Flange analysis

The code rules for bolted flange connections provide only for hydrostatic end loads (operating conditiong with pressure) and gasket seating(gasket seating condition with bolt tightening).

The design of a flange involves the selection of the gasket (material, type, and dimensions), flange facing, bolting, hub proportions, flange width, and flange thickness.

The required bolt load for the operating conditions Wm1 is determined in accordance with eq. (1).

Wm1 = H + Hp = 0.785G2P + (2b x 3.14GmP) (1)

Flange Design Bolt Load; W = Wm1

Before a tight joint can be obtained, it is necessary to seat the gasket or joint-contact surface properly by

applying a minimum initial load (under atmospheric temperature conditions without the presence of internal pressure), which is a function of the gasket material and the effective gasket area to be seated. The minimum initial bolt load required for this purposeWm2 shall be determined in accordance with eq. (2).

Wm2 = 3.14bGy (2)

Flange Design Bolt Load; W = (Am + Ab )Sa/2

The need for providing sufficient bolt load to seat the gasket or joint-contact surfaces in accordance with eq. (2) will prevail on many low-pressure designs and with facings and materials that require a high seating load, and where the bolt load computed by eq. (1) for the operating conditions is insufficient to seat the joint. Accordingly, it is necessary to furnish bolting and to pretighten the bolts to provide a bolt load sufficient to satisfy both of these requirements, each one being individually investigated.

When eq. (2) governs, flange proportions will be a function of the bolting instead of internal pressure.

Notations

G: diameter at location of gasket load reaction

b: effective gasket or joint-contact-surface seating width

m: gasket factor

y: gasket or joint-contact-surface unit seating load

H: total hydrostatic end force

Hp: total joint-contact surface compression load

P: internal design pressure

Ab: cross-sectional area of the bolts using the root diameter of the thread or least diameter of unthreaded position

Am: total required cross-sectional area of bolts, taken as the greater of Am1 and Am2

Am1: total cross-sectional area of bolts at root of thread or section of least diameter under stress, required for the operating conditions = Wm1 / Sb

Am2: total cross-sectional area of bolts at root of thread or section of least diameter under stress, required for gasket seating = Wm2 / Sa

Sa: allowable bolt stress at atmospheric temperature

Sb: allowable bolt stress at design temperature

Thus, the gasket seating condition is dependent on effective gasket width(b), gasket seating load(y) and gasket reaction diamaeter (G), but independent on pressure.

If your flanges do not meet code requirement for gasket seating conditon, it is because the selections of gasket (y, G, b) are not appropriate for the code requirements. (Frankly speaking, sometimes the low pressure rating flanges such as CLASS 150 - SWG gasket combination fail these criteria.)

Anyway if you want to meet the code requirements, you need to change gasket selection (y, G, b) OR shall take higher class flanges.

What Are m and y Gasket Design Constants, and How Are They Usedm

Consideration of Flanged Bend while modeling in Caesar II

http://www.whatispiping.com/flanged-bend

PARAMETRIC STUDY OF FLEXIBILITY FACTOR FOR CURVED PIPE AND WELDING ELBOWS,

Oleg Kireev, Dmitry Kireev and Alexey Berkovsky, Transactions, SMiRT-22, San Francisco, California, USA - August 18-23, 2013, Division III