I think that 36 liters is cutting it a bit close for a 20 liter batch. You need to figure on boil off volume, grain displacement and any process (trub) losses you may have in your process.

For example, I brew 10 liter BIAB batches in a 20 liter kettle. It fits, given that I plan on 11 liters post boil (10 liters in fermentor, 1 liter trub), my boil off rate is 2.2 liters/hr, so for a 90 minute boil, I am losing 3.3 liters. That gives me 14.3 liters pre-boil volume at room temperature. My grain absorption is .5 liters/kg of grain and a high gravity brew for me takes around 3.5 kilos of grain. So that adds another 1.8 liters of water needed for mashing in. Since we have established that the water displacement of the grain is 0.65 liters/kg of grain, this adds 2.3 liters of volume needed in the kettle. So my volume of 14.3l plus 1.8l for grain absorption and 2.3l for grain displacement gives me a total required volume for my kettle of 18.4 liters. Just fits, but leaves not so much room for mixing and stirring in the grains.

Luckily, I get pretty good mash/lauter efficiency from my system. If I had a lower mash efficiency, say 70%, I would be overflowing my kettle with the increased grain bill.

Take similar figures from your brewing and size up what you might need for volume for a 21 to 22 liter batch and then add a bit of a safety factor of a couple of liters just to be on the good side of things.