All of the answers you have been given above are correct for the theory of a "spec break" in a piping system.
Now let's look at the mechanical placement of a "spec break"
First a "spec break" happens where there is a change from one significant design condition to another.
One example of this is a Pressure Safety Valve (PSV). Up stream of the PSV there is a design condition with a higher pressure normally requiring a higher "spec". Down stream of the PSV there is a flare collection system (or atmosphere) with a much lower design condition. Therefore there would be a "spec break" at the downstream side of the PSV.
There is also the situation that occurs at control valves. If it is a Pressure Control Valve (PCV) there will be a higher pressure on the upstream side and a lower pressure on the down stream side. With this control valve there are other valves involved. There are the up stream and down stream block valves which allow for the removal or maintenance on the PCV without shutting down the plant. There is also the bypass (globe) valve used to throttle the flow when the PCV is out of operation. For a control valve situation the "spec break" shall be on the down-stream side of the bypass globe valve and on the down-stream side of the downstream block valve. To say it another way, all valves in this control valve station would be the higher spec.
Pennpiper, I agree completely with your statements, but would also point out that in a proper design you would have a PSV downstream of the control valve (set appropriately) if you want to have a spec break to a lower-rated materials.
Another way to answer your question is: due to costs.
You can go with a higher pressure spec for all of your piping system, even when you don't need it, but it would not be cost effective.
So, after a control valve where the pressure is dropped (from 600# - 300# ANSI), you can go with a lower pressure spec, because it is more cost effective.
"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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