A company used a flammable solvent in part of its process. For many years the cylindrical steel tank was mounted vertically with a level instrument that passed an electronic signal to the plant control room in terms of the percentage depth of the solvent in the tank. There was a high level trip switch and an open vent pipe.



The tank had a 10,000 litre capacity and it had become standard practice to place an order for additional solvent when the tank reached 25% full (i.e. had 2500 litres remaining). Each order was for 7500 litres of solvent and was delivered in three loads, each of 2500 litres. With solvent continuing to be used between the date of order and the date of delivery, the tank never reached 100% full.

Eventually, the tank corroded to such an extent that it needed replacement. It was decided to replace it with a horizontally mounted tank and the level instrument and high level trip were simply transferred across to the new tank. The new tank was used successfully for 18 months.

One morning the solvent supplier telephoned the Plant Manager and told him that they had been let down by one of their customers and that they had 2500 litres of solvent on a tanker that they did not want to take back to their yard. The tanker was close to his plant and could the Manager help by taking this extra delivery at a slightly reduced price? The Plant Manager checked the level of solvent in the tank via the control room screens and noted that the tank showed 73% full. This he calculated meant he had 2700 litres capacity in the tank so he said he would take this additional delivery.

The tanker duly arrived and delivered the solvent, but towards the end of the delivery, solvent over-flowed from the vent pipe on the tank and was ignited. The fire engulfed the nearby plant and damaged the delivery tanker, but fortunately did not cause the solvent tank to fail or ignite. However, the damage to the plant cost £50,000 and halted production for eight weeks while it was repaired. The lost production was far more costly than the material damage.

The investigation ruled out the possibilities that the tanker was trying to deliver more than the stated 2500 litres and established that the level instrument was reading accurately. It was established that the high level trip switch did not work, having not been wired correctly when the new tank was installed and routine trip testing had not been carried out since then.

While testing the level instrument it was established that it gave a simple linear read out as would be required for a vertically orientated cylindrical tank. However, the level in a horizontally orientated cylindrical tank does not simply rise linearly because the cross section of such a tank is not constant at all depths. While in a vertically orientated cylindrical tank each 500 litres will show an increase in depth of 5% of the tank's capacity, in a horizontally orientated cylindrical tank the first 500 litres will show an increase in depth from 0% to 10% but when 500 litres is added when the tank is half full it will only increase the depth from 50% to 51%.



Thus, when the Plant Manager observed the tank showing 73% depth he rationalised it as 73% full and so there was 2700 litres of space capacity (i.e. sufficient for that delivery). In a horizontally orientated cylindrical tank with a level instrument responding linearly to the depth, 73% depth actually means there were not 7300 litres in the tank but 7800 litres. Hence, the last 300 litres of solvent would inevitably over-flow.