While calculating the wind load multiplier how the effect of ladders is taken?Is 1.2 is a conservative design for a 2.4m dia vessel with 60 mm insulation and a 500 dia pipe running from top to bottom with with 25 mm insulation on the pipe.Platforms are there at every 5 m with cagged ladder (effective width of 300 mm) running from bottom to top of the column is considered.Height of the column is 64m.

Hello Mr Dutta

The wind multiplier refers specifically to the shell, skirt and heads. The platforms are considered separately. In the Constraints screen of PVElite where you can see the button 'Miscellaneous Options'. You can select how the wind pressure on platforms is to be calculated (see Wind areal calculation method). As the external attachments and the method of performing the wind calculation is not in the pressure vessel codes, the designer has a choice. The default value of 1.2 seems to be a reasonable value which accounts for pipine and other wind obstructions, but there is no hard and fast rule. Using the 1.2 value and the default method of calculating the wind area of the platforms is conservative, and may produce higher wind pressures than would actually be encountered in practice.

Books such as 'Pressure Vessel Handbook' by Megyesy, and 'Pressure Vessel Design Manual' by Dennis Moss give some guidance. However, at COADE we have taken what we consider to be a conservative approach.

For the main insulated vessel envelope the actual area could be estimated, the the actual areas of the piping can be estimated. By taking this total area, and divding this into the actual area of the bare undressed vessel itself, you could derive a factor that could be more realistic. However, this is just a suggestion. Great care must be taken for a 64m tower.

In the platform dialogue screen in PVElite, two important values are considered. One is the wind coefficient Cf (usually between 1.2 and 1.8), and the actual wind area of the platform. Both these figures can be adjusted by the user.

Upper platforms in the region of the top of the tower have a marked effect on the wind inducesd stresses.