If you have live dead legs, just get rid of them. Its not in any code or standard I know of, but its one of those generally accepted practices in piping design that should be implimented.
Dead legs build up with corrosion products or corrosives and tend to forgotten and not looked at. I've had more than my fair share of leaks associated with them. I've even had a situation where another company supply feedstock had a very minor deadleg that didn't get purged properly. The O2 in the air was more than enough to cause an ethylene reactor to run away.
From Piping Inspection Code Inspection, Repair, Alteration, and Rerating
Of In-service Piping Systems API 570 about dead legs one can read.
“5.3.2 Dead legs
The corrosion rate in dead legs can vary significantly from adjacent active piping. The inspector should monitor wall thickness on selected dead legs, including both the stagnant end and at the connection to an active line. In hot piping systems, the high-point area may corrode due to convective currents set up in the dead leg. Consideration should be given to removing dead legs that serve no further process purpose.”
Dead legs are a havoc to burst out in future. It will make life miserable once the facilities start ageing. Having a spare stub or a valve & not using the same will only corrode the same. Adding a flushing program doesn't work much. The microbial SRB's work 24/7 & our flushing program works once in a while.Several process designers keep adding spare valves which is a waste of resource. Adding piping segments for future use is not a wise thing to do as most often the isolation valves will not hold after several years of idle service.
We can always add provisons using a hot tap or during a turnaround.
In discussion of the 2007 propane fire at the Valero Sunray refinery, the Chemical Safety Board mentioned an out-of-service elbow as part of a dead-leg posing a hazard.
"...did not identify hazards arising from the dead-leg when it was created in the 1990s and did not implement safeguards, such as removing the piping, isolating it from the process using metal plates known as blinds, or protecting it against freezing temperatures."
In some of our tank farms we have short dead legs (10 feet or so) and when I asked why we design them in, was told we replace the piping every 30 years anyway and have had no problems thus far. Which is not saying we should continue doing it! In fact I'm not aware of any time we have actually used these dead legs (ie, when expanding the size of tank farm) or, indeed, for any other purpose. Not sure whey they are even needed.
