This document is for discussion, comments and suggestions. The purpose is to demonstrate the dimensions of the 42” 150# flange in relation to bolt load requirements for leak tightness. The 300# flange is used to illustrate the differences and the cause for possible tightness problems in case a spiral wound gasket is used
Possible unfavorable leak tightness with spiral wound gasket
Description
The flange image below is to scale:
Figure: ASME B16.47A – 42” 150# and related ASME B16.20 spiral wound gasket
The required bolt load for seating gasket was difficult to reach or not possible during the calculation of the flange. The EN 1591-1 was used to calculate the bolt for minimum leak tightness of the gasket.
In the quest to find the cause, the 300# flange was also calculated and there was no problem to tighten the gasket. The 300# flange, to scale, is shown below:
Figure: ASME B16.47A – 42” 300# and related ASME B16.20 spiral wound gasket
The reader could notice that the 300# flange is much more compact than the 150# flange. It seams that the bolt circle for a 150# is much larger than the bolt circle for a 300# flange.
It can be calculated (by formula or by finite element analysis) that if the distance between the bolt circle and the seating face is larger, the stresses in the hub connecting to the flange ring will increase. Secondly the flange will rotate more with the same bolt load.
The following figure shows the same flanges 150# and 300# opposite of each other with the relevant gaskets to further see the dimensional differences.
Figure: ASME B16.47A – 42” 150# (red) and 300# (blue), dimensions in millimeter
The main dimension are shown according ASME B16.47A:2017 and ASME B16.20:2017. Please notice the outside raised face (RF) distance from the center and the spiral wound seating outside distance to the center.
The lever arm for the bolt load in relation with the gasket seating face is shown in a box. The 150# lever arm is 66.65 mm and the 300# is 42.8 mm. Due to this larger lever arm the 150# flange will already rotate more than the 300# if other dimensions would be the same.
The next image shows the seating area in more detail.
Figure: ASME B16.47A – 42” 150# (red) and 300# (blue), seating area
The figure show that in both cases, 150# and 300#, the raised face (RF) is close to the bolt holes. The seating face (SPW) of the gasket for the 300# (blue) close to the edge of the raised face but the 150# (red) seating face is quite a distance from the raised face edge. A large part of the raised face to the bolt is covered by the outer ring of the gasket. The reason for having the seating close to the inside bore of the flange is most probably to have a minimum area for holding fluid, for contamination reasons and also to have minimum internal pressure on the raised face working in the opposite direction of the bolt load.
The current design of the 150# flange introduces possible problems with the spiral wound gasket, refer to below image:
Figure: ASME B16.47A – 42” 150# (red) and 300# (blue), location of forces
The above image shows the forces of the gasket seating and the bolt load with the red and blue arrows. The influence and forces of internal pressure and outside (pipe stress) forces are omitted for simplicity.
The tightening of the bolts will compress the gasket seating area for example from 4.50 mm to 3.67 mm thickness. The outer ring of the spiral wound gasket according the standard is 2.97 mm. Due to the rotation of the flange ring, the raised face of the 150# flange might touch the outer ring of the gasket before the required assembly bolt load is reached.
For example: if the flange rotates 0.6 degree and the distance between the gasket seating outer diameter and the raised face outer diameter is 35 mm the edge raised face will come 0.37 mm closer to the outer gasket ring. This is problematic in this example because (3.67 mm – 2.97 mm) / 2 = 0.35 mm is less than 0.37 mm. With even higher bolt loads, and thus more compression of the gasket and more rotation of the flange ring, the flange will touch the outer ring of the gasket and the flange will be further lifted from the seating area of the gasket.
Another problematic issue is the stress in the hub connecting to the flange ring in the 150# flange. If the seating face of the spiral wound would be at the edge of the raised face the stress in the flange would be at least 35% lower allowing for a similar higher bolt load, less rotation and better leak tightness of the gasket. This assumption includes the influence of the internal pressure on the larger exposed raised face area to the fluid pressure.
Conclusion
This document compares the dimensional design of ASME B16.47A – 42” 150# and 300# flange with the corresponding ASME B16.20 spiral wound gasket.
There are two potential problems highlighted for the 150# flange:
- Possible contact of the flange with outer ring of the gasket
- High stresses in flange hub/ring area due to the larger diameter of the bolt circle
There is room for improvement of the design of the ASME B16.47A – 42” 150# flange. A more compact design like the 300# flange could be a step in the right direction and provide better leak tightness with a spiral wound gasket
(This report has been sent to the ASME B16 committee)