can we connect two flange w/ different class in one pipe

Sure you can do that - however, your pipe system will only be designed for the lower class rating (pressure/temperature)

If you intend to couple a valve of 900 lbs. class to a pipe of 600 lbs. class it is not possible, because the dimension of valve that have 900 lbs. class is different by pipe flange that have 600 lbs. class. Hope to be clear!

I think this is a similar post to the other question of "mating two different flange classes" - however, this question adds a little more information such as:

A class 900 valve mated to a pipe end with class 900 flange, where the other end of the pipe (and the rest of the pipe system has all class 600 flanges).

In other words - you would have a length of pipe with one end class 900, the other end class 600. It is possible, but the pipe rating would be as per the temperature/pressure rating of the class 600 (assuming the pipe is also rated to the same temperature/pressure of the 600 flange)

Didn't you post the same question yesterday, Guest? Please be patient in awaiting a response rather than double-posting. Also, how about registering with CR4? One of the benefits of registering is that you can be notified automatically whenever someone responds to your post. So, you'll never need to double-post again!

Best,

Moose

moose your correct that i post one yesterday but i made a wrong way of expresing my question. but thanks to your reply. guest and aurel answered already what i ment to ask.

one last question... is there a supporting standard that can clearly state that situation. so i can read it, thanks so much again.

ASME B31.3 Paragraph 302.2.1 → Table 326.1 → ASME B16.5

I guess I should mention also that, contrary to what others have said, many of the small bore flanges <3.5" in different classes do in fact match up. And this situation is covered under ASME B31.3 Para. 312.1