Complex sentence: The reactors were constructed from mild steel with a stainless steel lining; when operating they held in total about 145 tonnes of flammable liquid at a working pressure of 8.6 bar gauge (0.86 MPa gauge; 125 psig).
Split into simpler sentences: The reactors were constructed from mild steel with a stainless steel lining; when operating they held in total about 145 tonnes of flammable liquid at a working pressure of 8. ' 6 bar gauge (0.86 MPa gauge; 125 psig).