Safety Issues Flashcards
(5 cards)
What is the conventional method for measuring level in distillation columns?
Pressure sensors placed near top and bottom of column.
Determine delta pressure
Using P = rho g h to determine liquid height h
Why does the vessel vapour pressure not affect measuring the level?
Changes in vapour pressure affect both pressure sensors evenly so delta P is still the same
What is the disadvantage of using delta pressure during start up?
During start up, liquid is required at bottom of column to warm up and start circulations.
Densities of the liquid inside the column may be incorrect in accordance with calibration.
What will happen if the density of the start up material is less than the expected density?
The instrument reads a lower dP for the same liquid height (since dP = ρ g h).
Calibrated for 775 kg/m³ but actual is 700 kg/m³ over 3 m:
Expected full‐scale dP = 775×9.81×3
Actual dP = 700×9.81×3 → same dP would come from only 2.7 m of 775 kg/m³ fluid
Therefore, when the column is truly full to the top tapping, the gauge reads 90% of range.
As level rises above the taps, dP doesn’t change, so the instrument never reads beyond 90%, risking unnoticed overfill.
What happened at BP Texas City?
Densities of liquid in column was less than density determined at calibration.
Level instruments read less than 100% despite being full and alarms did not go off.
Column continued to be filled and then overflowed.
Column had no flare system so overflowing stream was sent to vent stack.
Liquid hydrocarbons leaked out of vent stack producing vapour cloud which then ignited by nearby diesel vehicle, causing explosion.
