The tube sheet is on both sides of flange fastening bolts, they are the same bolts, and the pressure on the gasket on both sides of the tube sheet is the same, and they are under force at same time. If different gaskets are selected on the side of the tube sheet, it will lead to uneven pressure on the gasket on one side, resulting in a seal or too much or too little pressure on one side resulting in failure.
So when we select the gasket on both sides of the tube sheet, we must use the same material
