How does pipe diameter affect pressure in fluid flow?

[DinjiP]

How does change in pipe diameter affect pressure? For example, ethylene in a 6" pipe at 400 psi, enters a 1" pipe. How does the pressure change?

I am fairly sure that pressure decreases and velocity increases. However, I need to know how pressure decreases? At what rate? Is it linear?

 

[DinjiP]

I should point out, I do know how to calculate pressure loss across the length of a pipe. That is not what I am interested in.

 

[billy_joule]

I should point out, I do know how to calculate pressure loss across the length of a pipe. That is not what I am interested in.

Pipe friction is termed major losses, pressure drop due to fittings like valves, tees, or in your case a reducer (specifically a 6" to 1" reducer) are termed minor losses and are even easier to calculate. You can find an overview here:
http://www.engineeringtoolbox.com/minor-loss-coefficients-pipes-d_626.html
You'll have to do some more searching to find the minor loss coefficient for your specific fitting.

 

[russ_watters]

Pressure change, both due to velocity change and due to friction losses is a function of velocity. Since your starting pressure is high, if your velocity happens to be low, the pressure change may well be negligible.