Apr. 16, 2018
If you look a water heater in a large building or possibly a home, you might notice a lot smaller tank near by. That small tank is a surge tank. An surge tank in this application is a vessel design to take extra system volume caused by heating. Water is incompressible and, when it is heated, it expands. Should this happen in a system using a fixed volume of pipes and equipment, you will soon find the weakest point in the system as water begins to spray through it. (Hopefully, it's the pressure relief valve releasing water!)
To guard the water heater, pipes, and other system elements from excessive pressure when heating water, a potable water surge tank is additional. A simple device, it is not anything more than a tank using a bladder (thick balloon) inside. The distance between the bladder and the interior of the tank is pre-charged with air to closely match the neutral pressure of the system BEFORE the water is heated. The air outside the bladder compresses from the tank walls, giving the cushion required to safeguard the rest of the system.