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Flanges with Different Bending Moments:

The flange design moments differ from the norm for external pressure, reverse flanges, and flat flanges.

Under external pressure only the end load and flange pressure are included in the design, and their signs are reversed. For reverse flanges all the moments are present, but the moment arm hd is negative, making MD negative. The load HT is negative, and the moment arm ht may be either positive or negative. The absolute value of the moment is used in the calculations.

For flat faced flanges an alternate value of hg (h''g) is used to calculate a reverse moment at the bolt circle. No calculations for seating conditions for full faced flanges are required.

Blind Flanges and Channel Covers:

The ASME Code formula for a circular blind flange is:

t = d * SQRT( C * P / S * E + 1.9 * W * Hg / S * E * d3 )

The first term in this formula is the bending of a flat plate under pressure. The second term is the bending of the plate due to an edge moment. The stress is limited to 1.5 times the allowable stress, but the 1.5 factor is already built into the equation. For seating conditions the first term is zero - the thickness of the flange depends only on the edge bending.

For non-circular blind flanges the term Z is added to the first term in the square root. Once again, Z is a simple function of the ratio of the large dimension to the small dimension of the flange. It is interesting to note that the Code covers non-circular blind flanges, but no other type of non-circular flanges (not even in the rectangular vessel appendix).

Channel covers designed to TEMA must meet at least the minimum thickness requirements of the Code. In addition, if there is a pass partition groove, the cover deflection is limited.

The formula for flange deflection limitation is found in paragraph 9.21 of TEMA. The deflection is, of course, a function of t3 and G3. Thus, a very small increase in flange thickness will decrease the deflection significantly. The Seventh Edition of TEMA also gives recommended deflections as a function of flange size. The previous editions hid the actual deflection you were working toward in a thickness equation.

Allowable Flange Stresses:

Allowable flange stresses are based on the ASME Code Allowable Stress for the flange material at the Ambient and Operating design temperatures. In the case of bending stresses, these allowable are multiplied by 1.5. This takes into account the higher maximum strain required to yield a section in bending versus pure tension. The stresses calculated and the allowable stresses are as follows:

Operating

Ambient

Longitudinal Hub Stress (bending)

1.5 x Sfo

1.5 x Sfa

Radial Flange Stress

1.0 x Sfo

1.0 x Sfa

Tangential Flange Stress

1.0 x Sfo

1.0 x Sfa

Maximum Average Stress

1.0 x Sfo

1.0 x Sfa

Stress in Bolts

1.0 x Sbo

1.0 x Sba

Stress in Reverse Flanges

1.0 x Sfo

1.0 x Sfa

Stress in Full Faced Gasket Flanges

1.0 x Sfo

1.0 x Sfa

Where:

Sfo

=

ASME Code Allowable Stress for flange material at operating temperature.

Sfa

=

ASME Code Allowable Stress for flange material at ambient temperature.

Sbo

=

ASME Code Allowable Stress for bolt material at ambient temperature.

Sba

=

ASME Code Allowable Stress for bolt material at ambient temperature.

Maximum Allowable Working Pressure:

The following graph shows conceptually how the software extrapolates for the Maximum Allowable Working Pressure:

1. For Operating Pressure MAWP

The software calculates the stresses at the pressure given and calculates the slope between the stress at zero pressure and the stress at the given pressure.

The software extrapolates the slope out to the point where the stress is equal to the allowable stress. The pressure at this point is the maximum allowable working pressure.

2. For Gasket Seating MAWP

Note that at low pressures the stress due to gasket seating is not a function of the design pressure. At higher pressures the stress is a function of pressure, and the MAWP can be calculated as described above, except that the extrapolation is from the point where pressure comes into the calculation of the seating stress.

The software calculates the Gasket Seating MAWP and Operating MAWP based on the input geometry and pressure. In theory both MAWPs should be independent of the input pressure. However, because of the extrapolation algorithm, the estimate of the MAWP may depend on the pressure slightly (when the pressure is very small). Please note that in Partial or Design mode, the software calculates MAWP based on the required flange thickness.

Flange Rigidity Calculations

Appendix 2 also contains equations that attempt to determine whether or not a given flange geometry will leak. The cases considered are ambient and operating. If the computed rigidity factor is > 1.0, then leakage is predicted.

Appendix 2 calculations are mandatory as of Addenda-2005.