3) Always Follow Up on Temporary MOCs

Such as:

1. Inspection, testing, and preventive maintenance tasks and frequencies
2. Monitoring the temporary change on operator rounds
3. Permanent supports for piping or equipment
4. Some personal protection methods
5. Permanent barricades
6. Emergency response systems, i.e., deluge coverage
7. Changes to emergency response plans and procedures due to the presence of the temporary change
8. Materials of construction suitable for long term service
9. Additional permanent instrumentation and controls.

1. The following incident is an example of why temporary changes must be addressed in the MOC process. Even though a MOC was not performed in this case, stress on the temporary piping was a factor in the system failure. Flixborough (Nypro UK) Explosion 1st June 1974. At about 16:53 hours on Saturday 1 June 1974, the Nypro (UK) site at Flixborough was severely damaged by a large explosion. Twenty-eight workers were killed and 36 suffered injuries. Offsite consequences resulted in fifty-three reported injuries. Property in the surrounding area was damaged to a varying degree. Prior to the explosion, on 27 March 1974, it was discovered that a vertical crack in reactor No.5 was leaking cyclohexane. The plant was subsequently shutdown for an investigation. The investigation that followed identified a serious problem with the reactor and the decision was taken to remove it and install a temporary connection (bypass assembly) to connect reactors No .4 and No. 6 so that the plant could continue production. The 20-inch diameter temporary connection used piping bellow units. The temporary assembly had only been pneumatically tested at pressures below the relief valve pressure. There was not an engineering analysis on the piping supports. The welding on the temporary pipework connections had not been tested or checked. During the late afternoon on 1 June 1974, the 20-inch bypass system ruptured at the bellows. This resulted in the escape of a large quantity of cyclohexane. The cyclohexane formed a flammable mixture and subsequently found a source of ignition. At about 16:53 hours, there was a massive vapor cloud explosion which caused extensive damage and started numerous fires on the site. The investigation concluded that the rupture of the temporary piping was due to excessive fatigue on the bellows which was only designed to work in compression-expansion and not with transverse loads. Also, a piping support analysis which typically would have been performed for a permanent change was not done for this temporary change. (1).

1.  Develop and monitor metrics to ensure temporary MOCs are not going past due. (6)
2.  Management review and auditing to ensure temporary MOC’s requirements are being met.

Supplemental Reading- CCPS book - Guidelines for Risk Based Process Safety - MOC and Measure and Metrics chapters 15,20.