The Fluidised Catalytic Cracker Unit (FCCU), which was originally installed in the early 1950’s, is one of the process units in the Oil Refinery on the North Side of the Complex.
FCCU’s are standard installations widely used throughout the world on oil refineries for converting the heaviest components of crude oil into a range of useful products such as motor fuels. There is a significant history of operational incidents involving FCCU’s throughout the world and also at the Complex and information is widely publicised and readily available.
For a general introduction to the basic design and important features of the FCCU and the layout of the facility at BP Grangemouth see Appendix 1.
Since the original construction of the FCCU several sections had been significantly modified. Major modifications to the "front end" reactor and catalyst handling sections (where the heavy crude products are "cracked" into a number of lighter components) had taken place in major revamp projects in 1996 and 1998. The "back end" fractionation section (where the lighter components are subsequently separated) however remained predominantly the original design and installation with only a few minor modifications and improvements.
Following the last major revamp project in 1998 the FCCU had been experiencing considerable difficulties and had not been able to operate consistently. This was due to a combination of technical problems with the "front end" catalyst handling section and other issues such as loss of utility supplies (steam, power etc.). This had resulted in the plant being subjected to numerous start-ups and shutdowns over the intervening two year period and especially during the few months preceding the fire. In the 11 weeks preceding the incident, 19 start-up attempts had been made, of which 7 reached the stage of starting to bring the Vapour Recovery Unit on line.
FCCU start-ups are periods of intense activity for the operators whilst the plant is brought up to the steady-state design and normal operating conditions from ambient and hydrocarbons are gradually introduced into the system. This requires significant manual involvement to open/close valves as necessary on the plant as well as assistance from the control room operators monitoring the plant from the DCS plant control system.
Starting up a process unit results in significant changes (operating temperature and pressure etc.) on the pipework and vessels as they are brought up to the required operating conditions from ambient. Increasing the frequency of start-ups results in fluctuations in conditions and increased cyclic stresses on mechanical systems as a result of these. Under such cyclic conditions the pipework can become "fatigued" with defects developing that can subsequently lead to failure.
The FCCU was shut down on the 29th May following the power distribution failure and was being restarted after an 11 day shutdown. The shut down of 29th May was preceded by a short period of steady operation after the last successful start-up on 24th May.