Seeker88
Mechanical
- Jul 29, 2003
- 22
Good afternoon folks,
I would like to ask a question if possible please relating to the use of ASME VIII Div 1 for calculating component thicknesses at hydrotest.
We are currently designing a shell and tube heat exchanger to ASME VIII Division 1 and the design pressure is 2980 psi on the shellside of the unit. We perform all of the usual calculations for the shells, nozzles, tubeplates and tubes and all the thicknesses seem to fall in line with what we would expect. The tube thickness under the external pressure of the shellside stream is required to be 2.4 mm.
At hydrotest (3874 psi) we are utilising 90% of the yield as the allowable stress and the thickness sees quite an increase compared to design. Even with the increased design stress the tube thickness increases to almost 2.72 mm
Added to this is a client requirement to also manufacture to the PED where the hydrotest factor (1.43) increases the test pressure to 4,261 psi. However, this increase in test pressure results in a huge jump in the tube thickness to 3.5 mm (an increase of 28% in thickness for a corresponding lift of 10% in test pressure).
One suggestion from our engineers is that we are really in the wrong territory using Div 1 for thickness calculations above 3,000 psi. Indeed, when our engineers actually perform the calculations to Div 2 (using Div 1 stresses) the tube thickness at the increased test pressure of 4,261 psi is only 2.3 mm.
Am I being too simplistic in how I seem to be selectively using the codes or is there logic in the fact that above the 3,000 psi we are so far away from thin shell theory that we can expect calculations to produce disproportionate effects.
I do hope this makes sense.
Charles
I would like to ask a question if possible please relating to the use of ASME VIII Div 1 for calculating component thicknesses at hydrotest.
We are currently designing a shell and tube heat exchanger to ASME VIII Division 1 and the design pressure is 2980 psi on the shellside of the unit. We perform all of the usual calculations for the shells, nozzles, tubeplates and tubes and all the thicknesses seem to fall in line with what we would expect. The tube thickness under the external pressure of the shellside stream is required to be 2.4 mm.
At hydrotest (3874 psi) we are utilising 90% of the yield as the allowable stress and the thickness sees quite an increase compared to design. Even with the increased design stress the tube thickness increases to almost 2.72 mm
Added to this is a client requirement to also manufacture to the PED where the hydrotest factor (1.43) increases the test pressure to 4,261 psi. However, this increase in test pressure results in a huge jump in the tube thickness to 3.5 mm (an increase of 28% in thickness for a corresponding lift of 10% in test pressure).
One suggestion from our engineers is that we are really in the wrong territory using Div 1 for thickness calculations above 3,000 psi. Indeed, when our engineers actually perform the calculations to Div 2 (using Div 1 stresses) the tube thickness at the increased test pressure of 4,261 psi is only 2.3 mm.
Am I being too simplistic in how I seem to be selectively using the codes or is there logic in the fact that above the 3,000 psi we are so far away from thin shell theory that we can expect calculations to produce disproportionate effects.
I do hope this makes sense.
Charles
