O F or below –50 O F if severe thermal gradients or thermal cycling are involved.

At temperature in the creep range, gradual relaxation of flanges, bolts & gaskets may progressively reduce bolt loads. It may be necessary to arrange for periodic tightening of bolts to prevent leakage.

When used above 400 O F, class 150 flanged joints may develop leakage unless care is taken to avoid imposing severe external loads and/or severe thermal gradients. For other classes, similar consideration should be given above 750° F