Site stress relieving at Mathura Refinery

Our first opportunity for stress relieving of sphere was given by the management of Mathura Refinery. We had built a number of spheres for storage of LPG. Each sphere was 18 meters in diameter with a wall thickness of 65mm.
When we arrived at Mathura refinery our team was interviewed by the management to find out whether we knew our business or not. After satisfying themselves we were given permission to go ahead and start the work.
LPG was supplied by an LPG tanker. Since the withdrawal of LPG was very high natural evaporation from the tanker was inadequate.Hence we connected the LPG tanker to an evaporator which drew the liquid from the tank and the liquid passed through heat exchanger converting the liquid in to gas. Air for combustion was supplied by a blower. We had a stand by blower in case the blower failed during operation.
A number of thermocouples were fixed on the sphere and these thermocouples were connected to strip chart recorders. The entire sphere was insulated.We fired the burner out side the sphere to test its performance at various gas and air flows. After ascertaining the performance we fixed the burner at the bottom of the sphere. The top exit of the sphere was kept partially open for the combustion gases to escape. After checking all connections we fired the burner. The gas and air flow was adjusted to give a temperature raise of around 50 deg C per hour.As the temperature reached the specified stress relieving temperature the rate of heating was brought down considerably and once the temperature reached the stress relieving temperature the flame was adjusted to give a heat input which was just sufficient to compensate for heat loss from the sphere.After maintaining the temperature constant for specified time the heat input was slowly reduced and when the temperature reached around 300 deg C the burner was was switched off and the sphere was allowed to cool to room temperature. The cycle took nearly 20 hours.
Full credit of successful stress relieving should go to the head of chemical engineering and his dedicated team. Later a number spheres in Mathura Refinery were stress relieved using the same technology. We stopped requesting foreign help. More than 100 spheres have been stress relieved by now using the technology developed by R&D.