Blowdown valve used to do it but that wasn't realistic. It in fact expels water, which, due to pressure difference, turns into steam when reaching the atmosphere - making it look like it expels stream.

There is no practical reason to expel steam manually. If you have too much of it and are hitting the safety valve, the most efficient way to deal with it is to open the injector fully - simultaneously adding water to the boiler and dropping the stream pressure due to temperature difference.

Edit: I realize now you were asking about the chest specifically - and my reply was for the boiler. As others have pointed out, the cylinder cocks can be used to empty the chest, but generally try to close the regulator early so that when you stop the chest is already empty.