Question

why there is a bucket of sand put up at all petrol pumps and tagged fire

Answer 1

Its there yo put off any accidental fire in the petrol units. Sand cuts off the oxygen supply to burning petrol thus putting it off Water won't work as petrol being less denser than water will start floating on it ,as a result still getting enough oxygen to burn

Answer 2

Sand acts as an layer between fire and air and cuts off air supply as a result fire extinguishes