A friend of mine told me about his client that "required" PSV's too. When he pointed out that for piping there is no code or standard for a fire case, blocked in or not, and that the 2 - 8" by 10" where going to cost $MM they accepted the recommendation.
The fire relief in your case will have to go into a flare header. And after some time period the line WILL fail. You mentioned that the liquid was "monomer", not noing which one, ther may be other issues with keeping it from escaping uncontrolled so the relief valve may be for that case more than for protecting the piping.
Where I worked on time, Exxon saw our check valve arrangement and did the same thing on their butadiene lines and meters because they hated all the little reliefs and vents and flares. They still ended up with 1- 3/4" relief, but it lowered their risk overall.
When you state 25', the code says vessels surface area more than 25' above ground need not be considered in the calculation for fire. If you have pipe you would have to consider the topology and the area where a pooling liquid fire would subject the pipe to. It could be more than 25 feet.
I've seen flowing lines fail when there was a fire near them and there was no pressure rise in the line that a relief valve would open on. You best defence for a pooling fire near pipes is to shut in the lines and let the pipe fail. If its stuff you do not want out of the line, then even with a relief valve you need a way to push the material, like a nitrogen system.