In one of my design calculation, our client specification mentions the blast load as 70kPa. As mentioned in the discussion forum clarifications dated 30/07/07, I have converted the wind details (wind speed) into wind pressure and then added the wind pressure with the blast pressure. The total value of the same is around 72 kPa. While I entering 72kPa in user defined wind pressure as mentioned in the discussion forum, PVElite is not accepting the same and giving the comment as ‘Please enter number between 0 to 47.879’.


Kindly clarify how to specify the blast pressure. Thanking in advance.

Jayakumar,

It looks as if the maximum user defined value is set to 1000lb/ft². I think I have had this problem before but I used the global scalars to cheat. If you use exactly half of your required value (i.e. 36 kPa) then this will be below the 1000lb/ft² limit and be allowed. If you then go to the Load Cases tab and use a value of 2 for Global Scalar for WI loads then this will double your wind load back up to what it would have been if 72 kPa had been allowed. Remember and click the Set Load Cases to show WI and EQ scalars button which will apply the factor.

That should solve your current problem but I think this should be a PVElite fix.

Stan,

Thanks. Thanks a lot for your valuable information. I checked, the wind pressure by changing Global Scalar for WI loads. Obviously, stress due to combined loads is increasing wherever the Wind load and wind moment facor is considered in the load cases and deflection due to wind is also increased.

Thank you once again and let me know where I can find the wind pressure applied except Input echo of the PVElite results.

While running the calculation, the input echo shows only the applied wind pressure (Not multiplied by global scalar values) and stress due to combined load area the load cases shown as NP+EW+WI+FW+BW, NP+EW+EQ+FS+BS, etc only. I mean the Wind data are not shown as multiplied by factor given by us. Hence, it is difficult to convince client & Authorized Inspector.(I Believe only option is to edit the value indicated in Input echo of PVElite results). If you know any other way to come out, please let me know.

One more clarification Mr. Stan, experts in PVElite are saying the for blast load analysis we have to consider blast load as external pressure (For example my blast load 70 kPa to be applied as external pressure)and in wind details we have to add the same with wind pressure(Here 70kPa Blast load + 2kPa actual wind pressure). In my opinion, this is very conservative. Can you put your comments?

Thanks & Regards,

Jayakumar.

Jayakumar,

The simplest method to demonstrate the effect of applying the Global Scalar may be to produce 'before & after' output. That way the unfactored load can be compared with the factored load.

It took me some time to consider your other point about applying the blast load as an external pressure. It will be conservative to apply this load as an external pressure on a cylindrical shell since, in practice, the load will not be acting around the full circumference. However, a dished end on a horizontal vessel would be different. Also, if you design a large vessel for a low internal pressure it may well offer little resistance to external load and suffer damage under blast. Of course, this would only apply whilst the external pressure controlled (i.e. when the vessel was at very low or atmospheric pressure).

Alternatively, if your design conditions also include full vacuum then this would suggest a combined worst case effective external pressure of 172kPa!

It would be appropriate for the client to specify the acceptance criteria. They have specified a blast load but they should also specify whether the vessel is to be designed to survive blast and remain in service. Unfortunately, you are still left with the problem of evaluating the effect of the blast load. If alternative analysis techniques are not available to you then I believe the only option will be to run it as an external pressure case but it would be correct to take advantage of all parameters that are beneficial (e.g. increased allowable stress for occasional load).