Article. Published in PTQ MAgazine. — Q2 — 2010 — 9 p.
Emergency relief in the process industries aims to protect equipment, the environment and operating personnel from abnormal conditions. Appropriate estimation of relief loads under extreme conditions is important for the correct sizing of relief valves and flare headers, and for the selection of disposal media. In addition, during debottlenecking or revamping of process units, adding a new relief valve and modifying the relief system can be very costly and, in terms of construction, difficult to implement.
Estimating accurate relief loads for distillation columns under various conditions is more complex because of compositional changes along the column height. The conventional method of estimating relief load (unbalanced heat method) is normally conservative and leads to bigger relief valves and flare headers, but it is the approach most widely practised. With increasing computing speed and software reliability, process simulation is increasingly used as an important tool for estimating relief load and properties. Steady-state simulation can also be used to estimate the relief load within limitations and can overcome some of the assumptions envisaged in the conventional method. Dynamic simulation provides an alternative method for determining relief load under abnormal conditions.
This article considers different methods for estimating relief load for a distillation column — a debutaniser in this case — and discusses the strengths and weaknesses of each method. There are many emergency cases that apply to a distillation column, and estimation of the maximum possible relief load requires an understanding of plant behaviour and identification of the worst case.
