There are cases with manhole blind flanges where the number of bolts are not enough for a proper leak tight connection with the flange at the storage tank side. The blind flange takes the shape as an ‘umbrella’ around the bolts.
Description
The EN 1591-1:2001+A1:2009 has the Annex A which is informative. The Annex A gives the ‘Requirements for limitation of non-uniformity of gasket stress’. Below image is the content of the annex:
Image: content of EN 1591-1:2001+A1:2009 Annex A
The above formula gives a good solution to detect widely spaced bolts. However for some small flanges with a low rating like PN6 to PN16 and 150#, and sizes DN50, DN100 and NPS 2”, 3” the bolts are also too widely spaced. A quick review gives the following result:
|
Flange Number |
NomOD |
Rating |
Bolt Size |
NoB |
eF >= e_BSL |
spacing |
|
F_BOLTSPACING001 |
DN100 |
PN2.5/6 |
M16 |
4 |
14.1924 >= 28.4283 |
fail |
|
F_BOLTSPACING002 |
DN100 |
PN10/16 |
M16 |
8 |
18.3525 >= 11.5469 |
ok |
|
F_BOLTSPACING005 |
DN50 |
PN2.5/6 |
M12 |
4 |
12.2456 >= 16.2311 |
fail |
|
F_BOLTSPACING006 |
DN50 |
PN10/16 |
M16 |
4 |
16.2896 >= 19.4416 |
fail |
|
F_BOLTSPACING007 |
DN50 |
PN25/40 |
M16 |
4 |
18.2896 >= 19.8908 |
fail |
|
F_BOLTSPACING008 |
DN50 |
PN63 |
M20 |
4 |
24.1355 >= 21.6334 |
ok |
|
F_BOLTSPACING021 |
NPS 3" |
150# |
0.625" |
4 |
23.176 >= 25.5376 |
fail |
|
F_BOLTSPACING022 |
NPS 3" |
300# |
0.75" |
8 |
27.7434 >= 11.2169 |
ok |
|
F_BOLTSPACING025 |
NPS 2" |
150# |
0.625" |
4 |
18.3123 >= 18.8236 |
fail |
|
F_BOLTSPACING027 |
NPS 1 1/2" |
150# |
0.5" |
4 |
16.6634 >= 13.8332 |
ok |
Table: bolt spacing results according EN 1591-1:2001+A1::2009 Annex A
The above table is based on regular flanges which normally should be Ok. Although these flanges are now not Ok according the spacing of the formula of Annex A, it is the only way to detect the widely spaced bolts in the manhole case as indicated in the introduction. Otherwise a manhole blind flange of 24” might be acceptable with only 4 bolts. In reality we have found many problematic manholes with not enough bolts. This low number of bolts appeared to be the cause of leakage the problem. This would be detected with the subject Annex A of the EN 1591-1 2009 version.
The proposal is to include the Annex A again, and perhaps as ‘informative’, in the EN 1591-1 and to include a statement that some small standard piping flanges might fail.
Image: flange F_BOLTSPACING001 DN100 PN2.5/6
Note: this document has been sent to the CEN/TC 74 WG10 committee.