Hindustan Petroleum Corporation Ltd (HPCL) on Thursday said it has signed a pact with Bharat Petroleum Corporation Ltd (BPCL) for synergy in purchase and sale of hydrogen between the refineries the two firms operate in Mumbai. Hydrogen is an essential utility required for treating of diesel and petrol to produce low-emission fuel of Euro-VI or Bharat Stage VI specifications, and is vital for reliable and safe refinery operations.