Spec Break

"Guest" (OP) you wrote:

I am drawing an isometric from Piping Layout and came across an issue. There is a piping spec break shown on the P&ID. The Layout indicates the spec break at the nearest Gate Valve... The two specs are class 300 and Class 150. Obviously in this case Valve would be of class 300. I talked to my colleague on the issue and he told me that there would be an Out-of-Spec flange for the side facing the lower Class line rating. The flange would be of the same material as specified in Class 150 Spec but its class rating (O.D., Bolt Circle, number of Bolts and size of Bolts, etc) would match the Class 300 Valve.

(Note: I took the liberty of modifying your statement for clarity)

I need to ask:

Q1, Is it mandatory (code requirement) to break the piping spec at nearest valve?

A1, I don't think there is anything in the Code that specifically addresses this issue. It is more of a long standing Industry practice based on pure logic.

Q2, Can't we show spec break in continuous piping?

A2, If the Valve and the up-stream line material were to be some very high cost Alloy you would not want to carry the Alloy Spec and added cost, 3 0r 4 meters down stream of the valve.

If you are really, really clever, you might be able to find the point along a pipeline where pressure drop from pipe friction reduces the pressure from one class to the next lower. And then do weird stuff like weld a stick of Sch 80 to the next of Sch 40.

_{Says Groucho Marx with cocked eyebrow.}