Pipe bend for high temperature line

Dear all,

Recently i have got an opportunity to study high temperature lines of thermal power plant critical lines, here i have observed that the bend radius is 5D in some cases and 1.5D in some other cases 3D in other conditions. Till now i had the chance to work with elbows pertaining to ASME B16.9 specification itself which contains 1.5R and R bend radius. I would like to know in what conditions we use bend radius greater than 1.5R. What are the limitations that we dont use bends with bend radius greater than 1.5R in general applications.

Thanks ....

In addition to the above post...selection criteria for R and 1.5R elbows was pressure drop, turbulence, space constraint. what is the selection criteria for using a 3D or 5D bend...

Thanks again....

In addition to the above post...selection criteria for R and 1.5R elbows was pressure drop, turbulence, space constraint. what is the selection criteria for using a 3D or 5D bend...

Thanks again....