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Flange Leakage Checking


Flange Leakage app
Flange Leakage app

Flanges are designed to remain leak-free under hydrostatic test pressure (cold condition) and under operating pressure (hot condition). Flange Leakage is a function of the relative stiffnesses of the flange, gasket, and bolts.

Flange leakage is considered a serious issue in the process plant operation. It has a tremendous potential to cause a severe hazard in the process plants. Therefore, flange leakage needs to be investigated during the design stage to reduce the possibility of leakage during the testing and operation stage.

The design of ASME flanges (ASME B16.5, B16.47) does not take into account the external bending moment and axial force in the pipe. This will cause a wire drawing effect on the mating surface of the flange. Therefore, additional flexibility is to be provided when a flange joint is located near a spot with a high bending moment. So, flange leakage checking is really required.

ASME Piping Flanges (ASME B16.5, B16.47 …) are designed in accordance with ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, Appendix 2, but using allowable stress and temperature limits of ASME B 31.3 code.


In general practice, the following cases need to be considered for flange leakage:

· Flanges with pressure rating 600 and above

· Flanges with pressure rating 300 and above, pipe size 16 inches and above

· Pipe flanges carrying category M fluid service

· Pipe flanges carrying hydrogen or other flammable fluid

· Flanges in PSV lines with NPS 4 inches and above

· Flanges in jacketed piping

· Flanges with stress analysis output of very high bending moment


There are several methods to determine if the flange joint is leaking, including:

- Pressure Equivalent method based on ASME B 16.5 P-T rating

- ASME BPVC Sec VIII Div.1 Appendix 2 method.

- NC 3658.3 method


In this article, we'll explore into how to check for flange leaking by ASME BPVC Sec VIII Div.1 Appendix 2 method together with the application of pressure equivalent method to take into account external force and moment.

Kellogg Method converts piping axial forces and bending moments into an equivalent pressure on the flange.

Peq = 16Mb /πG³+ 4Fa/πG²+ Pd

Where:

Peq = total equivalent pressure

Mb = calculated bending moment on flange

G = diameter of effective gasket reaction

Fa = axial force of flange

Pd = design pressure

Before we go to the detailed calculation, we need to point out here two different cases for the gasket category:

- case 1 for spiral-wound and ring joint gasket

- case 2 for the full-face gasket.


1. For spiral-wound and ring joint gasket.