Warning: There are pitfalls in the SW6 external pressure calculation!
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This post was last edited by yuqinglin876 on 2020-10-27 at 13:33: There are issues with SW6 external pressure calculations! Some pressure vessels operate under both internal pressure and vacuum conditions. When using SW6 software for design calculations, the designer typically first conducts calculations for internal pressure; once these are complete, the nominal thickness obtained is used to carry out calculations for vacuum conditions. In these vacuum calculations, the pressure value is simply modified to a negative value of -0.1 MPa. We know that when calculating internal pressure, it is the correct approach to sometimes take into account the effect of the hydrostatic column in vertical vessels containing liquid materials. However, the situation is different when calculating external pressures; if the hydrostatic head is still included when using SW6, it will lead to design errors. That’s how SW6 works: it adds the hydrostatic pressure of the liquid column to the designed external pressure (adding the positive and negative values), resulting in a very low value for the designed external pressure; if the allowable external pressure is greater than this value, then the calculation is considered successful. Example: Diameter 1600, cylinder length 8000, design pressure under internal pressure condition is 0.2 MPa, vacuum condition (-0.1 MPa), design temperature 60 degrees, hydrostatic pressure of liquid column 0.05 MPa, C2=2. The calculated pressure for internal pressure is 0.25 MPa; entering 8 for thickness is sufficient ； Vacuum calculation (design pressure of -0.1 MPa, with the hydrostatic pressure of 0.05 MPa remaining unchanged): a thickness of 8 mm, along with the addition of reinforcing rings, also passes the calculation, as the calculated pressure for SW6 is -0.1 + 0.05 = -0.05 MPa! And if the allowable external pressure > -0.05, it is considered passed! In fact, the calculated pressure is -0.1 MPa; the hydrostatic pressure of the liquid column should not be taken into account. If the hydrostatic pressure of the liquid column is set to 0, the calculation will fail! Because the originally calculated allowable external pressure
I remember encountering this issue before when reviewing drawings of a vertical unit with its equipment flanges; when calculating the negative pressure, the static pressure of the liquid column was not taken into account.
The teacher explained it during the audit and renewal process for pressure vessels
You need to be extremely careful when using SW6, otherwise you’ll regret it: lol
To be honest, SW6 has many bugs and unreasonable aspects; the central station isn’t updated or adjusted either. For example, there are too many openings in the equipment, and the amount of material left for reinforcement rings is too small. Reinforcement rings have to be added in every case, and it’s necessary to check individually whether each ring should be retained. Can’t there be a list format for this? Or conduct a single-hole reinforcement calculation. Another example is the floating head heat exchanger: its outer cover is simply called an outer cover. New employees who are not familiar with its structure tend to enter all the data based on the tube side parameters. GB 29465 has been in use for many years now – can’t there be a database similar to that for flanges specified in standard 47023? There are too many of these; I’m too exhausted to complain – it really isn’t as user-friendly as PVDESKTOP.
I never noticed this before; I’ve learned from it, and I’ll be more careful going forward.
SW6 is not on the same level as PVDESKTOP; for example, when it comes to external pressure, PVDESKTOP accepts inputs for both internal and external pressures and calculates them separately automatically. The allowable external pressure is clearly specified as >0.1 MPa, which is simple and straightforward – there is no need to combine different pressure values at all. With SW6 combined, the concepts are all wrong!
Yes, using SW6 can lead to errors if you’re not careful
Yes, using SW6 can lead to errors if you’re not careful
There is no problem with the relevant sections in 150; the allowable external pressure is greater than the design external pressure. The problem with SW6 when dealing with this computational load is that it applies the concept of internal pressure calculation to external pressure calculations, which is incorrect