Tarks
Mechanical
- May 19, 2023
- 1
I am quite used to using our companies calculators to design pressure vessels to PD5500, but in this case we have been handed a project from a customer that another company started but was unable to finish. There is a real potential saving in using their design/calculations, but we need to be able to get behind them as well. I'm currently a bit unsure of how they have come to their conclusions ad was hoping someone could clarify.
The vessel is a cylindrical shell, P = 192 MPa, 17-4 vessel and end cap (T=125°C). A blank flange that is bolted with 8 x 1.5" x 8 UN bolts (SA-354 BD). Test pressure 275 MPa, using a BX gasket ring (316 SS).
I have been able to hand check the majority of their workings and for the most part I am able to agree with their answers. They have used Fbtest figures (763 MPa) to arrive at their bolt load ratios, which I have been told by the notified body that assessed them is fine to do. The assembly load ratio on the bolts (G.7-3) is 0.94, which seems large but is fine. The load ratio that I want to be able to check, but don't seem to be able to find, is the stress on the threads in the vessel. The Vessel is just a cylindrical shell with 8 tapped holes in each end. The flange is connected by fully threaded studs. The stress that is being applied to the bolts on assembly and during test is directly transferred to the vessel. 6.3.2 & 6.3.3 are where the design stresses for the vessel are derived, and there is a little note under table 11.4-2 that caught my eye "the assembly condition an operating conditions are both normal design conditions...".
This is making me a little wary, the ftest figures for 17-4 are 621 MPa, and fd of 434.67, which are quite a lot lower than the bolting. Does EN 13445 have a direct check for the stress in the threads of the vessel at all conditions? They are squeezing everything they can out of their bolting, which is fine as they can have a separate fatigue life. But if the fatigue life of the vessel is governed by the same number, then it wll be a huge expense (if it can even get through the test!)
For reference the bolt torque is 4553 Nm.
Checking the bolt stress another way at hydro test, the load/area is 960.7 MPa, which fails.
Any insight would be greatly appreciated.
The vessel is a cylindrical shell, P = 192 MPa, 17-4 vessel and end cap (T=125°C). A blank flange that is bolted with 8 x 1.5" x 8 UN bolts (SA-354 BD). Test pressure 275 MPa, using a BX gasket ring (316 SS).
I have been able to hand check the majority of their workings and for the most part I am able to agree with their answers. They have used Fbtest figures (763 MPa) to arrive at their bolt load ratios, which I have been told by the notified body that assessed them is fine to do. The assembly load ratio on the bolts (G.7-3) is 0.94, which seems large but is fine. The load ratio that I want to be able to check, but don't seem to be able to find, is the stress on the threads in the vessel. The Vessel is just a cylindrical shell with 8 tapped holes in each end. The flange is connected by fully threaded studs. The stress that is being applied to the bolts on assembly and during test is directly transferred to the vessel. 6.3.2 & 6.3.3 are where the design stresses for the vessel are derived, and there is a little note under table 11.4-2 that caught my eye "the assembly condition an operating conditions are both normal design conditions...".
This is making me a little wary, the ftest figures for 17-4 are 621 MPa, and fd of 434.67, which are quite a lot lower than the bolting. Does EN 13445 have a direct check for the stress in the threads of the vessel at all conditions? They are squeezing everything they can out of their bolting, which is fine as they can have a separate fatigue life. But if the fatigue life of the vessel is governed by the same number, then it wll be a huge expense (if it can even get through the test!)
For reference the bolt torque is 4553 Nm.
Checking the bolt stress another way at hydro test, the load/area is 960.7 MPa, which fails.
Any insight would be greatly appreciated.