TWO workers died in an explosion caused by air entering two high pressure hydrogen storage receivers in a Castle Peak power plant, a coroner's court heard yesterday. Dr Richard Jack, scientific services officer for China Light and Power Company Ltd, was testifying in the inquest into the deaths of two engineers with the company. Mr Wong Kwong-yu, 28, and Mr Yip Ka-pui, 41, died in the explosion on August 28 last year. Coroner Mr Warner Banks heard that hydrogen coolant was either produced on site or stored in bottles. The explosion occurred during the manufacturing of hydrogen on site. During a site visit on Monday, Dr Jack briefed Mr Banks, a jury and legal representatives on the operation of the plant and how the explosion occurred. He said a board of inquiry set up after the accident concluded that the explosion was caused by a mixture of air and hydrogen in the high pressure receivers. This arose because the hydrogen plant had supplied air rather than hydrogen to the two high pressure storage receivers between 6.30 am on August 27 and 2.30 am on August 28. He said the air ingress was most likely caused by a combination of water in pipes between these systems and an inadvertent failure to close two vent valves. No alarms in the plant could indicate there was air inside the hydrogen vessels at the time. He said it was most likely that the pressure ratio across a reduction valve in the distribution panel would generate temperatures high enough to ignite a flammable or explosive mixture. Assistant charge engineer Mr Ben Chau said he started his shift at about 10.30 pm on August 27. He finished work at 8 am. He said it was noted that the hydrogen purity in the casing of generator two in plant B had dropped to 85 per cent, which cast doubt on the purity in the high pressure vessels. He added that the normal hydrogen purity level should be more than 95 per cent. He was asked by his superior, Mr David Ford, to change the hydrogen supply from the high pressure vessels to hydrogen bottles in the early hours of August 28. At the same time, he said Mr Ford asked chemists to take gas samples from the high pressure vessels. Results of the analysis, which indicated that the hydrogen in the receivers had a purity of about 98 per cent, were faxed to the station at about 8.45 am. On receipt of the results, and with an intent to improve the rate of supply of hydrogen to the B2 generator, it was decided at 9.15 am that it would be reconnected to the high power receiver. The hydrogen in the casing of generator two in plant B was changed from the bottle supply to the receiver half an hour later. ''Almost immediately a reduction in B2 generator casing hydrogen purity was seen,'' Dr Jack said. He said at 10.05 am, the two high pressure receivers exploded, resulting in the deaths of the two engineers. Eighteen China Light and Power and contractor's employees were injured. The hearing continues.