UK HSE: Workers injured by unplanned release of mud slurry

What happened

An electricity power station company was prosecuted after three contractors were injured by a sudden release of water and sludge, while carrying out maintenance work. Three crew were removing a recirculation pump for maintenance when approximately 39 cubic metres of diluted slurry and associated stored energy was released, while the pump was being removed.  The contractors were swept off the concrete plinth where they had been working and sustained multiple fracture injuries.

What were the causes/what went wrong?

An investigation found that the pressurised contents of the pump and associated pipework had not been discharged prior to work starting, because the drain pipe was blocked. This resulted in the release of stored energy during the removal of the pump, instead of during the isolation process. There had been a failure to undertake a suitable and sufficient assessment of the hazards and risks associated with the work, and their employer had not implemented a safe system of work.