Explosion in a urea ammonium nitrate (UAN) fertiliser transfer pump

OG status: Fully open
Author unit/section: HID CI4

A dangerous incident occurred at a facility storing urea ammonium nitrate (UAN) fertiliser in solution. Following a tanker loading operation a UAN transfer pump was inadvertently left running. The transfer pump continued to run for several hours until an explosion occurred. Fragments were projected over an extended distance and widespread damage resulted. One fragment punctured a nearby gas oil tank resulting in a substantial loss of product.

In solution UAN is normally considered low hazard. However, when the water in UAN evaporates, residue may include concentrated ammonium nitrate and urea. When sensitised or during decomposition, concentrated ammonium nitrate may become unstable and/or explosive. UAN pumps left operating for extended periods or left running against a blocked discharge have been known to detonate. In this incident the pump was left running, creating conditions for forming the concentrated ammonium nitrate and urea deposits which led to this explosion.

Operators are advised to ensure that UAN transfer pumps are fitted with suitable protection, for example a cut-out that operates in the event of high temperature and/or no or low liquid flow. It would also be advisable to instigate systems of work requiring checks by site staff to ensure that transfer pumps are switched off on completion of tanker loading or other transfer operations.