The yield strength of both B7 and B16 is 105,000psi at ambient temp. Changing the bolts will probably not solve the problem. You will need to look at the joint as a whole and check the following -
1. Flatness of gasket faces
2. Gasket type – jacketed/spiral wound/camprofile/flat metal versus joint type. An example of a poor combination would be a male/female joint with a jacketed gasket.
3. Tightening method – manual slogging/manual torque wrench/hydraulic torque/tension.
4. Torque or tension value
5. Sequence of tightening
6. Flange rotation
7. Surface finish of gasket face must suit the gasket type
8. Alignment of flanges
9. How the gasket is held in place – using tape or sealant
10. Lubricant used and corresponding torque value calculation
Of particular interest in my experience were joints in a steam reboiler where steam is on the tube side with fluctuating levels of condensate in the outlet section of the bonnet. Here, in spite of addressing all of the above, we still had leaks. Finally had to use live loading washers to stop the leaks.
Ashok
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of M S R K S Prasad
Sent: Friday, August 12, 2011 5:40 PM
To: materials-welding@googlegroups.com
Subject: [MW:12205] ASTM A 193 Gr B7 to B16
Dear members,
I am thinking of changing MOC of one of the exchanger channel cover bolts from B7 to B16 as the exchanger channel cover is leaking very frequently. Whenever it leaks we tighten it manually the leak minimizes after few days it resurfaces.In each tightening the nuts are moving by about 10 to 15 deg.
I request members to suggest whether the choice of B16 is OK are any other bolting material is better than B16 is available.
with regards
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