Rikakose:
From what little or un-important data you’ve given us, I can only conjecture that you have a potential serious problem. And based on that scarce information, the only way that I can see for you to continue using that storage tank the way you propose is to do a complete mechanical design investigation - including detailed design calculations (which I have to assume you don’t have).
I have written a lot of responses to threads such as yours through the years and I continue to be amazed as to how mixed up some storage tank owners and user are when the facts are there in front of you.
50 degrees Celsius should not be a concern for you. 1.0 barg should be. That is fairly obvious. Carbon steels are more than capable of 100 or even 200 oC without a problem. The problem is containing PRESSURE in a storage tank - not temperature.
Why don’t you tell us if you have the fabrication calculations for your tank? Or at least tell us the wall and other thicknesses as well as the tank’s configuration, anchors, and type. The fabrication name plate would even be or some help. The fact that you have been using the tank for years doesn’t surprise me. As I’ve stated many times before, I’ve seen much abuse of storage tanks in my 57 years of practice - in many countries. It is sad how little importance is given to storage tank safe design and operation.
I seriously recommend you obtain a thorough and detailed mechanical design rating on the tank before continuing to use it without having any hard and accurate information on its realistic and dependable MAWP and MAWV.