Hi Arshad, you can do these ...

Hi Arshad, you can do these following approximations:

1. Can you measure pressure differences and flowrate between 2 points?

If yes you can use simplified Bernoulli Equation to find the Area of the pipe with the following calculations (assume that there is no changes in area and no changes in height between those 2 points of measurements):

A (area of pipe) = sq root of (rho*g*(Q2 squared - Q1 squared) / 2*(P1-P2))

You can then easily find the diameter of “A” using area of circle formula.

2. You can use an approach by looking for flowmeters specs. Most flowmeters have a link between their average flowrate capacity and pipe diameter. As you mentioned flowrate of 40 m3/h, if this is the average/permanent/most dominant flowrate, then the closest diameter of flowmeter that will match this requirement is diameter (DN) between 50-65 mm.

Hope this help.