Im gonna try and not be too verbose here - If you want more info, then please ask.

I have a liquified vapour delivery system from a pressurised vessel. Im happy with the PRV ive designed to protect this vessel using API520.

The pressurised, liquified vapour is then pumped through a long pipe section (100m). This pipe section has a relief line that will return the chemical back to its tank.

Now the purpose of this line was really if the pump dead-headed or something similar - it was to carry over-pressure liquid; however, it's really just occurred to me that it may have to carry vapour in the event of sudden pressure loss or a fire or similar event.

Getting to the point API520 is for vessels, how do I design for relieving a pipe? Is there a code to use or am I being stupid and missing something obvious??

thanks for any help

The code that applies (and is the code to use) is your local safety code and/or regulation.

You have to protect your vessel. You have to protect it from any over-pressure source you attach to it (or allow to be attached to it). You haven’t stated what you designed your vessel PSV for – i.e., the worse, credible relief scenario. If you didn’t include a potential fire around your pump’s recycle valve (it’s there for capacity control of the pump’s discharge) as a potential contributor to your vessel’s vapor relief load, then you should seriously look at it if it is credible and possible.

You also must protect the piping. But if your piping is directly connected to the vessel, it is the vessel that is affected and must be relieved if it can’t take the pressure excursion.

All the possible, credible over-pressure cases have to be considered - and I have to presume that you've done that.

If you are indeed only concerned about the pipe, process piping falls under ASME B31.3 and will be Code only if your location has adapted this as such.